patch, 2 partsrefs: allow setting the reference directory

diff --git a/refs.c b/refs.c
index 965381367e..23f46867f2 100644
--- a/refs.c
+++ b/refs.c
@@ -2177,6 +2177,15 @@ void ref_store_release(struct ref_store *ref_store)
 	free(ref_store->gitdir);
 }
 
+static struct ref_store *get_ref_store_for_dir(struct repository *r,
+					       char *dir,
+					       enum ref_storage_format format)
+{
+	struct ref_store *ref_store = ref_store_init(r, format, dir,
+						     REF_STORE_ALL_CAPS);
+	return maybe_debug_wrap_ref_store(dir, ref_store);
+}
+
 struct ref_store *get_main_ref_store(struct repository *r)
 {
 	if (r->refs_private)
@@ -2185,9 +2194,7 @@ struct ref_store *get_main_ref_store(struct repository *r)
 	if (!r->gitdir)
 		BUG("attempting to get main_ref_store outside of repository");
 
-	r->refs_private = ref_store_init(r, r->ref_storage_format,
-					 r->gitdir, REF_STORE_ALL_CAPS);
-	r->refs_private = maybe_debug_wrap_ref_store(r->gitdir, r->refs_private);
+	r->refs_private = get_ref_store_for_dir(r, r->gitdir, r->ref_storage_format);
 	return r->refs_private;
 }
 

Documentation/git.adoc, environment.h, refs.c, t/meson.build, t/t1423-ref-backend.sh

diff --git a/Documentation/git.adoc b/Documentation/git.adoc
index ce099e78b8..a1d1078f42 100644
--- a/Documentation/git.adoc
+++ b/Documentation/git.adoc
@@ -584,6 +584,14 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REF_URI`::
+    Specify which reference backend and path to be used, if not specified the
+    backend is inferred from the configuration and $GIT_DIR is used as the
+    path.
++
+Expects the format '<ref_backend>://<path>', where the 'backend' specifies the
+reference backend and the 'path' specifies the directory used by the backend.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::
diff --git a/environment.h b/environment.h
index 51898c99cd..9bc380bba4 100644
--- a/environment.h
+++ b/environment.h
@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REF_URI_ENVIRONMENT "GIT_REF_URI"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the
diff --git a/refs.c b/refs.c
index 23f46867f2..0922f08c9f 100644
--- a/refs.c
+++ b/refs.c
@@ -2186,15 +2186,66 @@ static struct ref_store *get_ref_store_for_dir(struct repository *r,
 	return maybe_debug_wrap_ref_store(dir, ref_store);
 }
 
+static struct ref_store *get_ref_store_from_uri(struct repository *repo,
+						const char *uri)
+{
+	struct string_list ref_backend_info = STRING_LIST_INIT_DUP;
+	enum ref_storage_format format;
+	struct ref_store *store = NULL;
+	char *format_string;
+	char *dir;
+
+	if (!uri || !uri[0]) {
+		error("reference backend uri is empty");
+		goto cleanup;
+	}
+
+	if (string_list_split(&ref_backend_info, uri, ":", 2) != 2) {
+		error("invalid reference backend uri format '%s'", uri);
+		goto cleanup;
+	}
+
+	format_string = ref_backend_info.items[0].string;
+	dir = ref_backend_info.items[1].string + 2;
+
+	if (!dir || !dir[0]) {
+		error("invalid path in uri '%s'", uri);
+		goto cleanup;
+	}
+
+	format = ref_storage_format_by_name(format_string);
+	if (format == REF_STORAGE_FORMAT_UNKNOWN) {
+		error("unknown reference backend '%s'", format_string);
+		goto cleanup;
+	}
+
+	store = get_ref_store_for_dir(repo, dir, format);
+
+cleanup:
+	string_list_clear(&ref_backend_info, 0);
+	return store;
+}
+
 struct ref_store *get_main_ref_store(struct repository *r)
 {
+	char *ref_uri;
+
 	if (r->refs_private)
 		return r->refs_private;
 
 	if (!r->gitdir)
 		BUG("attempting to get main_ref_store outside of repository");
 
-	r->refs_private = get_ref_store_for_dir(r, r->gitdir, r->ref_storage_format);
+	ref_uri = getenv(GIT_REF_URI_ENVIRONMENT);
+	if (ref_uri) {
+		r->refs_private = get_ref_store_from_uri(r, ref_uri);
+		if (!r->refs_private)
+			die("failed to initialize ref store from URI: %s", ref_uri);
+
+	} else {
+		r->refs_private = get_ref_store_for_dir(r, r->gitdir,
+							r->ref_storage_format);
+	}
 	return r->refs_private;
 }
 
diff --git a/t/meson.build b/t/meson.build
index a5531df415..a66f8fafff 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -208,6 +208,7 @@ integration_tests = [
   't1420-lost-found.sh',
   't1421-reflog-write.sh',
   't1422-show-ref-exists.sh',
+  't1423-ref-backend.sh',
   't1430-bad-ref-name.sh',
   't1450-fsck.sh',
   't1451-fsck-buffer.sh',
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
new file mode 100755
index 0000000000..e271708e02
--- /dev/null
+++ b/t/t1423-ref-backend.sh
@@ -0,0 +1,109 @@
+#!/bin/sh
+
+test_description='Test different reference backend URIs'
+
+. ./test-lib.sh
+
+test_expect_success 'empty uri provided' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="" &&
+		export GIT_REF_URI &&
+		! git refs list 2>err &&
+		test_grep "reference backend uri is empty" err
+	)
+'
+
+test_expect_success 'invalid uri provided' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="reftable@/home/reftable" &&
+		export GIT_REF_URI &&
+		! git refs list 2>err &&
+		test_grep "invalid reference backend uri format" err
+	)
+'
+
+test_expect_success 'empty path in uri' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="reftable://" &&
+		export GIT_REF_URI &&
+		! git refs list 2>err &&
+		test_grep "invalid path in uri" err
+	)
+'
+
+test_expect_success 'unknown reference backend' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	(
+		cd repo &&
+		GIT_REF_URI="db://.git" &&
+		export GIT_REF_URI &&
+		! git refs list 2>err &&
+		test_grep "unknown reference backend" err
+	)
+'
+
+ref_formats="files reftable"
+for from_format in $ref_formats
+do
+	for to_format in $ref_formats
+	do
+		if test "$from_format" = "$to_format"
+		then
+			continue
+		fi
+
+		test_expect_success 'read from other reference backend' '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=files repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=reftable >out &&
+				REFTABLE_PATH=$(cat out | sed "s/.* ${SQ}\(.*\)${SQ}/\1/") &&
+				git refs list >expect &&
+				GIT_REF_URI="reftable://$REFTABLE_PATH" git refs list >actual &&
+				test_cmp expect actual
+			)
+		'
+
+		test_expect_success 'write to other reference backend' '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=files repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=reftable >out &&
+				git refs list >expect &&
+
+				REFTABLE_PATH=$(cat out | sed "s/.* ${SQ}\(.*\)${SQ}/\1/") &&
+				GIT_REF_URI="reftable://$REFTABLE_PATH" git tag -d 1 &&
+
+				git refs list >actual &&
+				test_cmp expect actual &&
+
+				GIT_REF_URI="reftable://$REFTABLE_PATH" git refs list >expect &&
+				git refs list >out &&
+				cat out | grep -v "refs/tags/1" >actual &&
+				test_cmp expect actual
+			)
+		'
+	done
+done
+
+test_done

builtin/clone.c, repository.c, repository.h, setup.c, setup.h

diff --git a/builtin/clone.c b/builtin/clone.c
index b40cee5968..28412576b3 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1442,7 +1442,8 @@ int cmd_clone(int argc,
 	hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 	initialize_repository_version(hash_algo, the_repository->ref_storage_format, 1);
 	repo_set_hash_algo(the_repository, hash_algo);
-	create_reference_database(the_repository->ref_storage_format, NULL, 1);
+	create_reference_database(the_repository->ref_storage_format,
+				  the_repository->ref_storage_payload, NULL, 1);
 
 	/*
 	 * Before fetching from the remote, download and install bundle
diff --git a/repository.c b/repository.c
index c7e75215ac..9815f081ef 100644
--- a/repository.c
+++ b/repository.c
@@ -193,9 +193,12 @@ void repo_set_compat_hash_algo(struct repository *repo, int algo)
 }
 
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format)
+				 enum ref_storage_format format,
+				 const char *payload)
 {
 	repo->ref_storage_format = format;
+	free(repo->ref_storage_payload);
+	repo->ref_storage_payload = xstrdup_or_null(payload);
 }
 
 /*
@@ -277,7 +280,8 @@ int repo_init(struct repository *repo,
 
 	repo_set_hash_algo(repo, format.hash_algo);
 	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
-	repo_set_ref_storage_format(repo, format.ref_storage_format);
+	repo_set_ref_storage_format(repo, format.ref_storage_format,
+				    format.ref_storage_payload);
 	repo->repository_format_worktree_config = format.worktree_config;
 	repo->repository_format_relative_worktrees = format.relative_worktrees;
 	repo->repository_format_precious_objects = format.precious_objects;
@@ -369,6 +373,7 @@ void repo_clear(struct repository *repo)
 	FREE_AND_NULL(repo->index_file);
 	FREE_AND_NULL(repo->worktree);
 	FREE_AND_NULL(repo->submodule_prefix);
+	FREE_AND_NULL(repo->ref_storage_payload);
 
 	odb_free(repo->objects);
 	repo->objects = NULL;
diff --git a/repository.h b/repository.h
index 6063c4b846..c648dab196 100644
--- a/repository.h
+++ b/repository.h
@@ -150,6 +150,8 @@ struct repository {
 
 	/* Repository's reference storage format, as serialized on disk. */
 	enum ref_storage_format ref_storage_format;
+	/* Reference storage information as needed for the backend. */
+	char *ref_storage_payload;
 
 	/* A unique-id for tracing purposes. */
 	int trace2_repo_id;
@@ -204,7 +206,8 @@ void repo_set_worktree(struct repository *repo, const char *path);
 void repo_set_hash_algo(struct repository *repo, int algo);
 void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format);
+				 enum ref_storage_format format,
+				 const char *payload);
 void initialize_repository(struct repository *repo);
 RESULT_MUST_BE_USED
 int repo_init(struct repository *r, const char *gitdir, const char *worktree);
diff --git a/setup.c b/setup.c
index b723f8b339..44e393c251 100644
--- a/setup.c
+++ b/setup.c
@@ -632,6 +632,21 @@ static enum extension_result handle_extension_v0(const char *var,
 		return EXTENSION_UNKNOWN;
 }
 
+static void parse_reference_uri(const char *value, char **format,
+				char **payload)
+{
+	char *schema_end;
+
+	schema_end = strstr(value, "://");
+	if (!schema_end) {
+		*format = xstrdup(value);
+		*payload = NULL;
+	} else {
+		*format = xstrndup(value, schema_end - value);
+		*payload = xstrdup_or_null(schema_end + 3);
+	}
+}
+
 /*
  * Record any new extensions in this function.
  */
@@ -674,10 +689,17 @@ static enum extension_result handle_extension(const char *var,
 		return EXTENSION_OK;
 	} else if (!strcmp(ext, "refstorage")) {
 		unsigned int format;
+		char *format_str;
 
 		if (!value)
 			return config_error_nonbool(var);
-		format = ref_storage_format_by_name(value);
+
+		parse_reference_uri(value, &format_str,
+				    &data->ref_storage_payload);
+
+		format = ref_storage_format_by_name(format_str);
+		free(format_str);
+
 		if (format == REF_STORAGE_FORMAT_UNKNOWN)
 			return error(_("invalid value for '%s': '%s'"),
 				     "extensions.refstorage", value);
@@ -850,6 +872,7 @@ void clear_repository_format(struct repository_format *format)
 	string_list_clear(&format->v1_only_extensions, 0);
 	free(format->work_tree);
 	free(format->partial_clone);
+	free(format->ref_storage_payload);
 	init_repository_format(format);
 }
 
@@ -1942,7 +1965,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			repo_set_compat_hash_algo(the_repository,
 						  repo_fmt.compat_hash_algo);
 			repo_set_ref_storage_format(the_repository,
-						    repo_fmt.ref_storage_format);
+						    repo_fmt.ref_storage_format,
+						    repo_fmt.ref_storage_payload);
 			the_repository->repository_format_worktree_config =
 				repo_fmt.worktree_config;
 			the_repository->repository_format_relative_worktrees =
@@ -2042,7 +2066,8 @@ void check_repository_format(struct repository_format *fmt)
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
 	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	repo_set_ref_storage_format(the_repository,
-				    fmt->ref_storage_format);
+				    fmt->ref_storage_format,
+				    fmt->ref_storage_payload);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_relative_worktrees =
@@ -2360,13 +2385,15 @@ static int is_reinit(void)
 }
 
 void create_reference_database(enum ref_storage_format ref_storage_format,
+			       const char *ref_storage_payload,
 			       const char *initial_branch, int quiet)
 {
 	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int reinit = is_reinit();
 
-	repo_set_ref_storage_format(the_repository, ref_storage_format);
+	repo_set_ref_storage_format(the_repository, ref_storage_format,
+				    ref_storage_payload);
 	if (ref_store_create_on_disk(get_main_ref_store(the_repository), 0, &err))
 		die("failed to set up refs db: %s", err.buf);
 
@@ -2645,7 +2672,8 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
-	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format);
+	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
+				    repo_fmt->ref_storage_payload);
 }
 
 int init_db(const char *git_dir, const char *real_git_dir,
@@ -2702,6 +2730,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 
 	if (!(flags & INIT_DB_SKIP_REFDB))
 		create_reference_database(repo_fmt.ref_storage_format,
+					  repo_fmt.ref_storage_payload,
 					  initial_branch, flags & INIT_DB_QUIET);
 	create_object_directory();
 
diff --git a/setup.h b/setup.h
index d55dcc6608..4d25a353ac 100644
--- a/setup.h
+++ b/setup.h
@@ -171,6 +171,7 @@ struct repository_format {
 	int hash_algo;
 	int compat_hash_algo;
 	enum ref_storage_format ref_storage_format;
+	char *ref_storage_payload;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;
@@ -241,6 +242,7 @@ void initialize_repository_version(int hash_algo,
 				   enum ref_storage_format ref_storage_format,
 				   int reinit);
 void create_reference_database(enum ref_storage_format ref_storage_format,
+			       const char *ref_storage_payload,
 			       const char *initial_branch, int quiet);
 
 /*

builtin/clone.c, refs.c, refs.h, refs/reftable-backend.c

diff --git a/builtin/clone.c b/builtin/clone.c
index 28412576b3..11854894d3 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1225,12 +1225,7 @@ int cmd_clone(int argc,
 	initialize_repository_version(GIT_HASH_UNKNOWN,
 				      the_repository->ref_storage_format, 1);
 
-	strbuf_addf(&buf, "%s/HEAD", git_dir);
-	write_file(buf.buf, "ref: refs/heads/.invalid");
-
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "%s/refs", git_dir);
-	safe_create_dir(the_repository, buf.buf, 1);
+	refs_create_refdir_stubs(the_repository, git_dir, NULL);
 
 	/*
 	 * additional config can be injected with -c, make sure it's included
diff --git a/refs.c b/refs.c
index 627b7f8698..32b4edaf2d 100644
--- a/refs.c
+++ b/refs.c
@@ -3402,3 +3402,27 @@ const char *ref_transaction_error_msg(enum ref_transaction_error err)
 		return "unknown failure";
 	}
 }
+
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_msg)
+{
+	struct strbuf path = STRBUF_INIT;
+
+
+	strbuf_addf(&path, "%s/HEAD", refdir);
+	write_file(path.buf, "ref: refs/heads/.invalid");
+	adjust_shared_perm(repo, path.buf);
+
+	strbuf_reset(&path);
+	strbuf_addf(&path, "%s/refs", refdir);
+	safe_create_dir(repo, path.buf, 1);
+
+	if (refs_heads_msg) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/refs/heads", refdir);
+		write_file(path.buf, "%s", refs_heads_msg);
+		adjust_shared_perm(repo, path.buf);
+	}
+
+	strbuf_release(&path);
+}
diff --git a/refs.h b/refs.h
index f0abfa1d93..9d8890fdff 100644
--- a/refs.h
+++ b/refs.h
@@ -1427,4 +1427,17 @@ void ref_iterator_free(struct ref_iterator *ref_iterator);
 int do_for_each_ref_iterator(struct ref_iterator *iter,
 			     each_ref_fn fn, void *cb_data);
 
+/*
+ * Git only recognizes a directory as a repository if it contains:
+ * - HEAD file
+ * - refs/ folder
+ * While it is necessary within the files backend, newer backends may not
+ * follow the same structure. To go around this, we create stubs as necessary.
+ *
+ * If provided with a 'refs_heads_msg', we create the 'refs/heads/head' file
+ * with the provided message.
+ */
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_msg);
+
 #endif /* REFS_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index fe74af73af..d8651fe779 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,18 +491,8 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	write_file(sb.buf, "ref: refs/heads/.invalid");
-	adjust_shared_perm(the_repository, sb.buf);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	safe_create_dir(the_repository, sb.buf, 1);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	write_file(sb.buf, "this repository uses the reftable format");
-	adjust_shared_perm(the_repository, sb.buf);
+	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
+				 "this repository uses the reftable format");
 
 	strbuf_release(&sb);
 	return 0;

Documentation/config/extensions.adoc, builtin/worktree.c, refs.c, refs/files-backend.c, refs/packed-backend.c, refs/packed-backend.h, refs/refs-internal.h, refs/reftable-backend.c, t/meson.build, t/t1423-ref-backend.sh

diff --git a/Documentation/config/extensions.adoc b/Documentation/config/extensions.adoc
index 532456644b..df86da6aa7 100644
--- a/Documentation/config/extensions.adoc
+++ b/Documentation/config/extensions.adoc
@@ -57,10 +57,24 @@ For historical reasons, this extension is respected regardless of the
 `core.repositoryFormatVersion` setting.
 
 refStorage:::
-	Specify the ref storage format to use. The acceptable values are:
+	Specify the ref storage format and location to use. The value can be
+	either a format name or a URI:
 +
 --
+* A format name alone (e.g., `reftable` or `files`) uses the default
+  location (the repository's common directory).
+
+* A URI format `<format>://<location>` explicitly specifies both the
+  format and payload (e.g., `reftable:///foo/bar`).
+
+Supported format names are:
++
 include::../ref-storage-format.adoc[]
++
+The payload is passed directly to the reference backend. For the files and
+reftable backends, this must be a filesystem path. Relative paths are resolved
+relative to the $GIT_DIR. Future backends may support other payload schemes,
+e.g., postgres://127.0.0.1:5432?database=myrepo.
 --
 +
 Note that this setting should only be set by linkgit:git-init[1] or
diff --git a/builtin/worktree.c b/builtin/worktree.c
index fbdaf2eb2e..800a376ac5 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -425,6 +425,40 @@ static int make_worktree_orphan(const char * ref, const struct add_opts *opts,
 	return run_command(&cp);
 }
 
