# Chatgpt 5.6 re-design of GIT 1.0 for more performance (pack file/protocol issue) (also a small hint towards "smart virtual git" idea) 1 messages from 2026-09-22 to 2026-09-22. Participants: Skybuck Flying. Thread: https://gitlist.dev/t/66364 ## Skybuck Flying, 2026-09-22 04:09 Subject: Chatgpt 5.6 re-design of GIT 1.0 for more performance (pack file/protocol issue) (also a small hint towards "smart virtual git" idea) Message-ID: ``` I read this article: https://cursor.com/blog/git-at-any-scale It points to the first version of git: https://github.com/git/git/commit/e83c5163316#diff-505e8cbcfb86ace34be6631e963814dc15014c8d5df255d323c155d11cf8c2e2 The cursor article mentions performance problems with GIT pack files/protocol and how the came out with solutions to try and make git more high performant. But instead of inventing all kinds of new/akward/work around solutions, how about tackling GIT as it's core. So I fed the initial version/idea of git/source code into Chatgpt 5.6 web/free chat, probably luna model and here is what it came up with: I post this here just for inspiration and a starting point, but running this multiple times through multiple AIs it can be re-fined/looked at, improved and ultimately developed into a new specification. (I might also build an AI agent specially/specifically for design/building/writing/develops specifications... this work is still in progess) I also have another idea I might like to work on: VirtualGit, it would present every commit of git to a windows explorer like interface/file system driver/whatever so that each commit looks like a checkout to the windows explorer, without actually having to check it out... so that's why I like to call to "virtual" git... It's simply computed... this might be possible with/for systems with lots of memory and compute/cpu... though for 1 gigabyte repositories and up I can imagine that it would quickly run out of memory, however, by sharing objects wisely, keeping it compressed in memory it might still function pretty well, so it would have to be "smart"... so perhaps a better name for it would be: "smart virtual git" :) At least for super large repos, it could at least show some checkouts/last few commits ;) Anyway, here is ChatGPT re-design ideas for GIT to solve the packfile/protocol performance problem: Yes. After looking at the original Git commit, the current protocol-v2 design, `upload-pack`/`fetch-pack`, pack heuristics, bitmaps, MIDX, partial clone, and the current pack format, I think the best redesign is **not “make Git negotiation a little smarter.”** It is to separate Git's excellent object model from the transport/storage decisions that accumulated around packfiles. The key idea would be: > **Keep Git's content-addressed DAG. Replace negotiated, request-specific pack streams with immutable, indexed, independently transferable object segments.** The original 2005 design already had the right fundamental abstraction: objects are content-addressed, and objects reference other objects by hash. ([GitHub][1]) Modern Git's performance problems largely come from optimizations layered on top of that abstraction rather than from the DAG itself. ## 1. What I would keep I would retain almost all of Git's semantic model: ˋˋˋtext blob tree commit tag object-id = Hash(canonical-object) ˋˋˋ Commits remain immutable DAG nodes. Trees remain immutable snapshots. Refs remain mutable names pointing into the DAG. You could continue using SHA-1 repositories and SHA-256 repositories exactly as today, although new repositories should obviously use SHA-256. So: ˋˋˋtext Git object semantics KEEP commit/tree/blob model KEEP content addressing KEEP distributed operation KEEP branch/ref semantics KEEP ˋˋˋ What I would replace is: ˋˋˋtext fetch negotiation REPLACE pack stream REPLACE pack indexing REPLACE delta representation REDESIGN clone/fetch protocol REDESIGN server-side pack building MOSTLY ELIMINATE ˋˋˋ --- # 2. The fundamental problem with