+/*
+ * References for worktress are generally stored in '$GIT_DIR/worktrees/<wt_id>'.
+ * But when using alternate reference directories, we want to store the worktree
+ * references in '$ALTERNATE_REFERENCE_DIR/worktrees/<wt_id>'.
+ *
+ * Create the necessary folder structure to facilitate the same. But to ensure
+ * that the former path is still considered a Git directory, add stubs (similar
+ *  to how we do in the reftable backend).
+ */
+static void setup_alternate_ref_dir(struct worktree *wt, const char *wt_git_path)
+{
+	struct strbuf sb = STRBUF_INIT;
+	char *path;
+
+	path = wt->repo->ref_storage_payload;
+	if (!path)
+		return;
+
+	if (!is_absolute_path(path))
+		strbuf_addf(&sb, "%s/", wt->repo->commondir);
+
+	strbuf_addf(&sb, "%s/worktrees", path);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_addf(&sb, "/%s", wt->id);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "this worktree stores references in %s/worktrees/%s",
+		   path, wt->id);
+	refs_create_refdir_stubs(wt->repo, wt_git_path, sb.buf);
+
+	strbuf_release(&sb);
+}
+
 static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
@@ -518,6 +552,7 @@ static int add_worktree(const char *path, const char *refname,
 		ret = error(_("could not find created worktree '%s'"), name);
 		goto done;
 	}
+	setup_alternate_ref_dir(wt, sb_repo.buf);
 	wt_refs = get_worktree_ref_store(wt);
 
 	ret = ref_store_create_on_disk(wt_refs, REF_STORE_CREATE_ON_DISK_IS_WORKTREE, &sb);
diff --git a/refs.c b/refs.c
index 32b4edaf2d..c1d69082a9 100644
--- a/refs.c
+++ b/refs.c
@@ -5,6 +5,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "advice.h"
 #include "config.h"
 #include "environment.h"
@@ -2224,7 +2225,11 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, gitdir, flags);
+	/*
+	 * TODO Send in a 'struct worktree' instead of a 'gitdir', and
+	 * allow the backend to handle how it wants to deal with worktrees.
+	 */
+	refs = be->init(repo, repo->ref_storage_payload, gitdir, flags);
 	return refs;
 }
 
@@ -3426,3 +3431,33 @@ void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
 
 	strbuf_release(&path);
 }
+
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	strbuf_addstr(refdir, gitdir);
+	*is_worktree = get_common_dir_noenv(ref_common_dir, gitdir);
+
+	if (!payload)
+		return;
+
+	if (!is_absolute_path(payload)) {
+		strbuf_addf(&sb, "%s/%s", ref_common_dir->buf, payload);
+		strbuf_realpath(ref_common_dir, sb.buf, 1);
+	} else {
+		strbuf_realpath(ref_common_dir, payload, 1);
+	}
+
+	strbuf_reset(refdir);
+	strbuf_addbuf(refdir, ref_common_dir);
+
+	if (*is_worktree) {
+		char *wt_id = strrchr(gitdir, '/') + 1;
+		strbuf_addf(refdir, "/worktrees/%s", wt_id);
+	}
+
+	strbuf_release(&sb);
+}
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 240d3c3b26..160ecb53b7 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -106,19 +106,24 @@ static void clear_loose_ref_cache(struct files_ref_store *refs)
  * set of caches.
  */
 static struct ref_store *files_ref_store_init(struct repository *repo,
+					      const char *payload,
 					      const char *gitdir,
 					      unsigned int flags)
 {
 	struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
 	struct ref_store *ref_store = (struct ref_store *)refs;
-	struct strbuf sb = STRBUF_INIT;
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
+	bool is_worktree;
+
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
 
-	base_ref_store_init(ref_store, repo, gitdir, &refs_be_files);
+	base_ref_store_init(ref_store, repo, refdir.buf, &refs_be_files);
 	refs->store_flags = flags;
-	get_common_dir_noenv(&sb, gitdir);
-	refs->gitcommondir = strbuf_detach(&sb, NULL);
+	refs->gitcommondir = xstrdup(ref_common_dir.buf);
 	refs->packed_ref_store =
-		packed_ref_store_init(repo, refs->gitcommondir, flags);
+		packed_ref_store_init(repo, payload, ref_common_dir.buf, flags);
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
 	repo_config_get_bool(repo, "core.prefersymlinkrefs", &refs->prefer_symlink_refs);
 
@@ -126,6 +131,9 @@ static struct ref_store *files_ref_store_init(struct repository *repo,
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
 			      &refs->gitcommondir);
 
+	strbuf_release(&ref_common_dir);
+	strbuf_release(&refdir);
+
 	return ref_store;
 }
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..028fbc0585 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -212,6 +212,7 @@ static size_t snapshot_hexsz(const struct snapshot *snapshot)
 }
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload UNUSED,
 					const char *gitdir,
 					unsigned int store_flags)
 {
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index 9481d5e7c2..2c2377a356 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -14,6 +14,7 @@ struct ref_transaction;
  */
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload,
 					const char *gitdir,
 					unsigned int store_flags);
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index c7d2a6e50b..bd09b1280c 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -389,6 +389,7 @@ struct ref_store;
  * the ref_store and to record the ref_store for later lookup.
  */
 typedef struct ref_store *ref_store_init_fn(struct repository *repo,
+					    const char *payload,
 					    const char *gitdir,
 					    unsigned int flags);
 /*
@@ -666,4 +667,18 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
 					  unsigned int initial_transaction,
 					  struct strbuf *err);
 
+/*
+ * Given a gitdir and the reference storage payload provided, retrieve the
+ * 'refdir' and 'ref_common_dir'. The former is where references should be
+ * stored for the current worktree, the latter is the common reference
+ * directory if working with a linked worktree. If working with the main
+ * worktree, both values will be the same.
+ *
+ * This is used by backends such as {files, reftable} which store references in
+ * dedicated filesystem paths.
+ */
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir);
+
 #endif /* REFS_REFS_INTERNAL_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index d8651fe779..964b0b50fc 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -372,18 +372,24 @@ static int reftable_be_fsync(int fd)
 }
 
 static struct ref_store *reftable_be_init(struct repository *repo,
+					  const char *payload,
 					  const char *gitdir,
 					  unsigned int store_flags)
 {
 	struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
 	struct strbuf path = STRBUF_INIT;
-	int is_worktree;
+	bool is_worktree;
 	mode_t mask;
 
 	mask = umask(0);
 	umask(mask);
 
-	base_ref_store_init(&refs->base, repo, gitdir, &refs_be_reftable);
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
+
+	base_ref_store_init(&refs->base, repo, refdir.buf, &refs_be_reftable);
 	strmap_init(&refs->worktree_backends);
 	refs->store_flags = store_flags;
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
@@ -419,14 +425,11 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.
 	 * This stack contains both the shared and the main worktree refs.
-	 *
-	 * Note that we don't try to resolve the path in case we have a
-	 * worktree because `get_common_dir_noenv()` already does it for us.
 	 */
-	is_worktree = get_common_dir_noenv(&path, gitdir);
+	strbuf_addbuf(&path, &ref_common_dir);
 	if (!is_worktree) {
 		strbuf_reset(&path);
-		strbuf_realpath(&path, gitdir, 0);
+		strbuf_realpath(&path, ref_common_dir.buf, 0);
 	}
 	strbuf_addstr(&path, "/reftable");
 	refs->err = reftable_backend_init(&refs->main_backend, path.buf,
@@ -443,10 +446,9 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	 * do it efficiently.
 	 */
 	if (is_worktree) {
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/reftable", gitdir);
+		strbuf_addstr(&refdir, "/reftable");
 
-		refs->err = reftable_backend_init(&refs->worktree_backend, path.buf,
+		refs->err = reftable_backend_init(&refs->worktree_backend, refdir.buf,
 						  &refs->write_options);
 		if (refs->err)
 			goto done;
@@ -456,6 +458,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 
 done:
 	assert(refs->err != REFTABLE_API_ERROR);
+	strbuf_release(&ref_common_dir);
+	strbuf_release(&refdir);
 	strbuf_release(&path);
 	return &refs->base;
 }
diff --git a/t/meson.build b/t/meson.build
index 459c52a489..11fc5a49ee 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -210,6 +210,7 @@ integration_tests = [
   't1420-lost-found.sh',
   't1421-reflog-write.sh',
   't1422-show-ref-exists.sh',
+  't1423-ref-backend.sh',
   't1430-bad-ref-name.sh',
   't1450-fsck.sh',
   't1451-fsck-buffer.sh',
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
new file mode 100755
index 0000000000..9c777b79f3
--- /dev/null
+++ b/t/t1423-ref-backend.sh
@@ -0,0 +1,159 @@
+#!/bin/sh
+
+test_description='Test reference backend URIs'
+
+. ./test-lib.sh
+
+# Run a git command with the provided reference storage. Reset the backend
+# post running the command.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <cmd> is the git subcommand to be run in the repository.
+run_with_uri() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	cmd=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1
+	git -C "$repo" config set extensions.refStorage "$uri" &&
+	git -C "$repo" $cmd &&
+	git -C "$repo" config set extensions.refStorage "$backend"
+}
+
+# Test a repository with a given reference storage by running and comparing
+# 'git refs list' before and after setting the new reference backend. If
+# err_msg is set, expect the command to fail and grep for the provided err_msg.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
+test_refs_backend() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	err_msg=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test -n "$err_msg";
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		test_must_fail git -C "$repo" refs list 2>err &&
+		test_grep "$err_msg" err
+	else
+		git -C "$repo" refs list >expect &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		test_cmp expect actual
+	fi
+}
+
+test_expect_success 'URI is invalid' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable@/home/reftable" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'URI ends with colon' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable:" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'unknown reference backend' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "db://.git" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+ref_formats="files reftable"
+for from_format in $ref_formats
+do
+
+for to_format in $ref_formats
+do
+	if test "$from_format" = "$to_format"
+	then
+		continue
+	fi
+
+
+	for dir in "$(pwd)/repo/.git" "./"
+	do
+
+		test_expect_success "$read from $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method"
+			)
+		'
+
+		test_expect_success "$write to $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+
+				git refs list >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				git refs list >actual &&
+				test_cmp expect actual &&
+
+				git refs list | grep -v "refs/tags/1" >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				test_cmp expect actual
+			)
+		'
+
+		test_expect_success "with worktree and $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo wt" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				git config set core.repositoryformatversion 1 &&
+				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+
+				git worktree add ../wt 2
+			) &&
+
+			git -C repo for-each-ref --include-root-refs >expect &&
+			git -C wt for-each-ref --include-root-refs >expect &&
+			! test_cmp expect actual &&
+
+			git -C wt rev-parse 2 >expect &&
+			git -C wt rev-parse HEAD >actual &&
+			test_cmp expect actual
+		'
+	done # closes dir
+done # closes to_format
+done # closes from_format
+
+test_done

Documentation/git.adoc, environment.h, setup.c, t/t1423-ref-backend.sh

diff --git a/Documentation/git.adoc b/Documentation/git.adoc
index ce099e78b8..ed3191e8f6 100644
--- a/Documentation/git.adoc
+++ b/Documentation/git.adoc
@@ -584,6 +584,11 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REFERENCE_BACKEND`::
+    Specify which reference backend to be used along with its URI.
+    See `extensions.refStorage` option in linkgit:git-config[1] for more
+    description. Overrides the config variable when used.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::
diff --git a/environment.h b/environment.h
index 27f657af04..540e0a7f6d 100644
--- a/environment.h
+++ b/environment.h
@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REFERENCE_BACKEND_ENVIRONMENT "GIT_REFERENCE_BACKEND"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the
diff --git a/setup.c b/setup.c
index 44e393c251..b4a7b82cca 100644
--- a/setup.c
+++ b/setup.c
@@ -1838,6 +1838,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	static struct strbuf cwd = STRBUF_INIT;
 	struct strbuf dir = STRBUF_INIT, gitdir = STRBUF_INIT, report = STRBUF_INIT;
 	const char *prefix = NULL;
+	const char *ref_backend_uri;
 	struct repository_format repo_fmt = REPOSITORY_FORMAT_INIT;
 
 	/*
@@ -1995,6 +1996,25 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	/*
+	 * The env variable should override the repository config
+	 * for 'extensions.refStorage'.
+	 */
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *location;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &location);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+		repo_set_ref_storage_format(the_repository, format, location);
+
+		free(backend);
+		free(location);
+	}
+
 	setup_original_cwd();
 
 	strbuf_release(&dir);
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
index 9c777b79f3..10a9bb1a9b 100755
--- a/t/t1423-ref-backend.sh
+++ b/t/t1423-ref-backend.sh
@@ -11,16 +11,25 @@ test_description='Test reference backend URIs'
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
 #   <cmd> is the git subcommand to be run in the repository.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 run_with_uri() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
 	cmd=$4 &&
+	via=$5 &&
 
-	git -C "$repo" config set core.repositoryformatversion 1
-	git -C "$repo" config set extensions.refStorage "$uri" &&
-	git -C "$repo" $cmd &&
-	git -C "$repo" config set extensions.refStorage "$backend"
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test "$via" = "env"
+	then
+		test_env GIT_REFERENCE_BACKEND="$uri" git -C "$repo" $cmd
+	elif test "$via" = "config"
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		git -C "$repo" $cmd &&
+		git -C "$repo" config set extensions.refStorage "$backend"
+	fi
 }
 
 # Test a repository with a given reference storage by running and comparing
@@ -30,44 +39,57 @@ run_with_uri() {
 #   <repo> is the relative path to the repo to run the command in.
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 #   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
 test_refs_backend() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
-	err_msg=$4 &&
+	via=$4 &&
+	err_msg=$5 &&
+
 
-	git -C "$repo" config set core.repositoryformatversion 1 &&
 	if test -n "$err_msg";
 	then
-		git -C "$repo" config set extensions.refStorage "$uri" &&
-		test_must_fail git -C "$repo" refs list 2>err &&
-		test_grep "$err_msg" err
+		if test "$via" = "env"
+		then
+			test_env GIT_REFERENCE_BACKEND="$uri" test_must_fail git -C "$repo" refs list 2>err
+		elif test "$via" = "config"
+		then
+			git -C "$repo" config set extensions.refStorage "$uri" &&
+			test_must_fail git -C "$repo" refs list 2>err &&
+			test_grep "$err_msg" err
+		fi
 	else
 		git -C "$repo" refs list >expect &&
-		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" "$via">actual &&
 		test_cmp expect actual
 	fi
 }
 
-test_expect_success 'URI is invalid' '
+methods="config"
+for method in $methods
+do
+
+test_expect_success "$method: URI is invalid" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable@/home/reftable" \
+	test_refs_backend repo files "reftable@/home/reftable" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'URI ends with colon' '
+test_expect_success "$method: URI ends with colon" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable:" \
+	test_refs_backend repo files "reftable:" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'unknown reference backend' '
+test_expect_success "$method: unknown reference backend" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "db://.git" \
+	test_refs_backend repo files "db://.git" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
@@ -86,7 +108,7 @@ do
 	for dir in "$(pwd)/repo/.git" "./"
 	do
 
-		test_expect_success "$read from $to_format backend, $dir dir" '
+		test_expect_success "$method: $read from $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -101,7 +123,7 @@ do
 			)
 		'
 
-		test_expect_success "$write to $to_format backend, $dir dir" '
+		test_expect_success "$method: $write to $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -113,20 +135,22 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method" &&
 
 				git refs list >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"tag -d 1" "$method" &&
 				git refs list >actual &&
 				test_cmp expect actual &&
 
 				git refs list | grep -v "refs/tags/1" >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"refs list" "$method" >actual &&
 				test_cmp expect actual
 			)
 		'
 
-		test_expect_success "with worktree and $to_format backend, $dir dir" '
+		test_expect_success "$method: with worktree and $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo wt" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -138,22 +162,26 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				git config set core.repositoryformatversion 1 &&
-				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"worktree add ../wt 2" "$method" &&
 
-				git worktree add ../wt 2
-			) &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >expect &&
+				! test_cmp expect actual &&
 
-			git -C repo for-each-ref --include-root-refs >expect &&
-			git -C wt for-each-ref --include-root-refs >expect &&
-			! test_cmp expect actual &&
-
-			git -C wt rev-parse 2 >expect &&
-			git -C wt rev-parse HEAD >actual &&
-			test_cmp expect actual
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse 2" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse HEAD" "$method" >expect &&
+				test_cmp expect actual
+			)
 		'
 	done # closes dir
 done # closes to_format
-done # closes from_format
+done # closes to_format
+
+done # closes method
 
 test_done

builtin/clone.c, refs.c, refs.h, refs/reftable-backend.c

diff --git a/builtin/clone.c b/builtin/clone.c
index b40cee5968..535a257b10 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1225,12 +1225,7 @@ int cmd_clone(int argc,
 	initialize_repository_version(GIT_HASH_UNKNOWN,
 				      the_repository->ref_storage_format, 1);
 
-	strbuf_addf(&buf, "%s/HEAD", git_dir);
-	write_file(buf.buf, "ref: refs/heads/.invalid");
-
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "%s/refs", git_dir);
-	safe_create_dir(the_repository, buf.buf, 1);
+	refs_create_refdir_stubs(the_repository, git_dir, NULL);
 
 	/*
 	 * additional config can be injected with -c, make sure it's included
diff --git a/refs.c b/refs.c
index 627b7f8698..36f3441632 100644
--- a/refs.c
+++ b/refs.c
@@ -3402,3 +3402,26 @@ const char *ref_transaction_error_msg(enum ref_transaction_error err)
 		return "unknown failure";
 	}
 }
+
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_content)
+{
+	struct strbuf path = STRBUF_INIT;
+
+	strbuf_addf(&path, "%s/HEAD", refdir);
+	write_file(path.buf, "ref: refs/heads/.invalid");
+	adjust_shared_perm(repo, path.buf);
+
+	strbuf_reset(&path);
+	strbuf_addf(&path, "%s/refs", refdir);
+	safe_create_dir(repo, path.buf, 1);
+
+	if (refs_heads_content) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/refs/heads", refdir);
+		write_file(path.buf, "%s", refs_heads_content);
+		adjust_shared_perm(repo, path.buf);
+	}
+
+	strbuf_release(&path);
+}
diff --git a/refs.h b/refs.h
index f0abfa1d93..9d8890fdff 100644
--- a/refs.h
+++ b/refs.h
@@ -1427,4 +1427,17 @@ void ref_iterator_free(struct ref_iterator *ref_iterator);
 int do_for_each_ref_iterator(struct ref_iterator *iter,
 			     each_ref_fn fn, void *cb_data);
 
+/*
+ * Git only recognizes a directory as a repository if it contains:
+ * - HEAD file
+ * - refs/ folder
+ * While it is necessary within the files backend, newer backends may not
+ * follow the same structure. To go around this, we create stubs as necessary.
+ *
+ * If provided with a 'refs_heads_msg', we create the 'refs/heads/head' file
+ * with the provided message.
+ */
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_msg);
+
 #endif /* REFS_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index fe74af73af..d8651fe779 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,18 +491,8 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	write_file(sb.buf, "ref: refs/heads/.invalid");
-	adjust_shared_perm(the_repository, sb.buf);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	safe_create_dir(the_repository, sb.buf, 1);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	write_file(sb.buf, "this repository uses the reftable format");
-	adjust_shared_perm(the_repository, sb.buf);
+	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
+				 "this repository uses the reftable format");
 
 	strbuf_release(&sb);
 	return 0;

refs.c, refs/files-backend.c, refs/packed-backend.c, refs/packed-backend.h, refs/refs-internal.h, refs/reftable-backend.c

diff --git a/refs.c b/refs.c
index 36f3441632..d9df25d7c0 100644
--- a/refs.c
+++ b/refs.c
@@ -5,6 +5,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "advice.h"
 #include "config.h"
 #include "environment.h"
@@ -2224,7 +2225,7 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, gitdir, flags);
+	refs = be->init(repo, NULL, gitdir, flags);
 	return refs;
 }
 