current Git Modern Git fetch conceptually looks like this: ˋˋˋtext CLIENT SERVER capabilities <---------------------------------------- ls-refs ----------------------------------------> refs <---------------------------------------- want A have X have Y have Z ----------------------------------------> ACK Y <---------------------------------------- more have... ----------------------------------------> ACK ... <---------------------------------------- done ----------------------------------------> enumerate objects construct pack select/reuse deltas compress pack PACK stream <======================================== ˋˋˋ Protocol v2 improved many aspects of the original protocol, but commands are still sequential: only one command can be requested at a time, and the server waits for the complete request before responding. ([Git][2]) The fetch protocol explicitly has `want`, `have`, acknowledgements, `ready`, and `done` machinery for determining the common history. ([GitHub][3]) Git even provides: ˋˋˋtext fetch.negotiationAlgorithm=consecutive fetch.negotiationAlgorithm=skipping fetch.negotiationAlgorithm=noop ˋˋˋ The existence of `noop` is revealing: Git can eliminate negotiation, but the price is potentially transferring substantially more data. ([Git][4]) So Git is making a trade: ˋˋˋtext extra RTT / computation versus extra bytes ˋˋˋ I don't think that should be the fundamental choice anymore. --- # 3. Packfiles are the bigger architectural problem The current packfile has this conceptual form: ˋˋˋtext PACK version object-count object object object object ... checksum ˋˋˋ An object can be: ˋˋˋtext FULL_OBJECT or DELTA(base) ˋˋˋ and a delta can refer to its base by: ˋˋˋtext REF_DELTA -> object ID OFS_DELTA -> relative position inside this pack ˋˋˋ That latter mechanism means physical placement becomes part of the dependency structure. ([Git][5]) Git gets extremely good compression from this. But look at what we lose. A pack is fundamentally a sequential artifact: ˋˋˋtext +-------------------------------------------------------+ | header | obj | obj | delta | delta | ... | checksum | +-------------------------------------------------------+ ˋˋˋ Object 8 may require object 3. Object 3 may require object 1. And their physical offsets matter. The `.idx` then exists separately to recover random-access properties from this sequential representation. `index-pack` must construct that index after receiving a pack. ([Git][6]) That design made enormous sense for: ˋˋˋtext 2005 CPUs 2005 disks 2005 network connections 2005 filesystems ˋˋˋ It is much less attractive for: ˋˋˋtext HTTP/3 CDNs NVMe many-core CPUs parallel decompression range requests object caches edge caches cloud object stores interrupted transfers large monorepositories ˋˋˋ --- # 4. My replacement: Git Segment Store I'd call the format something like: ˋˋˋtext Git Object Segment Format GOSF/1 ˋˋˋ or, if you want to retain the terminology: ˋˋˋtext PACK/4 ˋˋˋ A repository becomes: ˋˋˋtext Repository Manifest | +--- Segment A +--- Segment B +--- Segment C +--- Segment D +--- Segment E ˋˋˋ Every segment is: * immutable * content-addressed * independently verifiable * independently downloadable * independently decompressible * internally indexed * resumable * cacheable forever For example: ˋˋˋtext segments/ 03/03b8....seg 17/178a....seg 62/62df....seg cc/cc12....seg ˋˋˋ The segment itself is hashed: ˋˋˋtext segment_id = SHA256(segment_bytes) ˋˋˋ That ID is **not** a Git object ID. It identifies the storage artifact. This distinction is important: ˋˋˋtext Git OID = identity of logical object Segment ID = identity of physical representation ˋˋˋ --- # 5. PACK/4 format Something like: ˋˋˋtext +--------------------------------------+ | PACK4 HEADER | +--------------------------------------+ | OBJECT DIRECTORY | +--------------------------------------+ | FRAME DIRECTORY | +--------------------------------------+ | DEPENDENCY TABLE | +--------------------------------------+ | COMPRESSED FRAME 0 | +--------------------------------------+ | COMPRESSED FRAME 1 | +--------------------------------------+ | COMPRESSED FRAME 2 | +--------------------------------------+ | ... | +--------------------------------------+ | MERKLE / SEGMENT CHECKSUM | +--------------------------------------+ ˋˋˋ The object directory could conceptually contain: ˋˋˋtext object_oid object_type canonical_size representation: FULL DELTA CHUNKED frame_number offset encoded_length base_object_index // DELTA only ˋˋˋ Notice what is gone: ˋˋˋtext absolute pack offset as semantic dependency relative pack offset as semantic dependency ˋˋˋ --- # 6. Delta bases must be local to a segment This is one of the strongest rules I would introduce. A transferable segment must be: ˋˋˋtext SELF CONTAINED ˋˋˋ Therefore: ˋˋˋtext delta object | +-- base object in SAME SEGMENT ˋˋˋ Never: ˋˋˋtext Segment B delta | +------> object buried in Segment A ˋˋˋ Yes, restricting delta choice can make compression slightly worse. But it buys something enormously valuable: ˋˋˋtext Segment X can be downloaded, verified, decoded, cached, copied, mirrored, or deleted without examining any other segment. ˋˋˋ Git already accepts similar compression-versus-operability compromises with delta islands: Git may accept somewhat larger packs so that server fetches can reuse stored delta relationships instead of recomputing deltas when the receiver lacks their bases. ([Git][7]) PACK/4 would make that idea architectural rather than heuristic. --- # 7. Make segments moderately sized Not: ˋˋˋtext one repository = one 80 GB pack ˋˋˋ and not: ˋˋˋtext one Git object = one network file ˋˋˋ Instead something around: ˋˋˋtext target compressed size: 4–32 MiB ˋˋˋ with perhaps: ˋˋˋtext 16 MiB default ˋˋˋ Large enough for good compression. Small enough for: ˋˋˋtext parallel downloads parallel decoding CDN distribution retry range requests incremental GC cache eviction ˋˋˋ So a 2 GB transfer might look like: ˋˋˋtext Worker 1 -> Segment 037 Worker 2 -> Segment 038 Worker 3 -> Segment 039 Worker 4 -> Segment 040 Worker 5 -> Segment 041 Worker 6 -> Segment 042 Worker 7 -> Segment 043 Worker 8 -> Segment 044 ˋˋˋ rather than: ˋˋˋtext 1 giant pack stream =========================================> ˋˋˋ --- # 8. Frames inside segments Even 16 MiB segments should have independently compressed frames. For example: ˋˋˋtext Segment frame 0 256 KB frame 1 512 KB frame 2 384 KB frame 3 640 KB ... ˋˋˋ I would probably use something in the Zstandard family rather than per-object zlib, though the exact codec should be negotiated/versioned. The important property isn't specifically Zstd. It is: ˋˋˋtext independent frames ˋˋˋ so four cores can do: ˋˋˋtext CPU0 -> frame 0 CPU1 -> frame 1 CPU2 -> frame 2 CPU3 -> frame 3 ˋˋˋ simultaneously. --- # 9. Large blobs should become chunk-addressable representations This is another major departure from present packfiles. Suppose Git has: ˋˋˋtext huge.iso 4 GB huge-v2.iso 4 GB, 5 MB modified ˋˋˋ Git's logical object model should still see: ˋˋˋtext blob A blob B ˋˋˋ But PACK/4 doesn't need to represent either blob as one monolithic compressed stream. Internally: ˋˋˋtext blob B = chunk A chunk B chunk C chunk D chunk E ... ˋˋˋ using content-defined chunk boundaries. Something like: ˋˋˋtext 64 KiB minimum 256 KiB target 1 MiB maximum ˋˋˋ could be a starting point. Then: ˋˋˋtext Version 1 A B C D E F G H Version 2 A B C X Y F G H ˋˋˋ requires transmitting: ˋˋˋtext X Y ˋˋˋ instead of computing an enormous blob delta. Important: The Git object ID still hashes the reconstructed canonical blob. Therefore existing Git semantics survive. --- # 10. The server should stop building bespoke packs Today `pack-objects` can exploit existing deltas, but there are circumstances where it must break or recompute delta relationships because the receiver doesn't have the required base. The Git documentation explicitly discusses this as a significant server CPU