@@ -3425,3 +3426,33 @@ void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
 
 	strbuf_release(&path);
 }
+
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	strbuf_addstr(refdir, gitdir);
+	*is_worktree = get_common_dir_noenv(ref_common_dir, gitdir);
+
+	if (!payload)
+		return;
+
+	if (!is_absolute_path(payload)) {
+		strbuf_addf(&sb, "%s/%s", ref_common_dir->buf, payload);
+		strbuf_realpath(ref_common_dir, sb.buf, 1);
+	} else {
+		strbuf_realpath(ref_common_dir, payload, 1);
+	}
+
+	strbuf_reset(refdir);
+	strbuf_addbuf(refdir, ref_common_dir);
+
+	if (*is_worktree) {
+		char *wt_id = strrchr(gitdir, '/') + 1;
+		strbuf_addf(refdir, "/worktrees/%s", wt_id);
+	}
+
+	strbuf_release(&sb);
+}
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 240d3c3b26..b192ce606d 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -106,19 +106,24 @@ static void clear_loose_ref_cache(struct files_ref_store *refs)
  * set of caches.
  */
 static struct ref_store *files_ref_store_init(struct repository *repo,
+					      const char *payload,
 					      const char *gitdir,
 					      unsigned int flags)
 {
 	struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
 	struct ref_store *ref_store = (struct ref_store *)refs;
-	struct strbuf sb = STRBUF_INIT;
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
+	bool is_worktree;
+
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
 
-	base_ref_store_init(ref_store, repo, gitdir, &refs_be_files);
+	base_ref_store_init(ref_store, repo, refdir.buf, &refs_be_files);
 	refs->store_flags = flags;
-	get_common_dir_noenv(&sb, gitdir);
-	refs->gitcommondir = strbuf_detach(&sb, NULL);
+	refs->gitcommondir = strbuf_detach(&ref_common_dir, NULL);
 	refs->packed_ref_store =
-		packed_ref_store_init(repo, refs->gitcommondir, flags);
+		packed_ref_store_init(repo, payload, refs->gitcommondir, flags);
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
 	repo_config_get_bool(repo, "core.prefersymlinkrefs", &refs->prefer_symlink_refs);
 
@@ -126,6 +131,8 @@ static struct ref_store *files_ref_store_init(struct repository *repo,
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
 			      &refs->gitcommondir);
 
+	strbuf_release(&refdir);
+
 	return ref_store;
 }
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..028fbc0585 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -212,6 +212,7 @@ static size_t snapshot_hexsz(const struct snapshot *snapshot)
 }
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload UNUSED,
 					const char *gitdir,
 					unsigned int store_flags)
 {
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index 9481d5e7c2..2c2377a356 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -14,6 +14,7 @@ struct ref_transaction;
  */
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload,
 					const char *gitdir,
 					unsigned int store_flags);
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index c7d2a6e50b..bd09b1280c 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -389,6 +389,7 @@ struct ref_store;
  * the ref_store and to record the ref_store for later lookup.
  */
 typedef struct ref_store *ref_store_init_fn(struct repository *repo,
+					    const char *payload,
 					    const char *gitdir,
 					    unsigned int flags);
 /*
@@ -666,4 +667,18 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
 					  unsigned int initial_transaction,
 					  struct strbuf *err);
 
+/*
+ * Given a gitdir and the reference storage payload provided, retrieve the
+ * 'refdir' and 'ref_common_dir'. The former is where references should be
+ * stored for the current worktree, the latter is the common reference
+ * directory if working with a linked worktree. If working with the main
+ * worktree, both values will be the same.
+ *
+ * This is used by backends such as {files, reftable} which store references in
+ * dedicated filesystem paths.
+ */
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir);
+
 #endif /* REFS_REFS_INTERNAL_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index d8651fe779..964b0b50fc 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -372,18 +372,24 @@ static int reftable_be_fsync(int fd)
 }
 
 static struct ref_store *reftable_be_init(struct repository *repo,
+					  const char *payload,
 					  const char *gitdir,
 					  unsigned int store_flags)
 {
 	struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
 	struct strbuf path = STRBUF_INIT;
-	int is_worktree;
+	bool is_worktree;
 	mode_t mask;
 
 	mask = umask(0);
 	umask(mask);
 
-	base_ref_store_init(&refs->base, repo, gitdir, &refs_be_reftable);
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
+
+	base_ref_store_init(&refs->base, repo, refdir.buf, &refs_be_reftable);
 	strmap_init(&refs->worktree_backends);
 	refs->store_flags = store_flags;
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
@@ -419,14 +425,11 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.
 	 * This stack contains both the shared and the main worktree refs.
-	 *
-	 * Note that we don't try to resolve the path in case we have a
-	 * worktree because `get_common_dir_noenv()` already does it for us.
 	 */
-	is_worktree = get_common_dir_noenv(&path, gitdir);
+	strbuf_addbuf(&path, &ref_common_dir);
 	if (!is_worktree) {
 		strbuf_reset(&path);
-		strbuf_realpath(&path, gitdir, 0);
+		strbuf_realpath(&path, ref_common_dir.buf, 0);
 	}
 	strbuf_addstr(&path, "/reftable");
 	refs->err = reftable_backend_init(&refs->main_backend, path.buf,
@@ -443,10 +446,9 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	 * do it efficiently.
 	 */
 	if (is_worktree) {
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/reftable", gitdir);
+		strbuf_addstr(&refdir, "/reftable");
 
-		refs->err = reftable_backend_init(&refs->worktree_backend, path.buf,
+		refs->err = reftable_backend_init(&refs->worktree_backend, refdir.buf,
 						  &refs->write_options);
 		if (refs->err)
 			goto done;
@@ -456,6 +458,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 
 done:
 	assert(refs->err != REFTABLE_API_ERROR);
+	strbuf_release(&ref_common_dir);
+	strbuf_release(&refdir);
 	strbuf_release(&path);
 	return &refs->base;
 }

Documentation/config/extensions.adoc, builtin/clone.c, builtin/worktree.c, refs.c, repository.c, repository.h, setup.c, setup.h, t/meson.build, t/t1423-ref-backend.sh

diff --git a/Documentation/config/extensions.adoc b/Documentation/config/extensions.adoc
index 532456644b..3e51da36d3 100644
--- a/Documentation/config/extensions.adoc
+++ b/Documentation/config/extensions.adoc
@@ -57,10 +57,24 @@ For historical reasons, this extension is respected regardless of the
 `core.repositoryFormatVersion` setting.
 
 refStorage:::
-	Specify the ref storage format to use. The acceptable values are:
+	Specify the ref storage format and a corresponding payload. The value
+	can be either a format name or a URI:
 +
 --
+* A format name alone (e.g., `reftable` or `files`).
+
+* A URI format `<format>://<payload>` explicitly specifies both the
+  format and payload (e.g., `reftable:///foo/bar`).
+
+Supported format names are:
++
 include::../ref-storage-format.adoc[]
++
+The payload is passed directly to the reference backend. For the files and
+reftable backends, this must be a filesystem path where the references will
+be stored. Defaulting to the commondir when no payload is provided. Relative
+paths are resolved relative to the $GIT_DIR. Future backends may support
+other payload schemes, e.g., postgres://127.0.0.1:5432?database=myrepo.
 --
 +
 Note that this setting should only be set by linkgit:git-init[1] or
diff --git a/builtin/clone.c b/builtin/clone.c
index 535a257b10..11854894d3 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1437,7 +1437,8 @@ int cmd_clone(int argc,
 	hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 	initialize_repository_version(hash_algo, the_repository->ref_storage_format, 1);
 	repo_set_hash_algo(the_repository, hash_algo);
-	create_reference_database(the_repository->ref_storage_format, NULL, 1);
+	create_reference_database(the_repository->ref_storage_format,
+				  the_repository->ref_storage_payload, NULL, 1);
 
 	/*
 	 * Before fetching from the remote, download and install bundle
diff --git a/builtin/worktree.c b/builtin/worktree.c
index fbdaf2eb2e..94480be5c4 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -425,6 +425,39 @@ static int make_worktree_orphan(const char * ref, const struct add_opts *opts,
 	return run_command(&cp);
 }
 
+/*
+ * References for worktress are generally stored in '$GIT_DIR/worktrees/<wt_id>'.
+ * But when using alternate reference directories, we want to store the worktree
+ * references in '$ALTERNATE_REFERENCE_DIR/worktrees/<wt_id>'.
+ *
+ * Create the necessary folder structure to facilitate the same. But to ensure
+ * that the former path is still considered a Git directory, add stubs.
+ */
+static void setup_alternate_ref_dir(struct worktree *wt, const char *wt_git_path)
+{
+	struct strbuf sb = STRBUF_INIT;
+	char *path;
+
+	path = wt->repo->ref_storage_payload;
+	if (!path)
+		return;
+
+	if (!is_absolute_path(path))
+		strbuf_addf(&sb, "%s/", wt->repo->commondir);
+
+	strbuf_addf(&sb, "%s/worktrees", path);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_addf(&sb, "/%s", wt->id);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "this worktree stores references in %s/worktrees/%s",
+		   path, wt->id);
+	refs_create_refdir_stubs(wt->repo, wt_git_path, sb.buf);
+
+	strbuf_release(&sb);
+}
+
 static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
@@ -518,6 +551,7 @@ static int add_worktree(const char *path, const char *refname,
 		ret = error(_("could not find created worktree '%s'"), name);
 		goto done;
 	}
+	setup_alternate_ref_dir(wt, sb_repo.buf);
 	wt_refs = get_worktree_ref_store(wt);
 
 	ret = ref_store_create_on_disk(wt_refs, REF_STORE_CREATE_ON_DISK_IS_WORKTREE, &sb);
diff --git a/refs.c b/refs.c
index d9df25d7c0..f520d64a9d 100644
--- a/refs.c
+++ b/refs.c
@@ -2225,7 +2225,11 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, NULL, gitdir, flags);
+	/*
+	 * TODO Send in a 'struct worktree' instead of a 'gitdir', and
+	 * allow the backend to handle how it wants to deal with worktrees.
+	 */
+	refs = be->init(repo, repo->ref_storage_payload, gitdir, flags);
 	return refs;
 }
 
diff --git a/repository.c b/repository.c
index c7e75215ac..9815f081ef 100644
--- a/repository.c
+++ b/repository.c
@@ -193,9 +193,12 @@ void repo_set_compat_hash_algo(struct repository *repo, int algo)
 }
 
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format)
+				 enum ref_storage_format format,
+				 const char *payload)
 {
 	repo->ref_storage_format = format;
+	free(repo->ref_storage_payload);
+	repo->ref_storage_payload = xstrdup_or_null(payload);
 }
 
 /*
@@ -277,7 +280,8 @@ int repo_init(struct repository *repo,
 
 	repo_set_hash_algo(repo, format.hash_algo);
 	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
-	repo_set_ref_storage_format(repo, format.ref_storage_format);
+	repo_set_ref_storage_format(repo, format.ref_storage_format,
+				    format.ref_storage_payload);
 	repo->repository_format_worktree_config = format.worktree_config;
 	repo->repository_format_relative_worktrees = format.relative_worktrees;
 	repo->repository_format_precious_objects = format.precious_objects;
@@ -369,6 +373,7 @@ void repo_clear(struct repository *repo)
 	FREE_AND_NULL(repo->index_file);
 	FREE_AND_NULL(repo->worktree);
 	FREE_AND_NULL(repo->submodule_prefix);
+	FREE_AND_NULL(repo->ref_storage_payload);
 
 	odb_free(repo->objects);
 	repo->objects = NULL;
diff --git a/repository.h b/repository.h
index 6063c4b846..95e2333bad 100644
--- a/repository.h
+++ b/repository.h
@@ -150,6 +150,11 @@ struct repository {
 
 	/* Repository's reference storage format, as serialized on disk. */
 	enum ref_storage_format ref_storage_format;
+	/*
+	 * Reference storage information as needed for the backend. This contains
+	 * only the payload from the reference URI without the schema.
+	 */
+	char *ref_storage_payload;
 
 	/* A unique-id for tracing purposes. */
 	int trace2_repo_id;
@@ -204,7 +209,8 @@ void repo_set_worktree(struct repository *repo, const char *path);
 void repo_set_hash_algo(struct repository *repo, int algo);
 void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format);
+				 enum ref_storage_format format,
+				 const char *payload);
 void initialize_repository(struct repository *repo);
 RESULT_MUST_BE_USED
 int repo_init(struct repository *r, const char *gitdir, const char *worktree);
diff --git a/setup.c b/setup.c
index b723f8b339..44e393c251 100644
--- a/setup.c
+++ b/setup.c
@@ -632,6 +632,21 @@ static enum extension_result handle_extension_v0(const char *var,
 		return EXTENSION_UNKNOWN;
 }
 
+static void parse_reference_uri(const char *value, char **format,
+				char **payload)
+{
+	char *schema_end;
+
+	schema_end = strstr(value, "://");
+	if (!schema_end) {
+		*format = xstrdup(value);
+		*payload = NULL;
+	} else {
+		*format = xstrndup(value, schema_end - value);
+		*payload = xstrdup_or_null(schema_end + 3);
+	}
+}
+
 /*
  * Record any new extensions in this function.
  */
@@ -674,10 +689,17 @@ static enum extension_result handle_extension(const char *var,
 		return EXTENSION_OK;
 	} else if (!strcmp(ext, "refstorage")) {
 		unsigned int format;
+		char *format_str;
 
 		if (!value)
 			return config_error_nonbool(var);
-		format = ref_storage_format_by_name(value);
+
+		parse_reference_uri(value, &format_str,
+				    &data->ref_storage_payload);
+
+		format = ref_storage_format_by_name(format_str);
+		free(format_str);
+
 		if (format == REF_STORAGE_FORMAT_UNKNOWN)
 			return error(_("invalid value for '%s': '%s'"),
 				     "extensions.refstorage", value);
@@ -850,6 +872,7 @@ void clear_repository_format(struct repository_format *format)
 	string_list_clear(&format->v1_only_extensions, 0);
 	free(format->work_tree);
 	free(format->partial_clone);
+	free(format->ref_storage_payload);
 	init_repository_format(format);
 }
 
@@ -1942,7 +1965,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			repo_set_compat_hash_algo(the_repository,
 						  repo_fmt.compat_hash_algo);
 			repo_set_ref_storage_format(the_repository,
-						    repo_fmt.ref_storage_format);
+						    repo_fmt.ref_storage_format,
+						    repo_fmt.ref_storage_payload);
 			the_repository->repository_format_worktree_config =
 				repo_fmt.worktree_config;
 			the_repository->repository_format_relative_worktrees =
@@ -2042,7 +2066,8 @@ void check_repository_format(struct repository_format *fmt)
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
 	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	repo_set_ref_storage_format(the_repository,
-				    fmt->ref_storage_format);
+				    fmt->ref_storage_format,
+				    fmt->ref_storage_payload);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_relative_worktrees =
@@ -2360,13 +2385,15 @@ static int is_reinit(void)
 }
 
 void create_reference_database(enum ref_storage_format ref_storage_format,
+			       const char *ref_storage_payload,
 			       const char *initial_branch, int quiet)
 {
 	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int reinit = is_reinit();
 
-	repo_set_ref_storage_format(the_repository, ref_storage_format);
+	repo_set_ref_storage_format(the_repository, ref_storage_format,
+				    ref_storage_payload);
 	if (ref_store_create_on_disk(get_main_ref_store(the_repository), 0, &err))
 		die("failed to set up refs db: %s", err.buf);
 
@@ -2645,7 +2672,8 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
-	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format);
+	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
+				    repo_fmt->ref_storage_payload);
 }
 
 int init_db(const char *git_dir, const char *real_git_dir,
@@ -2702,6 +2730,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 
 	if (!(flags & INIT_DB_SKIP_REFDB))
 		create_reference_database(repo_fmt.ref_storage_format,
+					  repo_fmt.ref_storage_payload,
 					  initial_branch, flags & INIT_DB_QUIET);
 	create_object_directory();
 
diff --git a/setup.h b/setup.h
index d55dcc6608..4d25a353ac 100644
--- a/setup.h
+++ b/setup.h
@@ -171,6 +171,7 @@ struct repository_format {
 	int hash_algo;
 	int compat_hash_algo;
 	enum ref_storage_format ref_storage_format;
+	char *ref_storage_payload;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;
@@ -241,6 +242,7 @@ void initialize_repository_version(int hash_algo,
 				   enum ref_storage_format ref_storage_format,
 				   int reinit);
 void create_reference_database(enum ref_storage_format ref_storage_format,
+			       const char *ref_storage_payload,
 			       const char *initial_branch, int quiet);
 
 /*
diff --git a/t/meson.build b/t/meson.build
index 459c52a489..11fc5a49ee 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -210,6 +210,7 @@ integration_tests = [
   't1420-lost-found.sh',
   't1421-reflog-write.sh',
   't1422-show-ref-exists.sh',
+  't1423-ref-backend.sh',
   't1430-bad-ref-name.sh',
   't1450-fsck.sh',
   't1451-fsck-buffer.sh',
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
new file mode 100755
index 0000000000..9c777b79f3
--- /dev/null
+++ b/t/t1423-ref-backend.sh
@@ -0,0 +1,159 @@
+#!/bin/sh
+
+test_description='Test reference backend URIs'
+
+. ./test-lib.sh
+
+# Run a git command with the provided reference storage. Reset the backend
+# post running the command.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <cmd> is the git subcommand to be run in the repository.
+run_with_uri() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	cmd=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1
+	git -C "$repo" config set extensions.refStorage "$uri" &&
+	git -C "$repo" $cmd &&
+	git -C "$repo" config set extensions.refStorage "$backend"
+}
+
+# Test a repository with a given reference storage by running and comparing
+# 'git refs list' before and after setting the new reference backend. If
+# err_msg is set, expect the command to fail and grep for the provided err_msg.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
+test_refs_backend() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	err_msg=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test -n "$err_msg";
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		test_must_fail git -C "$repo" refs list 2>err &&
+		test_grep "$err_msg" err
+	else
+		git -C "$repo" refs list >expect &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		test_cmp expect actual
+	fi
+}
+
+test_expect_success 'URI is invalid' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable@/home/reftable" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'URI ends with colon' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable:" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'unknown reference backend' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "db://.git" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+ref_formats="files reftable"
+for from_format in $ref_formats
+do
+
+for to_format in $ref_formats
+do
+	if test "$from_format" = "$to_format"
+	then
+		continue
+	fi
+
+
+	for dir in "$(pwd)/repo/.git" "./"
+	do
+
+		test_expect_success "$read from $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method"
+			)
+		'
+
+		test_expect_success "$write to $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+
+				git refs list >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				git refs list >actual &&
+				test_cmp expect actual &&
+
+				git refs list | grep -v "refs/tags/1" >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				test_cmp expect actual
+			)
+		'
+
+		test_expect_success "with worktree and $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo wt" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				git config set core.repositoryformatversion 1 &&
+				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+
+				git worktree add ../wt 2
+			) &&
+
+			git -C repo for-each-ref --include-root-refs >expect &&
+			git -C wt for-each-ref --include-root-refs >expect &&
+			! test_cmp expect actual &&
+
+			git -C wt rev-parse 2 >expect &&
+			git -C wt rev-parse HEAD >actual &&
+			test_cmp expect actual
+		'
+	done # closes dir
+done # closes to_format
+done # closes from_format
+
+test_done

Documentation/git.adoc, environment.h, setup.c, t/t1423-ref-backend.sh

diff --git a/Documentation/git.adoc b/Documentation/git.adoc
index ce099e78b8..66442735ea 100644
--- a/Documentation/git.adoc
+++ b/Documentation/git.adoc
@@ -584,6 +584,11 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REFERENCE_BACKEND`::
+    Specify which reference backend to be used along with its URI.
+    See `extensions.refStorage` option in linkgit:git-config[1] for more
+    details. Overrides the config variable when used.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::
diff --git a/environment.h b/environment.h
index 27f657af04..540e0a7f6d 100644
--- a/environment.h
+++ b/environment.h
@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REFERENCE_BACKEND_ENVIRONMENT "GIT_REFERENCE_BACKEND"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the
diff --git a/setup.c b/setup.c
index 44e393c251..b4a7b82cca 100644
--- a/setup.c
+++ b/setup.c
@@ -1838,6 +1838,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	static struct strbuf cwd = STRBUF_INIT;
 	struct strbuf dir = STRBUF_INIT, gitdir = STRBUF_INIT, report = STRBUF_INIT;
 	const char *prefix = NULL;
+	const char *ref_backend_uri;
 	struct repository_format repo_fmt = REPOSITORY_FORMAT_INIT;
 
 	/*
@@ -1995,6 +1996,25 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	/*
+	 * The env variable should override the repository config
+	 * for 'extensions.refStorage'.
+	 */
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *location;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &location);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+		repo_set_ref_storage_format(the_repository, format, location);
+
+		free(backend);
+		free(location);
+	}
+
 	setup_original_cwd();
 
 	strbuf_release(&dir);
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
index 9c777b79f3..10a9bb1a9b 100755
--- a/t/t1423-ref-backend.sh
+++ b/t/t1423-ref-backend.sh
@@ -11,16 +11,25 @@ test_description='Test reference backend URIs'
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
 #   <cmd> is the git subcommand to be run in the repository.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 run_with_uri() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
 	cmd=$4 &&
+	via=$5 &&
 
-	git -C "$repo" config set core.repositoryformatversion 1
-	git -C "$repo" config set extensions.refStorage "$uri" &&
-	git -C "$repo" $cmd &&
-	git -C "$repo" config set extensions.refStorage "$backend"
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test "$via" = "env"
+	then
+		test_env GIT_REFERENCE_BACKEND="$uri" git -C "$repo" $cmd
+	elif test "$via" = "config"
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		git -C "$repo" $cmd &&
+		git -C "$repo" config set extensions.refStorage "$backend"
+	fi
 }
 
 # Test a repository with a given reference storage by running and comparing
@@ -30,44 +39,57 @@ run_with_uri() {
 #   <repo> is the relative path to the repo to run the command in.
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 #   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
 test_refs_backend() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
-	err_msg=$4 &&
+	via=$4 &&
+	err_msg=$5 &&
+
 
-	git -C "$repo" config set core.repositoryformatversion 1 &&
 	if test -n "$err_msg";
 	then
-		git -C "$repo" config set extensions.refStorage "$uri" &&
-		test_must_fail git -C "$repo" refs list 2>err &&
-		test_grep "$err_msg" err
+		if test "$via" = "env"
+		then
+			test_env GIT_REFERENCE_BACKEND="$uri" test_must_fail git -C "$repo" refs list 2>err
+		elif test "$via" = "config"
+		then
+			git -C "$repo" config set extensions.refStorage "$uri" &&
+			test_must_fail git -C "$repo" refs list 2>err &&
+			test_grep "$err_msg" err
+		fi
 	else
 		git -C "$repo" refs list >expect &&
-		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" "$via">actual &&
 		test_cmp expect actual
 	fi
 }
 
-test_expect_success 'URI is invalid' '
+methods="config"
+for method in $methods
+do
+
+test_expect_success "$method: URI is invalid" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable@/home/reftable" \
+	test_refs_backend repo files "reftable@/home/reftable" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'URI ends with colon' '
+test_expect_success "$method: URI ends with colon" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable:" \
+	test_refs_backend repo files "reftable:" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'unknown reference backend' '
+test_expect_success "$method: unknown reference backend" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "db://.git" \
+	test_refs_backend repo files "db://.git" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
@@ -86,7 +108,7 @@ do
 	for dir in "$(pwd)/repo/.git" "./"
 	do
 
-		test_expect_success "$read from $to_format backend, $dir dir" '
+		test_expect_success "$method: $read from $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -101,7 +123,7 @@ do
 			)
 		'
 
-		test_expect_success "$write to $to_format backend, $dir dir" '
+		test_expect_success "$method: $write to $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -113,20 +135,22 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method" &&
 
 				git refs list >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"tag -d 1" "$method" &&
 				git refs list >actual &&
 				test_cmp expect actual &&
 
 				git refs list | grep -v "refs/tags/1" >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"refs list" "$method" >actual &&
 				test_cmp expect actual
 			)
 		'
 
-		test_expect_success "with worktree and $to_format backend, $dir dir" '
+		test_expect_success "$method: with worktree and $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo wt" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -138,22 +162,26 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				git config set core.repositoryformatversion 1 &&
-				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"worktree add ../wt 2" "$method" &&
 
-				git worktree add ../wt 2
-			) &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >expect &&
+				! test_cmp expect actual &&
 
-			git -C repo for-each-ref --include-root-refs >expect &&
-			git -C wt for-each-ref --include-root-refs >expect &&
-			! test_cmp expect actual &&
-
-			git -C wt rev-parse 2 >expect &&
-			git -C wt rev-parse HEAD >actual &&
-			test_cmp expect actual
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse 2" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse HEAD" "$method" >expect &&
+				test_cmp expect actual
+			)
 		'
 	done # closes dir
 done # closes to_format
-done # closes from_format
+done # closes to_format
+
+done # closes method
 
 test_done

builtin/clone.c, setup.c, setup.h

diff --git a/builtin/clone.c b/builtin/clone.c
index b40cee5968..cd43bb5aa2 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1442,7 +1442,7 @@ int cmd_clone(int argc,
 	hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 	initialize_repository_version(hash_algo, the_repository->ref_storage_format, 1);
 	repo_set_hash_algo(the_repository, hash_algo);
-	create_reference_database(the_repository->ref_storage_format, NULL, 1);
+	create_reference_database(NULL, 1);
 
 	/*
 	 * Before fetching from the remote, download and install bundle
diff --git a/setup.c b/setup.c
index b723f8b339..1fc9ae3872 100644
--- a/setup.c
+++ b/setup.c
@@ -2359,14 +2359,12 @@ static int is_reinit(void)
 	return ret;
 }
 
-void create_reference_database(enum ref_storage_format ref_storage_format,
-			       const char *initial_branch, int quiet)
+void create_reference_database(const char *initial_branch, int quiet)
 {
 	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int reinit = is_reinit();
 
-	repo_set_ref_storage_format(the_repository, ref_storage_format);
 	if (ref_store_create_on_disk(get_main_ref_store(the_repository), 0, &err))
 		die("failed to set up refs db: %s", err.buf);
 
@@ -2701,8 +2699,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 				      &repo_fmt, init_shared_repository);
 
 	if (!(flags & INIT_DB_SKIP_REFDB))
-		create_reference_database(repo_fmt.ref_storage_format,
-					  initial_branch, flags & INIT_DB_QUIET);
+		create_reference_database(initial_branch, flags & INIT_DB_QUIET);
 	create_object_directory();
 
 	if (repo_settings_get_shared_repository(the_repository)) {
diff --git a/setup.h b/setup.h
index d55dcc6608..ddb9f6701c 100644
--- a/setup.h
+++ b/setup.h
@@ -240,8 +240,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 void initialize_repository_version(int hash_algo,
 				   enum ref_storage_format ref_storage_format,
 				   int reinit);
-void create_reference_database(enum ref_storage_format ref_storage_format,
-			       const char *initial_branch, int quiet);
+void create_reference_database(const char *initial_branch, int quiet);
 
 /*
  * NOTE NOTE NOTE!!

builtin/clone.c, refs.c, refs.h, refs/reftable-backend.c

diff --git a/builtin/clone.c b/builtin/clone.c
index cd43bb5aa2..697c5bb5cb 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1225,12 +1225,7 @@ int cmd_clone(int argc,
 	initialize_repository_version(GIT_HASH_UNKNOWN,
 				      the_repository->ref_storage_format, 1);
 
-	strbuf_addf(&buf, "%s/HEAD", git_dir);
-	write_file(buf.buf, "ref: refs/heads/.invalid");
-
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "%s/refs", git_dir);
-	safe_create_dir(the_repository, buf.buf, 1);
+	refs_create_refdir_stubs(the_repository, git_dir, NULL);
 
 	/*
 	 * additional config can be injected with -c, make sure it's included
diff --git a/refs.c b/refs.c
index 627b7f8698..77b93d655b 100644
--- a/refs.c
+++ b/refs.c
@@ -2163,6 +2163,29 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 	return NULL;
 }
 
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_content)
+{
+	struct strbuf path = STRBUF_INIT;
+
+	strbuf_addf(&path, "%s/HEAD", refdir);
+	write_file(path.buf, "ref: refs/heads/.invalid");
+	adjust_shared_perm(repo, path.buf);
+
+	strbuf_reset(&path);
+	strbuf_addf(&path, "%s/refs", refdir);
+	safe_create_dir(repo, path.buf, 1);
+
+	if (refs_heads_content) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/refs/heads", refdir);
+		write_file(path.buf, "%s", refs_heads_content);
+		adjust_shared_perm(repo, path.buf);
+	}
+
+	strbuf_release(&path);
+}
+
 /* backend functions */
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
diff --git a/refs.h b/refs.h
index f0abfa1d93..9d8890fdff 100644
--- a/refs.h
+++ b/refs.h
@@ -1427,4 +1427,17 @@ void ref_iterator_free(struct ref_iterator *ref_iterator);
 int do_for_each_ref_iterator(struct ref_iterator *iter,
 			     each_ref_fn fn, void *cb_data);
 
+/*
+ * Git only recognizes a directory as a repository if it contains:
+ * - HEAD file
+ * - refs/ folder
+ * While it is necessary within the files backend, newer backends may not
+ * follow the same structure. To go around this, we create stubs as necessary.
+ *
+ * If provided with a 'refs_heads_msg', we create the 'refs/heads/head' file
+ * with the provided message.
+ */
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_msg);
+
 #endif /* REFS_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index fe74af73af..d8651fe779 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,18 +491,8 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	write_file(sb.buf, "ref: refs/heads/.invalid");
-	adjust_shared_perm(the_repository, sb.buf);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	safe_create_dir(the_repository, sb.buf, 1);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	write_file(sb.buf, "this repository uses the reftable format");
-	adjust_shared_perm(the_repository, sb.buf);
+	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
+				 "this repository uses the reftable format");
 
 	strbuf_release(&sb);
 	return 0;

refs.c, refs/files-backend.c, refs/packed-backend.c, refs/packed-backend.h, refs/refs-internal.h, refs/reftable-backend.c

diff --git a/refs.c b/refs.c
index 77b93d655b..11d028232b 100644
--- a/refs.c
+++ b/refs.c
@@ -5,6 +5,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "advice.h"
 #include "config.h"
 #include "environment.h"
@@ -2247,7 +2248,7 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, gitdir, flags);
+	refs = be->init(repo, NULL, gitdir, flags);
 	return refs;
 }
 
@@ -3425,3 +3426,40 @@ const char *ref_transaction_error_msg(enum ref_transaction_error err)
 		return "unknown failure";
 	}
 }
+
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	*is_worktree = get_common_dir_noenv(ref_common_dir, gitdir);
+
+	if (!payload) {
+		/*
+		 * We can use the 'gitdir' as the 'refdir' without appending the
+		 * worktree path, as the 'gitdir' here is already the worktree
+		 * path and is different from 'commondir' denoted by 'ref_common_dir'.
+		 */
+		strbuf_addstr(refdir, gitdir);
+		return;
+	}
+
+	if (!is_absolute_path(payload)) {
+		strbuf_addf(&sb, "%s/%s", ref_common_dir->buf, payload);
+		strbuf_realpath(ref_common_dir, sb.buf, 1);
+	} else {
+		strbuf_realpath(ref_common_dir, payload, 1);
+	}
+
+	strbuf_addbuf(refdir, ref_common_dir);
+
+	if (*is_worktree) {
+		const char *wt_id = strrchr(gitdir, '/');
+		if (!wt_id)
+			BUG("worktree path does not contain slash ");
+		strbuf_addf(refdir, "/worktrees/%s", wt_id + 1);
+	}
+
+	strbuf_release(&sb);
+}
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 240d3c3b26..b40d6feb1f 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -106,19 +106,24 @@ static void clear_loose_ref_cache(struct files_ref_store *refs)
  * set of caches.
  */
 static struct ref_store *files_ref_store_init(struct repository *repo,
+					      const char *payload,
 					      const char *gitdir,
 					      unsigned int flags)
 {
 	struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
 	struct ref_store *ref_store = (struct ref_store *)refs;
-	struct strbuf sb = STRBUF_INIT;
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
+	bool is_worktree;
+
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
 
-	base_ref_store_init(ref_store, repo, gitdir, &refs_be_files);
+	base_ref_store_init(ref_store, repo, refdir.buf, &refs_be_files);
 	refs->store_flags = flags;
-	get_common_dir_noenv(&sb, gitdir);
-	refs->gitcommondir = strbuf_detach(&sb, NULL);
+	refs->gitcommondir = strbuf_detach(&ref_common_dir, NULL);
 	refs->packed_ref_store =
-		packed_ref_store_init(repo, refs->gitcommondir, flags);
+		packed_ref_store_init(repo, NULL, refs->gitcommondir, flags);
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
 	repo_config_get_bool(repo, "core.prefersymlinkrefs", &refs->prefer_symlink_refs);
 
@@ -126,6 +131,8 @@ static struct ref_store *files_ref_store_init(struct repository *repo,
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
 			      &refs->gitcommondir);
 
+	strbuf_release(&refdir);
+
 	return ref_store;
 }
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..e7bb9f10f9 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -211,7 +211,12 @@ static size_t snapshot_hexsz(const struct snapshot *snapshot)
 	return snapshot->refs->base.repo->hash_algo->hexsz;
 }
 
+/*
+ * Since packed-refs is only stored in the common dir, don't parse the
+ * payload and rely on the files-backend to set 'gitdir' correctly.
+ */
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload UNUSED,
 					const char *gitdir,
 					unsigned int store_flags)
 {
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index 9481d5e7c2..2c2377a356 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -14,6 +14,7 @@ struct ref_transaction;
  */
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload,
 					const char *gitdir,
 					unsigned int store_flags);
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index c7d2a6e50b..9a635f4e6c 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -389,6 +389,7 @@ struct ref_store;
  * the ref_store and to record the ref_store for later lookup.
  */
 typedef struct ref_store *ref_store_init_fn(struct repository *repo,
+					    const char *payload,
 					    const char *gitdir,
 					    unsigned int flags);
 /*
@@ -666,4 +667,17 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
 					  unsigned int initial_transaction,
 					  struct strbuf *err);
 
+/*
+ * Given a gitdir and the reference storage payload provided, retrieve the
+ * 'refdir' and 'ref_common_dir'. The former is where references should be
+ * stored for the current worktree, the latter is the common reference
+ * directory if working with a linked worktree. If working with the main
+ * worktree, both values will be the same.
+ *
+ * This is used by backends that store store files in the repository directly.
+ */
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir);
+
 #endif /* REFS_REFS_INTERNAL_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index d8651fe779..964b0b50fc 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -372,18 +372,24 @@ static int reftable_be_fsync(int fd)
 }
 
 static struct ref_store *reftable_be_init(struct repository *repo,
+					  const char *payload,
 					  const char *gitdir,
 					  unsigned int store_flags)
 {
 	struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
 	struct strbuf path = STRBUF_INIT;
-	int is_worktree;
+	bool is_worktree;
 	mode_t mask;
 
 	mask = umask(0);
 	umask(mask);
 
-	base_ref_store_init(&refs->base, repo, gitdir, &refs_be_reftable);
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
+
+	base_ref_store_init(&refs->base, repo, refdir.buf, &refs_be_reftable);
 	strmap_init(&refs->worktree_backends);
 	refs->store_flags = store_flags;
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
@@ -419,14 +425,11 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.
 	 * This stack contains both the shared and the main worktree refs.
-	 *
-	 * Note that we don't try to resolve the path in case we have a
-	 * worktree because `get_common_dir_noenv()` already does it for us.
 	 */
-	is_worktree = get_common_dir_noenv(&path, gitdir);
+	strbuf_addbuf(&path, &ref_common_dir);
 	if (!is_worktree) {
 		strbuf_reset(&path);
-		strbuf_realpath(&path, gitdir, 0);
+		strbuf_realpath(&path, ref_common_dir.buf, 0);
 	}
 	strbuf_addstr(&path, "/reftable");
 	refs->err = reftable_backend_init(&refs->main_backend, path.buf,
@@ -443,10 +446,9 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	 * do it efficiently.
 	 */
 	if (is_worktree) {
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/reftable", gitdir);
+		strbuf_addstr(&refdir, "/reftable");
 
-		refs->err = reftable_backend_init(&refs->worktree_backend, path.buf,
+		refs->err = reftable_backend_init(&refs->worktree_backend, refdir.buf,
 						  &refs->write_options);
 		if (refs->err)
 			goto done;
@@ -456,6 +458,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 
 done:
 	assert(refs->err != REFTABLE_API_ERROR);
+	strbuf_release(&ref_common_dir);
+	strbuf_release(&refdir);
 	strbuf_release(&path);
 	return &refs->base;
 }

refs.c, refs/files-backend.c, refs/reftable-backend.c

diff --git a/refs.c b/refs.c
index 11d028232b..a24602c9bf 100644
--- a/refs.c
+++ b/refs.c
@@ -2190,12 +2190,59 @@ void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
 /* backend functions */
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
-	return refs->be->create_on_disk(refs, flags, err);
+	int ret = refs->be->create_on_disk(refs, flags, err);
+
+	if (!ret &&
+	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+		struct strbuf msg = STRBUF_INIT;
+
+		strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
+		refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
+		strbuf_release(&msg);
+	}
+
+	return ret;
 }
 
 int ref_store_remove_on_disk(struct ref_store *refs, struct strbuf *err)
 {
-	return refs->be->remove_on_disk(refs, err);
+	int ret = refs->be->remove_on_disk(refs, err);
+
+	if (!ret) {
+		enum ref_storage_format format = ref_storage_format_by_name(refs->be->name);
+		struct strbuf sb = STRBUF_INIT;
+
+		/* Backends apart from the files backend create stubs. */
+		if (format == REF_STORAGE_FORMAT_FILES)
+			return ret;
+
+		strbuf_addf(&sb, "%s/HEAD", refs->gitdir);
+		if (unlink(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete stub HEAD: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+		strbuf_reset(&sb);
+
+		strbuf_addf(&sb, "%s/refs/heads", refs->gitdir);
+		if (unlink(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete stub heads: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+		strbuf_reset(&sb);
+
+		strbuf_addf(&sb, "%s/refs", refs->gitdir);
+		if (rmdir(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete refs directory: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+
+		strbuf_release(&sb);
+	}
+
+	return ret;
 }
 
 int repo_resolve_gitlink_ref(struct repository *r,
diff --git a/refs/files-backend.c b/refs/files-backend.c
index b40d6feb1f..9cde3ba724 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -3707,7 +3707,11 @@ static int files_ref_store_remove_on_disk(struct ref_store *ref_store,
 	if (for_each_root_ref(refs, remove_one_root_ref, &data) < 0)
 		ret = -1;
 