issue. ([Git][7]) PACK/4 changes the serving model from: ˋˋˋtext request arrives ↓ inspect client's history ↓ walk DAG ↓ choose objects ↓ choose deltas ↓ generate pack ↓ compress ↓ stream ˋˋˋ to: ˋˋˋtext request arrives ↓ identify required immutable segments ↓ return segment IDs ˋˋˋ The expensive work is amortized when segments are created. Think: ˋˋˋtext repository write-time: expensive packing once repository read-time: cheap immutable serving millions of times ˋˋˋ That's enormously CDN-friendly. --- # 11. Repository state should have a manifest Introduce an immutable repository manifest. Conceptually: ˋˋˋtext RepositoryManifest { repository_id epoch refs_root commit_graph_root segment_set_root segments[] previous_manifest signatures... } ˋˋˋ and: ˋˋˋtext manifest_id = SHA256(canonical_manifest) ˋˋˋ A ref snapshot therefore has a cheap immutable identifier: ˋˋˋtext repo-state: 7c8512... ˋˋˋ --- # 12. Introduce sync receipts This is how I'd eliminate most negotiation. When a server sends data to a client it also sends: ˋˋˋtext SYNC_RECEIPT ˋˋˋ For example: ˋˋˋtext repository = github.com/git/git manifest = abc123 coverage = reachable(refs snapshot X) filters = blob:none epoch = 483918 ˋˋˋ The server cryptographically authenticates or otherwise validates this opaque token. The client does **not** have to tell the server: ˋˋˋtext have a have b have c have d have e ... ˋˋˋ next time. It just says: ˋˋˋtext I previously synchronized through RECEIPT XYZ. Give me refs/heads/master now. ˋˋˋ The server already knows exactly what the receipt means. That turns: ˋˋˋtext "Which objects do you possess?" ˋˋˋ into: ˋˋˋtext "I know what I previously gave you." ˋˋˋ Huge difference. --- # 13. Common fetch becomes one request The normal fetch would now be: ˋˋˋtext CLIENT SERVER FETCH { repo-state-token = ABC sync-receipt = XYZ want-ref = refs/heads/master filter = full } ---------------------------------------> resolve ref compare immutable epochs determine new segments RESULT { new-ref = ... manifest = ... segments = [ S123, S124, S131 ] receipt = NEWXYZ } SEGMENT S123 SEGMENT S124 SEGMENT S131 <======================================= ˋˋˋ That is **one application-level request/response**. No: ˋˋˋtext ls-refs RTT have RTT have RTT ACK RTT done RTT pack-generation delay ˋˋˋ --- # 14. And don't wait for the plan before transmitting Even better, the response should be multiplexable: ˋˋˋtext RESPONSE HEADER REF refs/heads/main = A829... SEGMENT S124 START MANIFEST ... SEGMENT S125 START SEGMENT S124 END SEGMENT S126 START ... ˋˋˋ In HTTP/3 terms, each segment can be a separate stream. So head-of-line blocking disappears: ˋˋˋtext Stream 0 metadata Stream 4 segment A Stream 8 segment B Stream 12 segment C Stream 16 segment D ˋˋˋ A slow segment doesn't stop everything behind it. --- # 15. Fresh clone becomes one request too Current Git often conceptually requires discovering refs before choosing what to fetch. PACK/4 should permit: ˋˋˋtext CLONE { ref = refs/heads/main filter = full } ˋˋˋ without knowing its OID. Server replies: ˋˋˋtext REF: refs/heads/main = 93824ab... REPOSITORY: manifest = ... REQUIRED_SEGMENTS: ... SEGMENT... SEGMENT... SEGMENT... ˋˋˋ So: ˋˋˋtext TCP/TLS/QUIC establishment + one Git request ˋˋˋ and the data starts flowing. With HTTP/3 session resumption, the transport itself can sometimes reduce handshake costs too, though that is separate from the Git protocol design. --- # 16. Historical clones become even better Static segment IDs mean Git hosting providers can put virtually all object payloads on CDNs: ˋˋˋtext git.example.com | +-- metadata / refs | +-- returns segment list | V CDN / object store ˋˋˋ The client downloads: ˋˋˋtext https://cdn/.../segment/abc https://cdn/.../segment/def https://cdn/.../segment/ghi ˋˋˋ Current Git already has concepts pointing this way: protocol v2 includes `packfile-uris`, and bundle URI support allows large static data to be downloaded while a smaller incremental negotiation