-	if (ref_store_remove_on_disk(refs->packed_ref_store, err) < 0)
+	/*
+	 * Directly access the cleanup functions for packed-refs as the generic function
+	 * would try to clear stubs which isn't required for the files backend.
+	 */
+	if (refs->packed_ref_store->be->remove_on_disk(refs->packed_ref_store, err) < 0)
 		ret = -1;
 
 	strbuf_release(&sb);
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 964b0b50fc..0e220d6bb5 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -495,9 +495,6 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
-				 "this repository uses the reftable format");
-
 	strbuf_release(&sb);
 	return 0;
 }
@@ -523,30 +520,6 @@ static int reftable_be_remove_on_disk(struct ref_store *ref_store,
 			    strerror(errno));
 		ret = -1;
 	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	if (unlink(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete stub HEAD: %s",
-			    strerror(errno));
-		ret = -1;
-	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	if (unlink(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete stub heads: %s",
-			    strerror(errno));
-		ret = -1;
-	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	if (rmdir(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete refs directory: %s",
-			    strerror(errno));
-		ret = -1;
-	}
 
 	strbuf_release(&sb);
 	return ret;

Documentation/config/extensions.adoc, builtin/worktree.c, refs.c, repository.c, repository.h, setup.c, setup.h, t/meson.build, t/t1423-ref-backend.sh

diff --git a/Documentation/config/extensions.adoc b/Documentation/config/extensions.adoc
index 532456644b..3e51da36d3 100644
--- a/Documentation/config/extensions.adoc
+++ b/Documentation/config/extensions.adoc
@@ -57,10 +57,24 @@ For historical reasons, this extension is respected regardless of the
 `core.repositoryFormatVersion` setting.
 
 refStorage:::
-	Specify the ref storage format to use. The acceptable values are:
+	Specify the ref storage format and a corresponding payload. The value
+	can be either a format name or a URI:
 +
 --
+* A format name alone (e.g., `reftable` or `files`).
+
+* A URI format `<format>://<payload>` explicitly specifies both the
+  format and payload (e.g., `reftable:///foo/bar`).
+
+Supported format names are:
++
 include::../ref-storage-format.adoc[]
++
+The payload is passed directly to the reference backend. For the files and
+reftable backends, this must be a filesystem path where the references will
+be stored. Defaulting to the commondir when no payload is provided. Relative
+paths are resolved relative to the $GIT_DIR. Future backends may support
+other payload schemes, e.g., postgres://127.0.0.1:5432?database=myrepo.
 --
 +
 Note that this setting should only be set by linkgit:git-init[1] or
diff --git a/builtin/worktree.c b/builtin/worktree.c
index fbdaf2eb2e..293e808379 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -425,6 +425,39 @@ static int make_worktree_orphan(const char * ref, const struct add_opts *opts,
 	return run_command(&cp);
 }
 
+/*
+ * References for worktrees are generally stored in '$GIT_DIR/worktrees/<wt_id>'.
+ * But when using alternate reference directories, we want to store the worktree
+ * references in '$ALTERNATE_REFERENCE_DIR/worktrees/<wt_id>'.
+ *
+ * Create the necessary folder structure to facilitate the same. But to ensure
+ * that the former path is still considered a Git directory, add stubs.
+ */
+static void setup_alternate_ref_dir(struct worktree *wt, const char *wt_git_path)
+{
+	struct strbuf sb = STRBUF_INIT;
+	char *path;
+
+	path = wt->repo->ref_storage_payload;
+	if (!path)
+		return;
+
+	if (!is_absolute_path(path))
+		strbuf_addf(&sb, "%s/", wt->repo->commondir);
+
+	strbuf_addf(&sb, "%s/worktrees", path);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_addf(&sb, "/%s", wt->id);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "this worktree stores references in %s/worktrees/%s",
+		    path, wt->id);
+	refs_create_refdir_stubs(wt->repo, wt_git_path, sb.buf);
+
+	strbuf_release(&sb);
+}
+
 static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
@@ -518,6 +551,7 @@ static int add_worktree(const char *path, const char *refname,
 		ret = error(_("could not find created worktree '%s'"), name);
 		goto done;
 	}
+	setup_alternate_ref_dir(wt, sb_repo.buf);
 	wt_refs = get_worktree_ref_store(wt);
 
 	ret = ref_store_create_on_disk(wt_refs, REF_STORE_CREATE_ON_DISK_IS_WORKTREE, &sb);
diff --git a/refs.c b/refs.c
index a24602c9bf..87ef54abd4 100644
--- a/refs.c
+++ b/refs.c
@@ -2295,7 +2295,11 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, NULL, gitdir, flags);
+	/*
+	 * TODO Send in a 'struct worktree' instead of a 'gitdir', and
+	 * allow the backend to handle how it wants to deal with worktrees.
+	 */
+	refs = be->init(repo, repo->ref_storage_payload, gitdir, flags);
 	return refs;
 }
 
diff --git a/repository.c b/repository.c
index c7e75215ac..9815f081ef 100644
--- a/repository.c
+++ b/repository.c
@@ -193,9 +193,12 @@ void repo_set_compat_hash_algo(struct repository *repo, int algo)
 }
 
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format)
+				 enum ref_storage_format format,
+				 const char *payload)
 {
 	repo->ref_storage_format = format;
+	free(repo->ref_storage_payload);
+	repo->ref_storage_payload = xstrdup_or_null(payload);
 }
 
 /*
@@ -277,7 +280,8 @@ int repo_init(struct repository *repo,
 
 	repo_set_hash_algo(repo, format.hash_algo);
 	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
-	repo_set_ref_storage_format(repo, format.ref_storage_format);
+	repo_set_ref_storage_format(repo, format.ref_storage_format,
+				    format.ref_storage_payload);
 	repo->repository_format_worktree_config = format.worktree_config;
 	repo->repository_format_relative_worktrees = format.relative_worktrees;
 	repo->repository_format_precious_objects = format.precious_objects;
@@ -369,6 +373,7 @@ void repo_clear(struct repository *repo)
 	FREE_AND_NULL(repo->index_file);
 	FREE_AND_NULL(repo->worktree);
 	FREE_AND_NULL(repo->submodule_prefix);
+	FREE_AND_NULL(repo->ref_storage_payload);
 
 	odb_free(repo->objects);
 	repo->objects = NULL;
diff --git a/repository.h b/repository.h
index 6063c4b846..95e2333bad 100644
--- a/repository.h
+++ b/repository.h
@@ -150,6 +150,11 @@ struct repository {
 
 	/* Repository's reference storage format, as serialized on disk. */
 	enum ref_storage_format ref_storage_format;
+	/*
+	 * Reference storage information as needed for the backend. This contains
+	 * only the payload from the reference URI without the schema.
+	 */
+	char *ref_storage_payload;
 
 	/* A unique-id for tracing purposes. */
 	int trace2_repo_id;
@@ -204,7 +209,8 @@ void repo_set_worktree(struct repository *repo, const char *path);
 void repo_set_hash_algo(struct repository *repo, int algo);
 void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format);
+				 enum ref_storage_format format,
+				 const char *payload);
 void initialize_repository(struct repository *repo);
 RESULT_MUST_BE_USED
 int repo_init(struct repository *r, const char *gitdir, const char *worktree);
diff --git a/setup.c b/setup.c
index 1fc9ae3872..d407f3347b 100644
--- a/setup.c
+++ b/setup.c
@@ -632,6 +632,21 @@ static enum extension_result handle_extension_v0(const char *var,
 		return EXTENSION_UNKNOWN;
 }
 
+static void parse_reference_uri(const char *value, char **format,
+				char **payload)
+{
+	const char *schema_end;
+
+	schema_end = strstr(value, "://");
+	if (!schema_end) {
+		*format = xstrdup(value);
+		*payload = NULL;
+	} else {
+		*format = xstrndup(value, schema_end - value);
+		*payload = xstrdup_or_null(schema_end + 3);
+	}
+}
+
 /*
  * Record any new extensions in this function.
  */
@@ -674,10 +689,17 @@ static enum extension_result handle_extension(const char *var,
 		return EXTENSION_OK;
 	} else if (!strcmp(ext, "refstorage")) {
 		unsigned int format;
+		char *format_str;
 
 		if (!value)
 			return config_error_nonbool(var);
-		format = ref_storage_format_by_name(value);
+
+		parse_reference_uri(value, &format_str,
+				    &data->ref_storage_payload);
+
+		format = ref_storage_format_by_name(format_str);
+		free(format_str);
+
 		if (format == REF_STORAGE_FORMAT_UNKNOWN)
 			return error(_("invalid value for '%s': '%s'"),
 				     "extensions.refstorage", value);
@@ -850,6 +872,7 @@ void clear_repository_format(struct repository_format *format)
 	string_list_clear(&format->v1_only_extensions, 0);
 	free(format->work_tree);
 	free(format->partial_clone);
+	free(format->ref_storage_payload);
 	init_repository_format(format);
 }
 
@@ -1942,7 +1965,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			repo_set_compat_hash_algo(the_repository,
 						  repo_fmt.compat_hash_algo);
 			repo_set_ref_storage_format(the_repository,
-						    repo_fmt.ref_storage_format);
+						    repo_fmt.ref_storage_format,
+						    repo_fmt.ref_storage_payload);
 			the_repository->repository_format_worktree_config =
 				repo_fmt.worktree_config;
 			the_repository->repository_format_relative_worktrees =
@@ -2042,7 +2066,8 @@ void check_repository_format(struct repository_format *fmt)
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
 	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	repo_set_ref_storage_format(the_repository,
-				    fmt->ref_storage_format);
+				    fmt->ref_storage_format,
+				    fmt->ref_storage_payload);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_relative_worktrees =
@@ -2643,7 +2668,8 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
-	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format);
+	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
+				    repo_fmt->ref_storage_payload);
 }
 
 int init_db(const char *git_dir, const char *real_git_dir,
diff --git a/setup.h b/setup.h
index ddb9f6701c..093af39e84 100644
--- a/setup.h
+++ b/setup.h
@@ -171,6 +171,7 @@ struct repository_format {
 	int hash_algo;
 	int compat_hash_algo;
 	enum ref_storage_format ref_storage_format;
+	char *ref_storage_payload;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;
diff --git a/t/meson.build b/t/meson.build
index 459c52a489..11fc5a49ee 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -210,6 +210,7 @@ integration_tests = [
   't1420-lost-found.sh',
   't1421-reflog-write.sh',
   't1422-show-ref-exists.sh',
+  't1423-ref-backend.sh',
   't1430-bad-ref-name.sh',
   't1450-fsck.sh',
   't1451-fsck-buffer.sh',
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
new file mode 100755
index 0000000000..9912433b8c
--- /dev/null
+++ b/t/t1423-ref-backend.sh
@@ -0,0 +1,159 @@
+#!/bin/sh
+
+test_description='Test reference backend URIs'
+
+. ./test-lib.sh
+
+# Run a git command with the provided reference storage. Reset the backend
+# post running the command.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <cmd> is the git subcommand to be run in the repository.
+run_with_uri() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	cmd=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1
+	git -C "$repo" config set extensions.refStorage "$uri" &&
+	git -C "$repo" $cmd &&
+	git -C "$repo" config set extensions.refStorage "$backend"
+}
+
+# Test a repository with a given reference storage by running and comparing
+# 'git refs list' before and after setting the new reference backend. If
+# err_msg is set, expect the command to fail and grep for the provided err_msg.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
+test_refs_backend() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	err_msg=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test -n "$err_msg";
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		test_must_fail git -C "$repo" refs list 2>err &&
+		test_grep "$err_msg" err
+	else
+		git -C "$repo" refs list >expect &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		test_cmp expect actual
+	fi
+}
+
+test_expect_success 'URI is invalid' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable@/home/reftable" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'URI ends with colon' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable:" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'unknown reference backend' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "db://.git" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+ref_formats="files reftable"
+for from_format in $ref_formats
+do
+
+for to_format in $ref_formats
+do
+	if test "$from_format" = "$to_format"
+	then
+		continue
+	fi
+
+
+	for dir in "$(pwd)/repo/.git" "."
+	do
+
+		test_expect_success "read from $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method"
+			)
+		'
+
+		test_expect_success "write to $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+
+				git refs list >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				git refs list >actual &&
+				test_cmp expect actual &&
+
+				git refs list | grep -v "refs/tags/1" >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				test_cmp expect actual
+			)
+		'
+
+		test_expect_success "with worktree and $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo wt" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				git config set core.repositoryformatversion 1 &&
+				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+
+				git worktree add ../wt 2
+			) &&
+
+			git -C repo for-each-ref --include-root-refs >expect &&
+			git -C wt for-each-ref --include-root-refs >expect &&
+			! test_cmp expect actual &&
+
+			git -C wt rev-parse 2 >expect &&
+			git -C wt rev-parse HEAD >actual &&
+			test_cmp expect actual
+		'
+	done # closes dir
+done # closes to_format
+done # closes from_format
+
+test_done

Documentation/git.adoc, environment.h, refs.c, setup.c, t/t1423-ref-backend.sh

diff --git a/Documentation/git.adoc b/Documentation/git.adoc
index ce099e78b8..66442735ea 100644
--- a/Documentation/git.adoc
+++ b/Documentation/git.adoc
@@ -584,6 +584,11 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REFERENCE_BACKEND`::
+    Specify which reference backend to be used along with its URI.
+    See `extensions.refStorage` option in linkgit:git-config[1] for more
+    details. Overrides the config variable when used.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::
diff --git a/environment.h b/environment.h
index 27f657af04..540e0a7f6d 100644
--- a/environment.h
+++ b/environment.h
@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REFERENCE_BACKEND_ENVIRONMENT "GIT_REFERENCE_BACKEND"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the
diff --git a/refs.c b/refs.c
index 87ef54abd4..6b3883a325 100644
--- a/refs.c
+++ b/refs.c
@@ -2192,16 +2192,21 @@ int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *e
 {
 	int ret = refs->be->create_on_disk(refs, flags, err);
 
-	if (!ret &&
-	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
-		struct strbuf msg = STRBUF_INIT;
-
-		strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
-		refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
-		strbuf_release(&msg);
+	if (!ret) {
+		/* Creation of stubs for linked worktrees are handled in the worktree code. */
+		if (!(flags & REF_STORE_CREATE_ON_DISK_IS_WORKTREE) && refs->repo->ref_storage_payload) {
+			refs_create_refdir_stubs(refs->repo, refs->repo->gitdir,
+						 "repository uses alternate refs storage");
+		} else if (ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+			struct strbuf msg = STRBUF_INIT;
+			strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
+			refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
+			strbuf_release(&msg);
+		}
 	}
 
 	return ret;
+
 }
 
 int ref_store_remove_on_disk(struct ref_store *refs, struct strbuf *err)
@@ -2216,6 +2221,10 @@ int ref_store_remove_on_disk(struct ref_store *refs, struct strbuf *err)
 		if (format == REF_STORAGE_FORMAT_FILES)
 			return ret;
 
+		/* Alternate refs backend require stubs in the gitdir. */
+		if (refs->repo->ref_storage_payload)
+			return ret;
+
 		strbuf_addf(&sb, "%s/HEAD", refs->gitdir);
 		if (unlink(sb.buf) < 0) {
 			strbuf_addf(err, "could not delete stub HEAD: %s",
diff --git a/setup.c b/setup.c
index d407f3347b..90cb9be578 100644
--- a/setup.c
+++ b/setup.c
@@ -1838,6 +1838,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	static struct strbuf cwd = STRBUF_INIT;
 	struct strbuf dir = STRBUF_INIT, gitdir = STRBUF_INIT, report = STRBUF_INIT;
 	const char *prefix = NULL;
+	const char *ref_backend_uri;
 	struct repository_format repo_fmt = REPOSITORY_FORMAT_INIT;
 
 	/*
@@ -1995,6 +1996,25 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	/*
+	 * The env variable should override the repository config
+	 * for 'extensions.refStorage'.
+	 */
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *payload;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &payload);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+		repo_set_ref_storage_format(the_repository, format, payload);
+
+		free(backend);
+		free(payload);
+	}
+
 	setup_original_cwd();
 
 	strbuf_release(&dir);
@@ -2337,7 +2357,8 @@ void initialize_repository_version(int hash_algo,
 	 * the remote repository's format.
 	 */
 	if (hash_algo != GIT_HASH_SHA1_LEGACY ||
-	    ref_storage_format != REF_STORAGE_FORMAT_FILES)
+	    ref_storage_format != REF_STORAGE_FORMAT_FILES ||
+	    the_repository->ref_storage_payload)
 		target_version = GIT_REPO_VERSION_READ;
 
 	if (hash_algo != GIT_HASH_SHA1_LEGACY && hash_algo != GIT_HASH_UNKNOWN)
@@ -2346,11 +2367,20 @@ void initialize_repository_version(int hash_algo,
 	else if (reinit)
 		repo_config_set_gently(the_repository, "extensions.objectformat", NULL);
 
-	if (ref_storage_format != REF_STORAGE_FORMAT_FILES)
+	if (the_repository->ref_storage_payload) {
+		struct strbuf ref_uri = STRBUF_INIT;
+
+		strbuf_addf(&ref_uri, "%s://%s",
+			    ref_storage_format_to_name(ref_storage_format),
+			    the_repository->ref_storage_payload);
+		repo_config_set(the_repository, "extensions.refstorage", ref_uri.buf);
+		strbuf_release(&ref_uri);
+	} else if (ref_storage_format != REF_STORAGE_FORMAT_FILES) {
 		repo_config_set(the_repository, "extensions.refstorage",
 				ref_storage_format_to_name(ref_storage_format));
-	else if (reinit)
+	} else if (reinit) {
 		repo_config_set_gently(the_repository, "extensions.refstorage", NULL);
+	}
 
 	if (reinit) {
 		struct strbuf config = STRBUF_INIT;
@@ -2623,6 +2653,7 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 		.ignore_repo = 1,
 		.ignore_worktree = 1,
 	};
+	const char *ref_backend_uri;
 	const char *env;
 
 	config_with_options(read_default_format_config, &cfg, NULL, NULL, &opts);
@@ -2668,6 +2699,24 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
+
+
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *payload;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &payload);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+
+		repo_fmt->ref_storage_format = format;
+		repo_fmt->ref_storage_payload = payload;
+
+		free(backend);
+	}
+
 	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
 				    repo_fmt->ref_storage_payload);
 }
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
index 9912433b8c..b743c03a59 100755
--- a/t/t1423-ref-backend.sh
+++ b/t/t1423-ref-backend.sh
@@ -11,16 +11,25 @@ test_description='Test reference backend URIs'
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
 #   <cmd> is the git subcommand to be run in the repository.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 run_with_uri() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
 	cmd=$4 &&
+	via=$5 &&
 
-	git -C "$repo" config set core.repositoryformatversion 1
-	git -C "$repo" config set extensions.refStorage "$uri" &&
-	git -C "$repo" $cmd &&
-	git -C "$repo" config set extensions.refStorage "$backend"
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test "$via" = "env"
+	then
+		test_env GIT_REFERENCE_BACKEND="$uri" git -C "$repo" $cmd
+	elif test "$via" = "config"
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		git -C "$repo" $cmd &&
+		git -C "$repo" config set extensions.refStorage "$backend"
+	fi
 }
 
 # Test a repository with a given reference storage by running and comparing
@@ -30,44 +39,57 @@ run_with_uri() {
 #   <repo> is the relative path to the repo to run the command in.
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 #   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
 test_refs_backend() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
-	err_msg=$4 &&
+	via=$4 &&
+	err_msg=$5 &&
+
 
-	git -C "$repo" config set core.repositoryformatversion 1 &&
 	if test -n "$err_msg";
 	then
-		git -C "$repo" config set extensions.refStorage "$uri" &&
-		test_must_fail git -C "$repo" refs list 2>err &&
-		test_grep "$err_msg" err
+		if test "$via" = "env"
+		then
+			test_env GIT_REFERENCE_BACKEND="$uri" test_must_fail git -C "$repo" refs list 2>err
+		elif test "$via" = "config"
+		then
+			git -C "$repo" config set extensions.refStorage "$uri" &&
+			test_must_fail git -C "$repo" refs list 2>err &&
+			test_grep "$err_msg" err
+		fi
 	else
 		git -C "$repo" refs list >expect &&
-		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" "$via">actual &&
 		test_cmp expect actual
 	fi
 }
 
-test_expect_success 'URI is invalid' '
+methods="config env"
+for method in $methods
+do
+
+test_expect_success "$method: URI is invalid" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable@/home/reftable" \
+	test_refs_backend repo files "reftable@/home/reftable" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'URI ends with colon' '
+test_expect_success "$method: URI ends with colon" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable:" \
+	test_refs_backend repo files "reftable:" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'unknown reference backend' '
+test_expect_success "$method: unknown reference backend" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "db://.git" \
+	test_refs_backend repo files "db://.git" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
@@ -86,7 +108,7 @@ do
 	for dir in "$(pwd)/repo/.git" "."
 	do
 
-		test_expect_success "read from $to_format backend, $dir dir" '
+		test_expect_success "$method: read from $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -101,7 +123,7 @@ do
 			)
 		'
 
-		test_expect_success "write to $to_format backend, $dir dir" '
+		test_expect_success "$method: write to $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -113,20 +135,22 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method" &&
 
 				git refs list >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"tag -d 1" "$method" &&
 				git refs list >actual &&
 				test_cmp expect actual &&
 
 				git refs list | grep -v "refs/tags/1" >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"refs list" "$method" >actual &&
 				test_cmp expect actual
 			)
 		'
 
-		test_expect_success "with worktree and $to_format backend, $dir dir" '
+		test_expect_success "$method: with worktree and $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo wt" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -138,22 +162,89 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				git config set core.repositoryformatversion 1 &&
-				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
-
-				git worktree add ../wt 2
-			) &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"worktree add ../wt 2" "$method" &&
 
-			git -C repo for-each-ref --include-root-refs >expect &&
-			git -C wt for-each-ref --include-root-refs >expect &&
-			! test_cmp expect actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >expect &&
+				! test_cmp expect actual &&
 
-			git -C wt rev-parse 2 >expect &&
-			git -C wt rev-parse HEAD >actual &&
-			test_cmp expect actual
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse 2" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse HEAD" "$method" >expect &&
+				test_cmp expect actual
+			)
 		'
 	done # closes dir
+
+	test_expect_success "migrating repository to $to_format with alternate refs directory" '
+		test_when_finished "rm -rf repo refdir" &&
+		mkdir refdir &&
+		GIT_REFERENCE_BACKEND="${from_format}://$(pwd)/refdir" git init repo &&
+		(
+			cd repo &&
+
+			test_commit 1 &&
+			test_commit 2 &&
+			test_commit 3 &&
+
+			git refs migrate --ref-format=$to_format &&
+			git refs list >out &&
+			test_grep "refs/tags/1"	out &&
+			test_grep "refs/tags/2"	out &&
+			test_grep "refs/tags/3"	out
+		)
+	'
+
 done # closes to_format
 done # closes from_format
 
+done # closes method
+
+test_expect_success 'initializing repository with alt ref directory' '
+	test_when_finished "rm -rf repo refdir" &&
+	mkdir refdir &&
+	BACKEND="$(test_detect_ref_format)://$(pwd)/refdir" &&
+	GIT_REFERENCE_BACKEND=$BACKEND git init repo &&
+	(
+		cd repo &&
+
+		git config get extensions.refstorage >expect &&
+		echo $BACKEND >actual &&
+		test_cmp expect actual &&
+
+		test_commit 1 &&
+		test_commit 2 &&
+		test_commit 3 &&
+		git refs list >out &&
+		test_grep "refs/tags/1"	out &&
+		test_grep "refs/tags/2"	out &&
+		test_grep "refs/tags/3"	out
+	)
+'
+
+test_expect_success 'cloning repository with alt ref directory' '
+	test_when_finished "rm -rf source repo refdir" &&
+	mkdir refdir &&
+
+	git init source &&
+	test_commit -C source 1 &&
+	test_commit -C source 2 &&
+	test_commit -C source 3 &&
+
+	BACKEND="$(test_detect_ref_format)://$(pwd)/refdir" &&
+	GIT_REFERENCE_BACKEND=$BACKEND git clone source repo &&
+
+	git -C repo config get extensions.refstorage >expect &&
+	echo $BACKEND >actual &&
+	test_cmp expect actual &&
+
+	git -C source for-each-ref refs/tags/ >expect &&
+	git -C repo for-each-ref refs/tags/ >actual &&
+	test_cmp expect actual
+'
+
 test_done

builtin/clone.c, setup.c, setup.h

diff --git a/builtin/clone.c b/builtin/clone.c
index b40cee5968..cd43bb5aa2 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1442,7 +1442,7 @@ int cmd_clone(int argc,
 	hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 	initialize_repository_version(hash_algo, the_repository->ref_storage_format, 1);
 	repo_set_hash_algo(the_repository, hash_algo);
-	create_reference_database(the_repository->ref_storage_format, NULL, 1);
+	create_reference_database(NULL, 1);
 
 	/*
 	 * Before fetching from the remote, download and install bundle
diff --git a/setup.c b/setup.c
index b723f8b339..1fc9ae3872 100644
--- a/setup.c
+++ b/setup.c
@@ -2359,14 +2359,12 @@ static int is_reinit(void)
 	return ret;
 }
 
-void create_reference_database(enum ref_storage_format ref_storage_format,
-			       const char *initial_branch, int quiet)
+void create_reference_database(const char *initial_branch, int quiet)
 {
 	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int reinit = is_reinit();
 
-	repo_set_ref_storage_format(the_repository, ref_storage_format);
 	if (ref_store_create_on_disk(get_main_ref_store(the_repository), 0, &err))
 		die("failed to set up refs db: %s", err.buf);
 
@@ -2701,8 +2699,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 				      &repo_fmt, init_shared_repository);
 
 	if (!(flags & INIT_DB_SKIP_REFDB))
-		create_reference_database(repo_fmt.ref_storage_format,
-					  initial_branch, flags & INIT_DB_QUIET);
+		create_reference_database(initial_branch, flags & INIT_DB_QUIET);
 	create_object_directory();
 
 	if (repo_settings_get_shared_repository(the_repository)) {
diff --git a/setup.h b/setup.h
index d55dcc6608..ddb9f6701c 100644
--- a/setup.h
+++ b/setup.h
@@ -240,8 +240,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 void initialize_repository_version(int hash_algo,
 				   enum ref_storage_format ref_storage_format,
 				   int reinit);
-void create_reference_database(enum ref_storage_format ref_storage_format,
-			       const char *initial_branch, int quiet);
+void create_reference_database(const char *initial_branch, int quiet);
 
 /*
  * NOTE NOTE NOTE!!

builtin/clone.c, refs.c, refs.h, refs/reftable-backend.c

diff --git a/builtin/clone.c b/builtin/clone.c
index cd43bb5aa2..697c5bb5cb 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1225,12 +1225,7 @@ int cmd_clone(int argc,
 	initialize_repository_version(GIT_HASH_UNKNOWN,
 				      the_repository->ref_storage_format, 1);
 
-	strbuf_addf(&buf, "%s/HEAD", git_dir);
-	write_file(buf.buf, "ref: refs/heads/.invalid");
-
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "%s/refs", git_dir);
-	safe_create_dir(the_repository, buf.buf, 1);
+	refs_create_refdir_stubs(the_repository, git_dir, NULL);
 
 	/*
 	 * additional config can be injected with -c, make sure it's included
diff --git a/refs.c b/refs.c
index 627b7f8698..77b93d655b 100644
--- a/refs.c
+++ b/refs.c
@@ -2163,6 +2163,29 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 	return NULL;
 }
 
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_content)
+{
+	struct strbuf path = STRBUF_INIT;
+
+	strbuf_addf(&path, "%s/HEAD", refdir);
+	write_file(path.buf, "ref: refs/heads/.invalid");
+	adjust_shared_perm(repo, path.buf);
+
+	strbuf_reset(&path);
+	strbuf_addf(&path, "%s/refs", refdir);
+	safe_create_dir(repo, path.buf, 1);
+
+	if (refs_heads_content) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/refs/heads", refdir);
+		write_file(path.buf, "%s", refs_heads_content);
+		adjust_shared_perm(repo, path.buf);
+	}
+
+	strbuf_release(&path);
+}
+
 /* backend functions */
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
diff --git a/refs.h b/refs.h
index f0abfa1d93..9d8890fdff 100644
--- a/refs.h
+++ b/refs.h
@@ -1427,4 +1427,17 @@ void ref_iterator_free(struct ref_iterator *ref_iterator);
 int do_for_each_ref_iterator(struct ref_iterator *iter,
 			     each_ref_fn fn, void *cb_data);
 
+/*
+ * Git only recognizes a directory as a repository if it contains:
+ * - HEAD file
+ * - refs/ folder
+ * While it is necessary within the files backend, newer backends may not
+ * follow the same structure. To go around this, we create stubs as necessary.
+ *
+ * If provided with a 'refs_heads_msg', we create the 'refs/heads/head' file
+ * with the provided message.
+ */
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_msg);
+
 #endif /* REFS_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index fe74af73af..d8651fe779 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,18 +491,8 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	write_file(sb.buf, "ref: refs/heads/.invalid");
-	adjust_shared_perm(the_repository, sb.buf);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	safe_create_dir(the_repository, sb.buf, 1);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	write_file(sb.buf, "this repository uses the reftable format");
-	adjust_shared_perm(the_repository, sb.buf);
+	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
+				 "this repository uses the reftable format");
 
 	strbuf_release(&sb);
 	return 0;

refs.c, refs/files-backend.c, refs/reftable-backend.c

diff --git a/refs.c b/refs.c
index 77b93d655b..c83af63dc5 100644
--- a/refs.c
+++ b/refs.c
@@ -2189,12 +2189,55 @@ void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
 /* backend functions */
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
-	return refs->be->create_on_disk(refs, flags, err);
+	int ret = refs->be->create_on_disk(refs, flags, err);
+
+	if (!ret &&
+	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+		struct strbuf msg = STRBUF_INIT;
+
+		strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
+		refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
+		strbuf_release(&msg);
+	}
+
+	return ret;
 }
 
 int ref_store_remove_on_disk(struct ref_store *refs, struct strbuf *err)
 {
-	return refs->be->remove_on_disk(refs, err);
+	int ret = refs->be->remove_on_disk(refs, err);
+
+	if (!ret &&
+	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+		struct strbuf sb = STRBUF_INIT;
+
+		strbuf_addf(&sb, "%s/HEAD", refs->gitdir);
+		if (unlink(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete stub HEAD: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+		strbuf_reset(&sb);
+
+		strbuf_addf(&sb, "%s/refs/heads", refs->gitdir);
+		if (unlink(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete stub heads: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+		strbuf_reset(&sb);
+
+		strbuf_addf(&sb, "%s/refs", refs->gitdir);
+		if (rmdir(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete refs directory: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+
+		strbuf_release(&sb);
+	}
+
+	return ret;
 }
 
 int repo_resolve_gitlink_ref(struct repository *r,
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 240d3c3b26..d3f6423261 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -3700,7 +3700,11 @@ static int files_ref_store_remove_on_disk(struct ref_store *ref_store,
 	if (for_each_root_ref(refs, remove_one_root_ref, &data) < 0)
 		ret = -1;
 
-	if (ref_store_remove_on_disk(refs->packed_ref_store, err) < 0)
+	/*
+	 * Directly access the cleanup functions for packed-refs as the generic function
+	 * would try to clear stubs which isn't required for the files backend.
+	 */
+	if (refs->packed_ref_store->be->remove_on_disk(refs->packed_ref_store, err) < 0)
 		ret = -1;
 
 	strbuf_release(&sb);
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index d8651fe779..6ce7f9bb8e 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,9 +491,6 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
-				 "this repository uses the reftable format");
-
 	strbuf_release(&sb);
 	return 0;
 }
@@ -519,30 +516,6 @@ static int reftable_be_remove_on_disk(struct ref_store *ref_store,
 			    strerror(errno));
 		ret = -1;
 	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	if (unlink(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete stub HEAD: %s",
-			    strerror(errno));
-		ret = -1;
-	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	if (unlink(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete stub heads: %s",
-			    strerror(errno));
-		ret = -1;
-	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	if (rmdir(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete refs directory: %s",
-			    strerror(errno));
-		ret = -1;
-	}
 
 	strbuf_release(&sb);
 	return ret;

refs.c, refs/files-backend.c, refs/packed-backend.c, refs/packed-backend.h, refs/refs-internal.h, refs/reftable-backend.c

diff --git a/refs.c b/refs.c
index c83af63dc5..ba2573eb7a 100644
--- a/refs.c
+++ b/refs.c
@@ -5,6 +5,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "advice.h"
 #include "config.h"
 #include "environment.h"
@@ -2290,7 +2291,7 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, gitdir, flags);
+	refs = be->init(repo, NULL, gitdir, flags);
 	return refs;
 }
 
@@ -3468,3 +3469,40 @@ const char *ref_transaction_error_msg(enum ref_transaction_error err)
 		return "unknown failure";
 	}
 }
+
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	*is_worktree = get_common_dir_noenv(ref_common_dir, gitdir);
+
+	if (!payload) {
+		/*
+		 * We can use the 'gitdir' as the 'refdir' without appending the
+		 * worktree path, as the 'gitdir' here is already the worktree
+		 * path and is different from 'commondir' denoted by 'ref_common_dir'.
+		 */
+		strbuf_addstr(refdir, gitdir);
+		return;
+	}
+
+	if (!is_absolute_path(payload)) {
+		strbuf_addf(&sb, "%s/%s", ref_common_dir->buf, payload);
+		strbuf_realpath(ref_common_dir, sb.buf, 1);
+	} else {
+		strbuf_realpath(ref_common_dir, payload, 1);
+	}
+
+	strbuf_addbuf(refdir, ref_common_dir);
+
+	if (*is_worktree) {
+		const char *wt_id = strrchr(gitdir, '/');
+		if (!wt_id)
+			BUG("worktree path does not contain slash");
+		strbuf_addf(refdir, "/worktrees/%s", wt_id + 1);
+	}
+
+	strbuf_release(&sb);
+}
diff --git a/refs/files-backend.c b/refs/files-backend.c
index d3f6423261..9cde3ba724 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -106,19 +106,24 @@ static void clear_loose_ref_cache(struct files_ref_store *refs)
  * set of caches.
  */
 static struct ref_store *files_ref_store_init(struct repository *repo,
+					      const char *payload,
 					      const char *gitdir,
 					      unsigned int flags)
 {
 	struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
 	struct ref_store *ref_store = (struct ref_store *)refs;
-	struct strbuf sb = STRBUF_INIT;
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
+	bool is_worktree;
+
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
 
-	base_ref_store_init(ref_store, repo, gitdir, &refs_be_files);
+	base_ref_store_init(ref_store, repo, refdir.buf, &refs_be_files);
 	refs->store_flags = flags;
-	get_common_dir_noenv(&sb, gitdir);
-	refs->gitcommondir = strbuf_detach(&sb, NULL);
+	refs->gitcommondir = strbuf_detach(&ref_common_dir, NULL);
 	refs->packed_ref_store =
-		packed_ref_store_init(repo, refs->gitcommondir, flags);
+		packed_ref_store_init(repo, NULL, refs->gitcommondir, flags);
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
 	repo_config_get_bool(repo, "core.prefersymlinkrefs", &refs->prefer_symlink_refs);
 
@@ -126,6 +131,8 @@ static struct ref_store *files_ref_store_init(struct repository *repo,
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
 			      &refs->gitcommondir);
 
+	strbuf_release(&refdir);
+
 	return ref_store;
 }
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..e7bb9f10f9 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -211,7 +211,12 @@ static size_t snapshot_hexsz(const struct snapshot *snapshot)
 	return snapshot->refs->base.repo->hash_algo->hexsz;
 }
 
+/*
+ * Since packed-refs is only stored in the common dir, don't parse the
+ * payload and rely on the files-backend to set 'gitdir' correctly.
+ */
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload UNUSED,
 					const char *gitdir,
 					unsigned int store_flags)
 {
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
index 9481d5e7c2..2c2377a356 100644
--- a/refs/packed-backend.h
+++ b/refs/packed-backend.h
@@ -14,6 +14,7 @@ struct ref_transaction;
  */
 
 struct ref_store *packed_ref_store_init(struct repository *repo,
+					const char *payload,
 					const char *gitdir,
 					unsigned int store_flags);
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index c7d2a6e50b..4fb8fdb872 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -389,6 +389,7 @@ struct ref_store;
  * the ref_store and to record the ref_store for later lookup.
  */
 typedef struct ref_store *ref_store_init_fn(struct repository *repo,
+					    const char *payload,
 					    const char *gitdir,
 					    unsigned int flags);
 /*
@@ -666,4 +667,17 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
 					  unsigned int initial_transaction,
 					  struct strbuf *err);
 
+/*
+ * Given a gitdir and the reference storage payload provided, retrieve the
+ * 'refdir' and 'ref_common_dir'. The former is where references should be
+ * stored for the current worktree, the latter is the common reference
+ * directory if working with a linked worktree. If working with the main
+ * worktree, both values will be the same.
+ *
+ * This is used by backends that store references in the repository directly.
+ */
+void refs_compute_filesystem_location(const char *gitdir, const char *payload,
+				      bool *is_worktree, struct strbuf *refdir,
+				      struct strbuf *ref_common_dir);
+
 #endif /* REFS_REFS_INTERNAL_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 6ce7f9bb8e..0e220d6bb5 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -372,18 +372,24 @@ static int reftable_be_fsync(int fd)
 }
 
 static struct ref_store *reftable_be_init(struct repository *repo,
+					  const char *payload,
 					  const char *gitdir,
 					  unsigned int store_flags)
 {
 	struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct strbuf ref_common_dir = STRBUF_INIT;
+	struct strbuf refdir = STRBUF_INIT;
 	struct strbuf path = STRBUF_INIT;
-	int is_worktree;
+	bool is_worktree;
 	mode_t mask;
 
 	mask = umask(0);
 	umask(mask);
 
-	base_ref_store_init(&refs->base, repo, gitdir, &refs_be_reftable);
+	refs_compute_filesystem_location(gitdir, payload, &is_worktree, &refdir,
+					 &ref_common_dir);
+
+	base_ref_store_init(&refs->base, repo, refdir.buf, &refs_be_reftable);
 	strmap_init(&refs->worktree_backends);
 	refs->store_flags = store_flags;
 	refs->log_all_ref_updates = repo_settings_get_log_all_ref_updates(repo);
@@ -419,14 +425,11 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.
 	 * This stack contains both the shared and the main worktree refs.
-	 *
-	 * Note that we don't try to resolve the path in case we have a
-	 * worktree because `get_common_dir_noenv()` already does it for us.
 	 */
-	is_worktree = get_common_dir_noenv(&path, gitdir);
+	strbuf_addbuf(&path, &ref_common_dir);
 	if (!is_worktree) {
 		strbuf_reset(&path);
-		strbuf_realpath(&path, gitdir, 0);
+		strbuf_realpath(&path, ref_common_dir.buf, 0);
 	}
 	strbuf_addstr(&path, "/reftable");
 	refs->err = reftable_backend_init(&refs->main_backend, path.buf,
@@ -443,10 +446,9 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	 * do it efficiently.
 	 */
 	if (is_worktree) {
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/reftable", gitdir);
+		strbuf_addstr(&refdir, "/reftable");
 
-		refs->err = reftable_backend_init(&refs->worktree_backend, path.buf,
+		refs->err = reftable_backend_init(&refs->worktree_backend, refdir.buf,
 						  &refs->write_options);
 		if (refs->err)
 			goto done;
@@ -456,6 +458,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 
 done:
 	assert(refs->err != REFTABLE_API_ERROR);
+	strbuf_release(&ref_common_dir);
+	strbuf_release(&refdir);
 	strbuf_release(&path);
 	return &refs->base;
 }

Documentation/config/extensions.adoc, builtin/worktree.c, refs.c, repository.c, repository.h, setup.c, setup.h, t/meson.build, t/t1423-ref-backend.sh

diff --git a/Documentation/config/extensions.adoc b/Documentation/config/extensions.adoc
index 532456644b..3e51da36d3 100644
--- a/Documentation/config/extensions.adoc
+++ b/Documentation/config/extensions.adoc
@@ -57,10 +57,24 @@ For historical reasons, this extension is respected regardless of the
 `core.repositoryFormatVersion` setting.
 
 refStorage:::
-	Specify the ref storage format to use. The acceptable values are:
+	Specify the ref storage format and a corresponding payload. The value
+	can be either a format name or a URI:
 +
 --
+* A format name alone (e.g., `reftable` or `files`).
+
+* A URI format `<format>://<payload>` explicitly specifies both the
+  format and payload (e.g., `reftable:///foo/bar`).
+
+Supported format names are:
++
 include::../ref-storage-format.adoc[]
++
+The payload is passed directly to the reference backend. For the files and
+reftable backends, this must be a filesystem path where the references will
+be stored. Defaulting to the commondir when no payload is provided. Relative
+paths are resolved relative to the $GIT_DIR. Future backends may support
+other payload schemes, e.g., postgres://127.0.0.1:5432?database=myrepo.
 --
 +
 Note that this setting should only be set by linkgit:git-init[1] or
diff --git a/builtin/worktree.c b/builtin/worktree.c
index fbdaf2eb2e..293e808379 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -425,6 +425,39 @@ static int make_worktree_orphan(const char * ref, const struct add_opts *opts,
 	return run_command(&cp);
 }
 
+/*
+ * References for worktrees are generally stored in '$GIT_DIR/worktrees/<wt_id>'.
+ * But when using alternate reference directories, we want to store the worktree
+ * references in '$ALTERNATE_REFERENCE_DIR/worktrees/<wt_id>'.
+ *
+ * Create the necessary folder structure to facilitate the same. But to ensure
+ * that the former path is still considered a Git directory, add stubs.
+ */
+static void setup_alternate_ref_dir(struct worktree *wt, const char *wt_git_path)
+{
+	struct strbuf sb = STRBUF_INIT;
+	char *path;
+
+	path = wt->repo->ref_storage_payload;
+	if (!path)
+		return;
+
+	if (!is_absolute_path(path))
+		strbuf_addf(&sb, "%s/", wt->repo->commondir);
+
+	strbuf_addf(&sb, "%s/worktrees", path);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_addf(&sb, "/%s", wt->id);
+	safe_create_dir(wt->repo, sb.buf, 1);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "this worktree stores references in %s/worktrees/%s",
+		    path, wt->id);
+	refs_create_refdir_stubs(wt->repo, wt_git_path, sb.buf);
+
+	strbuf_release(&sb);
+}
+
 static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
@@ -518,6 +551,7 @@ static int add_worktree(const char *path, const char *refname,
 		ret = error(_("could not find created worktree '%s'"), name);
 		goto done;
 	}
+	setup_alternate_ref_dir(wt, sb_repo.buf);
 	wt_refs = get_worktree_ref_store(wt);
 
 	ret = ref_store_create_on_disk(wt_refs, REF_STORE_CREATE_ON_DISK_IS_WORKTREE, &sb);
diff --git a/refs.c b/refs.c
index ba2573eb7a..ef1902e85c 100644
--- a/refs.c
+++ b/refs.c
@@ -2291,7 +2291,11 @@ static struct ref_store *ref_store_init(struct repository *repo,
 	if (!be)
 		BUG("reference backend is unknown");
 
-	refs = be->init(repo, NULL, gitdir, flags);
+	/*
+	 * TODO Send in a 'struct worktree' instead of a 'gitdir', and
+	 * allow the backend to handle how it wants to deal with worktrees.
+	 */
+	refs = be->init(repo, repo->ref_storage_payload, gitdir, flags);
 	return refs;
 }
 
diff --git a/repository.c b/repository.c
index c7e75215ac..9815f081ef 100644
--- a/repository.c
+++ b/repository.c
@@ -193,9 +193,12 @@ void repo_set_compat_hash_algo(struct repository *repo, int algo)
 }
 
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format)
+				 enum ref_storage_format format,
+				 const char *payload)
 {
 	repo->ref_storage_format = format;
+	free(repo->ref_storage_payload);
+	repo->ref_storage_payload = xstrdup_or_null(payload);
 }
 
 /*
@@ -277,7 +280,8 @@ int repo_init(struct repository *repo,
 
 	repo_set_hash_algo(repo, format.hash_algo);
 	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
-	repo_set_ref_storage_format(repo, format.ref_storage_format);
+	repo_set_ref_storage_format(repo, format.ref_storage_format,
+				    format.ref_storage_payload);
 	repo->repository_format_worktree_config = format.worktree_config;
 	repo->repository_format_relative_worktrees = format.relative_worktrees;
 	repo->repository_format_precious_objects = format.precious_objects;
@@ -369,6 +373,7 @@ void repo_clear(struct repository *repo)
 	FREE_AND_NULL(repo->index_file);
 	FREE_AND_NULL(repo->worktree);
 	FREE_AND_NULL(repo->submodule_prefix);
+	FREE_AND_NULL(repo->ref_storage_payload);
 
 	odb_free(repo->objects);
 	repo->objects = NULL;
diff --git a/repository.h b/repository.h
index 6063c4b846..95e2333bad 100644
--- a/repository.h
+++ b/repository.h
@@ -150,6 +150,11 @@ struct repository {
 
 	/* Repository's reference storage format, as serialized on disk. */
 	enum ref_storage_format ref_storage_format;
+	/*
+	 * Reference storage information as needed for the backend. This contains
+	 * only the payload from the reference URI without the schema.
+	 */
+	char *ref_storage_payload;
 
 	/* A unique-id for tracing purposes. */
 	int trace2_repo_id;
@@ -204,7 +209,8 @@ void repo_set_worktree(struct repository *repo, const char *path);
 void repo_set_hash_algo(struct repository *repo, int algo);
 void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format);
+				 enum ref_storage_format format,
+				 const char *payload);
 void initialize_repository(struct repository *repo);
 RESULT_MUST_BE_USED
 int repo_init(struct repository *r, const char *gitdir, const char *worktree);
diff --git a/setup.c b/setup.c
index 1fc9ae3872..d407f3347b 100644
--- a/setup.c
+++ b/setup.c
@@ -632,6 +632,21 @@ static enum extension_result handle_extension_v0(const char *var,
 		return EXTENSION_UNKNOWN;
 }
 
+static void parse_reference_uri(const char *value, char **format,
+				char **payload)
+{
+	const char *schema_end;
+
+	schema_end = strstr(value, "://");
+	if (!schema_end) {
+		*format = xstrdup(value);
+		*payload = NULL;
+	} else {
+		*format = xstrndup(value, schema_end - value);
+		*payload = xstrdup_or_null(schema_end + 3);
+	}
+}
+
 /*
  * Record any new extensions in this function.
  */
@@ -674,10 +689,17 @@ static enum extension_result handle_extension(const char *var,
 		return EXTENSION_OK;
 	} else if (!strcmp(ext, "refstorage")) {
 		unsigned int format;
+		char *format_str;
 
 		if (!value)
 			return config_error_nonbool(var);
-		format = ref_storage_format_by_name(value);
+
+		parse_reference_uri(value, &format_str,
+				    &data->ref_storage_payload);
+
+		format = ref_storage_format_by_name(format_str);
+		free(format_str);
+
 		if (format == REF_STORAGE_FORMAT_UNKNOWN)
 			return error(_("invalid value for '%s': '%s'"),
 				     "extensions.refstorage", value);
@@ -850,6 +872,7 @@ void clear_repository_format(struct repository_format *format)
 	string_list_clear(&format->v1_only_extensions, 0);
 	free(format->work_tree);
 	free(format->partial_clone);
+	free(format->ref_storage_payload);
 	init_repository_format(format);
 }
 
@@ -1942,7 +1965,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			repo_set_compat_hash_algo(the_repository,
 						  repo_fmt.compat_hash_algo);
 			repo_set_ref_storage_format(the_repository,
-						    repo_fmt.ref_storage_format);
+						    repo_fmt.ref_storage_format,
+						    repo_fmt.ref_storage_payload);
 			the_repository->repository_format_worktree_config =
 				repo_fmt.worktree_config;
 			the_repository->repository_format_relative_worktrees =
@@ -2042,7 +2066,8 @@ void check_repository_format(struct repository_format *fmt)
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
 	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	repo_set_ref_storage_format(the_repository,
-				    fmt->ref_storage_format);
+				    fmt->ref_storage_format,
+				    fmt->ref_storage_payload);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_relative_worktrees =
@@ -2643,7 +2668,8 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
-	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format);
+	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
+				    repo_fmt->ref_storage_payload);
 }
 
 int init_db(const char *git_dir, const char *real_git_dir,
diff --git a/setup.h b/setup.h
index ddb9f6701c..093af39e84 100644
--- a/setup.h
+++ b/setup.h
@@ -171,6 +171,7 @@ struct repository_format {
 	int hash_algo;
 	int compat_hash_algo;
 	enum ref_storage_format ref_storage_format;
+	char *ref_storage_payload;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;
diff --git a/t/meson.build b/t/meson.build
index 459c52a489..11fc5a49ee 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -210,6 +210,7 @@ integration_tests = [
   't1420-lost-found.sh',
   't1421-reflog-write.sh',
   't1422-show-ref-exists.sh',
+  't1423-ref-backend.sh',
   't1430-bad-ref-name.sh',
   't1450-fsck.sh',
   't1451-fsck-buffer.sh',
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
new file mode 100755
index 0000000000..9912433b8c
--- /dev/null
+++ b/t/t1423-ref-backend.sh
@@ -0,0 +1,159 @@
+#!/bin/sh
+
+test_description='Test reference backend URIs'
+
+. ./test-lib.sh
+
+# Run a git command with the provided reference storage. Reset the backend
+# post running the command.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <cmd> is the git subcommand to be run in the repository.
+run_with_uri() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	cmd=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1
+	git -C "$repo" config set extensions.refStorage "$uri" &&
+	git -C "$repo" $cmd &&
+	git -C "$repo" config set extensions.refStorage "$backend"
+}
+
+# Test a repository with a given reference storage by running and comparing
+# 'git refs list' before and after setting the new reference backend. If
+# err_msg is set, expect the command to fail and grep for the provided err_msg.
+# Usage: run_with_uri <repo> <backend> <uri> <cmd>
+#   <repo> is the relative path to the repo to run the command in.
+#   <backend> is the original ref storage of the repo.
+#   <uri> is the new URI to be set for the ref storage.
+#   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
+test_refs_backend() {
+	repo=$1 &&
+	backend=$2 &&
+	uri=$3 &&
+	err_msg=$4 &&
+
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test -n "$err_msg";
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		test_must_fail git -C "$repo" refs list 2>err &&
+		test_grep "$err_msg" err
+	else
+		git -C "$repo" refs list >expect &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		test_cmp expect actual
+	fi
+}
+
+test_expect_success 'URI is invalid' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable@/home/reftable" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'URI ends with colon' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "reftable:" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+test_expect_success 'unknown reference backend' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_refs_backend repo files "db://.git" \
+		"invalid value for ${SQ}extensions.refstorage${SQ}"
+'
+
+ref_formats="files reftable"
+for from_format in $ref_formats
+do
+
+for to_format in $ref_formats
+do
+	if test "$from_format" = "$to_format"
+	then
+		continue
+	fi
+
+
+	for dir in "$(pwd)/repo/.git" "."
+	do
+
+		test_expect_success "read from $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method"
+			)
+		'
+
+		test_expect_success "write to $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+
+				git refs list >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				git refs list >actual &&
+				test_cmp expect actual &&
+
+				git refs list | grep -v "refs/tags/1" >expect &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				test_cmp expect actual
+			)
+		'
+
+		test_expect_success "with worktree and $to_format backend, $dir dir" '
+			test_when_finished "rm -rf repo wt" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit 1 &&
+				test_commit 2 &&
+				test_commit 3 &&
+
+				git refs migrate --dry-run --ref-format=$to_format >out &&
+				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
+
+				git config set core.repositoryformatversion 1 &&
+				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+
+				git worktree add ../wt 2
+			) &&
+
+			git -C repo for-each-ref --include-root-refs >expect &&
+			git -C wt for-each-ref --include-root-refs >expect &&
+			! test_cmp expect actual &&
+
+			git -C wt rev-parse 2 >expect &&
+			git -C wt rev-parse HEAD >actual &&
+			test_cmp expect actual
+		'
+	done # closes dir
+done # closes to_format
+done # closes from_format
+
+test_done

diff --git a/Documentation/config/extensions.adoc
b/Documentation/config/extensions.adoc
index 3e51da36d3..329d02b3c4 100644
--- a/Documentation/config/extensions.adoc
+++ b/Documentation/config/extensions.adoc
@@ -67,13 +67,13 @@ refStorage:::
   format and payload (e.g., `reftable:///foo/bar`).

 Supported format names are:
-+
+
 include::../ref-storage-format.adoc[]
-+
+
 The payload is passed directly to the reference backend. For the files and
 reftable backends, this must be a filesystem path where the references will
 be stored. Defaulting to the commondir when no payload is provided. Relative
-paths are resolved relative to the $GIT_DIR. Future backends may support
+paths are resolved relative to the `$GIT_DIR`. Future backends may support
 other payload schemes, e.g., postgres://127.0.0.1:5432?database=myrepo.
 --
 +


>> ++
>> +The payload is passed directly to the reference backend. For the files
>>[snip]

Documentation/git.adoc, environment.h, refs.c, setup.c, t/t1423-ref-backend.sh

diff --git a/Documentation/git.adoc b/Documentation/git.adoc
index ce099e78b8..66442735ea 100644
--- a/Documentation/git.adoc
+++ b/Documentation/git.adoc
@@ -584,6 +584,11 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REFERENCE_BACKEND`::
+    Specify which reference backend to be used along with its URI.
+    See `extensions.refStorage` option in linkgit:git-config[1] for more
+    details. Overrides the config variable when used.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::
diff --git a/environment.h b/environment.h
index 27f657af04..540e0a7f6d 100644
--- a/environment.h
+++ b/environment.h
@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REFERENCE_BACKEND_ENVIRONMENT "GIT_REFERENCE_BACKEND"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the
diff --git a/refs.c b/refs.c
index ef1902e85c..a700a66f08 100644
--- a/refs.c
+++ b/refs.c
@@ -2192,13 +2192,17 @@ int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *e
 {
 	int ret = refs->be->create_on_disk(refs, flags, err);
 
-	if (!ret &&
-	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
-		struct strbuf msg = STRBUF_INIT;
-
-		strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
-		refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
-		strbuf_release(&msg);
+	if (!ret) {
+		/* Creation of stubs for linked worktrees are handled in the worktree code. */
+		if (!(flags & REF_STORE_CREATE_ON_DISK_IS_WORKTREE) && refs->repo->ref_storage_payload) {
+			refs_create_refdir_stubs(refs->repo, refs->repo->gitdir,
+						 "repository uses alternate refs storage");
+		} else if (ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+			struct strbuf msg = STRBUF_INIT;
+			strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
+			refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
+			strbuf_release(&msg);
+		}
 	}
 
 	return ret;
@@ -2208,10 +2212,18 @@ int ref_store_remove_on_disk(struct ref_store *refs, struct strbuf *err)
 {
 	int ret = refs->be->remove_on_disk(refs, err);
 
-	if (!ret &&
-	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+	if (!ret) {
+		enum ref_storage_format format = ref_storage_format_by_name(refs->be->name);
 		struct strbuf sb = STRBUF_INIT;
 
+		/* Backends apart from the files backend create stubs. */
+		if (format == REF_STORAGE_FORMAT_FILES)
+			return ret;
+
+		/* Alternate refs backend require stubs in the gitdir. */
+		if (refs->repo->ref_storage_payload)
+			return ret;
+
 		strbuf_addf(&sb, "%s/HEAD", refs->gitdir);
 		if (unlink(sb.buf) < 0) {
 			strbuf_addf(err, "could not delete stub HEAD: %s",
diff --git a/setup.c b/setup.c
index d407f3347b..90cb9be578 100644
--- a/setup.c
+++ b/setup.c
@@ -1838,6 +1838,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	static struct strbuf cwd = STRBUF_INIT;
 	struct strbuf dir = STRBUF_INIT, gitdir = STRBUF_INIT, report = STRBUF_INIT;
 	const char *prefix = NULL;
+	const char *ref_backend_uri;
 	struct repository_format repo_fmt = REPOSITORY_FORMAT_INIT;
 
 	/*
@@ -1995,6 +1996,25 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	/*
+	 * The env variable should override the repository config
+	 * for 'extensions.refStorage'.
+	 */
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *payload;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &payload);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+		repo_set_ref_storage_format(the_repository, format, payload);
+
+		free(backend);
+		free(payload);
+	}
+
 	setup_original_cwd();
 
 	strbuf_release(&dir);
@@ -2337,7 +2357,8 @@ void initialize_repository_version(int hash_algo,
 	 * the remote repository's format.
 	 */
 	if (hash_algo != GIT_HASH_SHA1_LEGACY ||
-	    ref_storage_format != REF_STORAGE_FORMAT_FILES)
+	    ref_storage_format != REF_STORAGE_FORMAT_FILES ||
+	    the_repository->ref_storage_payload)
 		target_version = GIT_REPO_VERSION_READ;
 
 	if (hash_algo != GIT_HASH_SHA1_LEGACY && hash_algo != GIT_HASH_UNKNOWN)
@@ -2346,11 +2367,20 @@ void initialize_repository_version(int hash_algo,
 	else if (reinit)
 		repo_config_set_gently(the_repository, "extensions.objectformat", NULL);
 
-	if (ref_storage_format != REF_STORAGE_FORMAT_FILES)
+	if (the_repository->ref_storage_payload) {
+		struct strbuf ref_uri = STRBUF_INIT;
+
+		strbuf_addf(&ref_uri, "%s://%s",
+			    ref_storage_format_to_name(ref_storage_format),
+			    the_repository->ref_storage_payload);
+		repo_config_set(the_repository, "extensions.refstorage", ref_uri.buf);
+		strbuf_release(&ref_uri);
+	} else if (ref_storage_format != REF_STORAGE_FORMAT_FILES) {
 		repo_config_set(the_repository, "extensions.refstorage",
 				ref_storage_format_to_name(ref_storage_format));
-	else if (reinit)
+	} else if (reinit) {
 		repo_config_set_gently(the_repository, "extensions.refstorage", NULL);
+	}
 
 	if (reinit) {
 		struct strbuf config = STRBUF_INIT;
@@ -2623,6 +2653,7 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 		.ignore_repo = 1,
 		.ignore_worktree = 1,
 	};
+	const char *ref_backend_uri;
 	const char *env;
 
 	config_with_options(read_default_format_config, &cfg, NULL, NULL, &opts);
@@ -2668,6 +2699,24 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
+
+
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *payload;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &payload);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+
+		repo_fmt->ref_storage_format = format;
+		repo_fmt->ref_storage_payload = payload;
+
+		free(backend);
+	}
+
 	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
 				    repo_fmt->ref_storage_payload);
 }
diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
index 9912433b8c..1a7b8eadba 100755
--- a/t/t1423-ref-backend.sh
+++ b/t/t1423-ref-backend.sh
@@ -11,16 +11,25 @@ test_description='Test reference backend URIs'
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
 #   <cmd> is the git subcommand to be run in the repository.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 run_with_uri() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
 	cmd=$4 &&
+	via=$5 &&
 
-	git -C "$repo" config set core.repositoryformatversion 1
-	git -C "$repo" config set extensions.refStorage "$uri" &&
-	git -C "$repo" $cmd &&
-	git -C "$repo" config set extensions.refStorage "$backend"
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test "$via" = "env"
+	then
+		test_env GIT_REFERENCE_BACKEND="$uri" git -C "$repo" $cmd
+	elif test "$via" = "config"
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		git -C "$repo" $cmd &&
+		git -C "$repo" config set extensions.refStorage "$backend"
+	fi
 }
 
 # Test a repository with a given reference storage by running and comparing
@@ -30,44 +39,84 @@ run_with_uri() {
 #   <repo> is the relative path to the repo to run the command in.
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 #   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
 test_refs_backend() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
-	err_msg=$4 &&
+	via=$4 &&
+	err_msg=$5 &&
+
 
-	git -C "$repo" config set core.repositoryformatversion 1 &&
 	if test -n "$err_msg";
 	then
-		git -C "$repo" config set extensions.refStorage "$uri" &&
-		test_must_fail git -C "$repo" refs list 2>err &&
-		test_grep "$err_msg" err
+		if test "$via" = "env"
+		then
+			test_env GIT_REFERENCE_BACKEND="$uri" test_must_fail git -C "$repo" refs list 2>err
+		elif test "$via" = "config"
+		then
+			git -C "$repo" config set extensions.refStorage "$uri" &&
+			test_must_fail git -C "$repo" refs list 2>err &&
+			test_grep "$err_msg" err
+		fi
 	else
 		git -C "$repo" refs list >expect &&
-		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" "$via">actual &&
 		test_cmp expect actual
 	fi
 }
 
-test_expect_success 'URI is invalid' '
+# Verify that the expected files are present in the gitdir and the refsdir.
+# Usage: verify_files_exist <gitdir> <refdir>
+#   <gitdir> is the path for the gitdir.
+#   <refdir> is the path for the refdir.
+verify_files_exist() {
+	gitdir=$1 &&
+	refdir=$2 &&
+
+	# verify that the stubs were added to the $GITDIR.
+	echo "repository uses alternate refs storage" >expect &&
+	test_cmp expect $gitdir/refs/heads &&
+	echo "ref: refs/heads/.invalid" >expect &&
+	test_cmp expect $gitdir/HEAD
+
+	# verify that backend specific files exist.
+	case "$GIT_DEFAULT_REF_FORMAT" in
+	files)
+		test_path_is_dir $refdir/refs/heads &&
+		test_path_is_file $refdir/HEAD;;
+	reftable)
+		test_path_is_dir $refdir/reftable &&
+		test_path_is_file $refdir/reftable/tables.list;;
+	*)
+		BUG "unhandled ref format $GIT_DEFAULT_REF_FORMAT";;
+	esac
+}
+
+methods="config env"
+for method in $methods
+do
+
+test_expect_success "$method: URI is invalid" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable@/home/reftable" \
+	test_refs_backend repo files "reftable@/home/reftable" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'URI ends with colon' '
+test_expect_success "$method: URI ends with colon" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable:" \
+	test_refs_backend repo files "reftable:" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'unknown reference backend' '
+test_expect_success "$method: unknown reference backend" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "db://.git" \
+	test_refs_backend repo files "db://.git" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
@@ -86,7 +135,7 @@ do
 	for dir in "$(pwd)/repo/.git" "."
 	do
 
-		test_expect_success "read from $to_format backend, $dir dir" '
+		test_expect_success "$method: read from $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -101,7 +150,7 @@ do
 			)
 		'
 
-		test_expect_success "write to $to_format backend, $dir dir" '
+		test_expect_success "$method: write to $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -113,20 +162,22 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method" &&
 
 				git refs list >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"tag -d 1" "$method" &&
 				git refs list >actual &&
 				test_cmp expect actual &&
 
 				git refs list | grep -v "refs/tags/1" >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"refs list" "$method" >actual &&
 				test_cmp expect actual
 			)
 		'
 
-		test_expect_success "with worktree and $to_format backend, $dir dir" '
+		test_expect_success "$method: with worktree and $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo wt" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -138,22 +189,92 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				git config set core.repositoryformatversion 1 &&
-				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
-
-				git worktree add ../wt 2
-			) &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"worktree add ../wt 2" "$method" &&
 
-			git -C repo for-each-ref --include-root-refs >expect &&
-			git -C wt for-each-ref --include-root-refs >expect &&
-			! test_cmp expect actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >expect &&
+				! test_cmp expect actual &&
 
-			git -C wt rev-parse 2 >expect &&
-			git -C wt rev-parse HEAD >actual &&
-			test_cmp expect actual
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse 2" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse HEAD" "$method" >expect &&
+				test_cmp expect actual
+			)
 		'
 	done # closes dir
+
+	test_expect_success "migrating repository to $to_format with alternate refs directory" '
+		test_when_finished "rm -rf repo refdir" &&
+		mkdir refdir &&
+		GIT_REFERENCE_BACKEND="${from_format}://$(pwd)/refdir" git init repo &&
+		(
+			cd repo &&
+
+			test_commit 1 &&
+			test_commit 2 &&
+			test_commit 3 &&
+
+			git refs migrate --ref-format=$to_format &&
+			git refs list >out &&
+			test_grep "refs/tags/1"	out &&
+			test_grep "refs/tags/2"	out &&
+			test_grep "refs/tags/3"	out
+		)
+	'
+
 done # closes to_format
 done # closes from_format
 
+done # closes method
+
+test_expect_success 'initializing repository with alt ref directory' '
+	test_when_finished "rm -rf repo refdir" &&
+	mkdir refdir &&
+	BACKEND="$(test_detect_ref_format)://$(pwd)/refdir" &&
+	GIT_REFERENCE_BACKEND=$BACKEND git init repo &&
+	verify_files_exist repo/.git refdir &&
+	(
+		cd repo &&
+
+		git config get extensions.refstorage >expect &&
+		echo $BACKEND >actual &&
+		test_cmp expect actual &&
+
+		test_commit 1 &&
+		test_commit 2 &&
+		test_commit 3 &&
+		git refs list >out &&
+		test_grep "refs/tags/1"	out &&
+		test_grep "refs/tags/2"	out &&
+		test_grep "refs/tags/3"	out
+	)
+'
+
+test_expect_success 'cloning repository with alt ref directory' '
+	test_when_finished "rm -rf source repo refdir" &&
+	mkdir refdir &&
+
+	git init source &&
+	test_commit -C source 1 &&
+	test_commit -C source 2 &&
+	test_commit -C source 3 &&
+
+	BACKEND="$(test_detect_ref_format)://$(pwd)/refdir" &&
+	GIT_REFERENCE_BACKEND=$BACKEND git clone source repo &&
+
+	git -C repo config get extensions.refstorage >expect &&
+	echo $BACKEND >actual &&
+	test_cmp expect actual &&
+
+	verify_files_exist repo/.git refdir &&
+
+	git -C source for-each-ref refs/tags/ >expect &&
+	git -C repo for-each-ref refs/tags/ >actual &&
+	test_cmp expect actual
+'
+
 test_done

builtin/clone.c, setup.c, setup.h

diff --git a/builtin/clone.c b/builtin/clone.c
index b40cee5968..cd43bb5aa2 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1442,7 +1442,7 @@ int cmd_clone(int argc,
 	hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 	initialize_repository_version(hash_algo, the_repository->ref_storage_format, 1);
 	repo_set_hash_algo(the_repository, hash_algo);
-	create_reference_database(the_repository->ref_storage_format, NULL, 1);
+	create_reference_database(NULL, 1);
 
 	/*
 	 * Before fetching from the remote, download and install bundle
diff --git a/setup.c b/setup.c
index b723f8b339..1fc9ae3872 100644
--- a/setup.c
+++ b/setup.c
@@ -2359,14 +2359,12 @@ static int is_reinit(void)
 	return ret;
 }
 
-void create_reference_database(enum ref_storage_format ref_storage_format,
-			       const char *initial_branch, int quiet)
+void create_reference_database(const char *initial_branch, int quiet)
 {
 	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int reinit = is_reinit();
 
-	repo_set_ref_storage_format(the_repository, ref_storage_format);
 	if (ref_store_create_on_disk(get_main_ref_store(the_repository), 0, &err))
 		die("failed to set up refs db: %s", err.buf);
 
@@ -2701,8 +2699,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 				      &repo_fmt, init_shared_repository);
 
 	if (!(flags & INIT_DB_SKIP_REFDB))
-		create_reference_database(repo_fmt.ref_storage_format,
-					  initial_branch, flags & INIT_DB_QUIET);
+		create_reference_database(initial_branch, flags & INIT_DB_QUIET);
 	create_object_directory();
 
 	if (repo_settings_get_shared_repository(the_repository)) {
diff --git a/setup.h b/setup.h
index d55dcc6608..ddb9f6701c 100644
--- a/setup.h
+++ b/setup.h
@@ -240,8 +240,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 void initialize_repository_version(int hash_algo,
 				   enum ref_storage_format ref_storage_format,
 				   int reinit);
-void create_reference_database(enum ref_storage_format ref_storage_format,
-			       const char *initial_branch, int quiet);
+void create_reference_database(const char *initial_branch, int quiet);
 
 /*
  * NOTE NOTE NOTE!!

builtin/clone.c, refs.c, refs.h, refs/reftable-backend.c

diff --git a/builtin/clone.c b/builtin/clone.c
index cd43bb5aa2..697c5bb5cb 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -1225,12 +1225,7 @@ int cmd_clone(int argc,
 	initialize_repository_version(GIT_HASH_UNKNOWN,
 				      the_repository->ref_storage_format, 1);
 
-	strbuf_addf(&buf, "%s/HEAD", git_dir);
-	write_file(buf.buf, "ref: refs/heads/.invalid");
-
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "%s/refs", git_dir);
-	safe_create_dir(the_repository, buf.buf, 1);
+	refs_create_refdir_stubs(the_repository, git_dir, NULL);
 
 	/*
 	 * additional config can be injected with -c, make sure it's included
diff --git a/refs.c b/refs.c
index 627b7f8698..77b93d655b 100644
--- a/refs.c
+++ b/refs.c
@@ -2163,6 +2163,29 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 	return NULL;
 }
 
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_content)
+{
+	struct strbuf path = STRBUF_INIT;
+
+	strbuf_addf(&path, "%s/HEAD", refdir);
+	write_file(path.buf, "ref: refs/heads/.invalid");
+	adjust_shared_perm(repo, path.buf);
+
+	strbuf_reset(&path);
+	strbuf_addf(&path, "%s/refs", refdir);
+	safe_create_dir(repo, path.buf, 1);
+
+	if (refs_heads_content) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/refs/heads", refdir);
+		write_file(path.buf, "%s", refs_heads_content);
+		adjust_shared_perm(repo, path.buf);
+	}
+
+	strbuf_release(&path);
+}
+
 /* backend functions */
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
diff --git a/refs.h b/refs.h
index f0abfa1d93..a35fdc6642 100644
--- a/refs.h
+++ b/refs.h
@@ -1427,4 +1427,17 @@ void ref_iterator_free(struct ref_iterator *ref_iterator);
 int do_for_each_ref_iterator(struct ref_iterator *iter,
 			     each_ref_fn fn, void *cb_data);
 
+/*
+ * Git only recognizes a directory as a repository if it contains:
+ * - HEAD file
+ * - refs/ folder
+ * While it is necessary within the files backend, newer backends may not
+ * follow the same structure. To go around this, we create stubs as necessary.
+ *
+ * If provided with a 'refs_heads_content', we create the 'refs/heads/head' file
+ * with the provided message.
+ */
+void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
+			      const char *refs_heads_content);
+
 #endif /* REFS_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index fe74af73af..d8651fe779 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,18 +491,8 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	write_file(sb.buf, "ref: refs/heads/.invalid");
-	adjust_shared_perm(the_repository, sb.buf);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	safe_create_dir(the_repository, sb.buf, 1);
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	write_file(sb.buf, "this repository uses the reftable format");
-	adjust_shared_perm(the_repository, sb.buf);
+	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
+				 "this repository uses the reftable format");
 
 	strbuf_release(&sb);
 	return 0;

refs.c, refs/files-backend.c, refs/reftable-backend.c

diff --git a/refs.c b/refs.c
index 77b93d655b..c83af63dc5 100644
--- a/refs.c
+++ b/refs.c
@@ -2189,12 +2189,55 @@ void refs_create_refdir_stubs(struct repository *repo, const char *refdir,
 /* backend functions */
 int ref_store_create_on_disk(struct ref_store *refs, int flags, struct strbuf *err)
 {
-	return refs->be->create_on_disk(refs, flags, err);
+	int ret = refs->be->create_on_disk(refs, flags, err);
+
+	if (!ret &&
+	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+		struct strbuf msg = STRBUF_INIT;
+
+		strbuf_addf(&msg, "this repository uses the %s format", refs->be->name);
+		refs_create_refdir_stubs(refs->repo, refs->gitdir, msg.buf);
+		strbuf_release(&msg);
+	}
+
+	return ret;
 }
 
 int ref_store_remove_on_disk(struct ref_store *refs, struct strbuf *err)
 {
-	return refs->be->remove_on_disk(refs, err);
+	int ret = refs->be->remove_on_disk(refs, err);
+
+	if (!ret &&
+	    ref_storage_format_by_name(refs->be->name) != REF_STORAGE_FORMAT_FILES) {
+		struct strbuf sb = STRBUF_INIT;
+
+		strbuf_addf(&sb, "%s/HEAD", refs->gitdir);
+		if (unlink(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete stub HEAD: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+		strbuf_reset(&sb);
+
+		strbuf_addf(&sb, "%s/refs/heads", refs->gitdir);
+		if (unlink(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete stub heads: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+		strbuf_reset(&sb);
+
+		strbuf_addf(&sb, "%s/refs", refs->gitdir);
+		if (rmdir(sb.buf) < 0) {
+			strbuf_addf(err, "could not delete refs directory: %s",
+				    strerror(errno));
+			ret = -1;
+		}
+
+		strbuf_release(&sb);
+	}
+
+	return ret;
 }
 
 int repo_resolve_gitlink_ref(struct repository *r,
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 240d3c3b26..d3f6423261 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -3700,7 +3700,11 @@ static int files_ref_store_remove_on_disk(struct ref_store *ref_store,
 	if (for_each_root_ref(refs, remove_one_root_ref, &data) < 0)
 		ret = -1;
 
-	if (ref_store_remove_on_disk(refs->packed_ref_store, err) < 0)
+	/*
+	 * Directly access the cleanup functions for packed-refs as the generic function
+	 * would try to clear stubs which isn't required for the files backend.
+	 */
+	if (refs->packed_ref_store->be->remove_on_disk(refs->packed_ref_store, err) < 0)
 		ret = -1;
 
 	strbuf_release(&sb);
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index d8651fe779..6ce7f9bb8e 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -491,9 +491,6 @@ static int reftable_be_create_on_disk(struct ref_store *ref_store,
 	safe_create_dir(the_repository, sb.buf, 1);
 	strbuf_reset(&sb);
 
-	refs_create_refdir_stubs(the_repository, refs->base.gitdir,
-				 "this repository uses the reftable format");
-
 	strbuf_release(&sb);
 	return 0;
 }
@@ -519,30 +516,6 @@ static int reftable_be_remove_on_disk(struct ref_store *ref_store,
 			    strerror(errno));
 		ret = -1;
 	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
-	if (unlink(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete stub HEAD: %s",
-			    strerror(errno));
-		ret = -1;
-	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
-	if (unlink(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete stub heads: %s",
-			    strerror(errno));
-		ret = -1;
-	}
-	strbuf_reset(&sb);
-
-	strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
-	if (rmdir(sb.buf) < 0) {
-		strbuf_addf(err, "could not delete refs directory: %s",
-			    strerror(errno));
-		ret = -1;
-	}
 
 	strbuf_release(&sb);
 	return ret;