proceeds concurrently. ([Git][8]) I would make that **the primary architecture**, rather than an optional acceleration path. --- # 17. What about clients with arbitrary local history? That's the difficult case. For instance: ˋˋˋtext Alice cloned from GitHub. Bob cloned Alice's laptop. Bob added commits. Bob now fetches GitHub. ˋˋˋ GitHub cannot rely entirely on its old receipt. I would support two mechanisms. ### First: commit frontier Client sends selected local graph tips: ˋˋˋtext local-frontier: A B C ˋˋˋ The server's commit graph/bitmap tells it whether those are known. One request still suffices. For most cases: ˋˋˋtext one known ancestor found ˋˋˋ and no iterative negotiation is needed. ### Second: object-set reconciliation For pathological cases, use a set-reconciliation structure rather than thousands of `have` messages. Candidates include techniques in the family of: ˋˋˋtext IBLT Golomb-coded sets XOR filters Merkle set reconciliation ˋˋˋ But I'd be extremely careful here. A probabilistic "client has this object" result must **never** silently create an incomplete repository. So probabilistic structures may only optimize transfer selection. Correctness still comes from: ˋˋˋtext object identity verification + DAG connectivity verification + missing-object recovery ˋˋˋ --- # 18. Missing-object recovery becomes normal and cheap Suppose optimization produces: ˋˋˋtext 99.999% correct object set ˋˋˋ but misses three objects. That should not mean: ˋˋˋtext fatal clone corruption start over ˋˋˋ Client performs: ˋˋˋtext GET_OBJECTS { oid1 oid2 oid3 } ˋˋˋ Server returns the corresponding segments or object frames. This makes aggressive low-latency negotiation possible because correctness no longer requires perfect prediction before transmission begins. --- # 19. Stop treating a fetch as one atomic pack This is probably my largest criticism of the current pack abstraction. Current conceptual model: ˋˋˋtext Fetch result = PACK ˋˋˋ New model: ˋˋˋtext Fetch result = repository metadata + set of independently verifiable segments ˋˋˋ Consequently interrupted fetch: ˋˋˋtext Downloaded: S1 ✓ S2 ✓ S3 ✓ S4 61% S5 pending S6 pending ˋˋˋ Resume: ˋˋˋtext keep S1 keep S2 keep S3 restart/range-resume S4 get S5 get S6 ˋˋˋ No need to treat a partially obtained giant pack as a special temporary state. --- # 20. Garbage collection becomes LSM-like Git already has MIDX and geometric repacking ideas. A multi-pack-index lets Git address objects across multiple packs, and geometric repack attempts to maintain a progression of pack sizes instead of constantly rewriting everything. ([Git][9]) I'd take that much further. Think: ˋˋˋtext Level 0: newest small segments Level 1: compacted segments Level 2: larger stable segments Archive: very stable historical segments ˋˋˋ Almost like an LSM tree: ˋˋˋtext new objects ↓ small immutable segment ↓ small immutable segment ↓ compaction ↓ larger immutable segment ˋˋˋ Old segment IDs remain valid for some retention period, so CDN caches and clients do not suddenly lose usefulness after server GC. --- # 21. Segments should be organized by expected transfer locality Current Git's delta-selection heuristics try to group likely-similar objects. ([GitHub][10]) PACK/4 could explicitly optimize segments for two dimensions: ˋˋˋtext compression similarity + fetch locality ˋˋˋ For example, objects reachable from nearby commits could tend to live together. Instead of maximizing: ˋˋˋtext smallest possible repository ˋˋˋ the objective becomes: ˋˋˋtext Minimize: storage_bytes + network_bytes + server_CPU + client_CPU + random_IO + RTT_cost + cache_miss_cost ˋˋˋ That's a much better objective function for 2026. --- # 22. Push should use the exact same representation Push currently has a related problem: ˋˋˋtext client computes pack client uploads pack server indexes/verifies pack ˋˋˋ I'd make push: ˋˋˋtext PUSH_BEGIN { old-ref = ABC new-ref = XYZ } OBJECT_SEGMENTS: S1 S2 S3 ˋˋˋ Server says: ˋˋˋtext HAVE S1 NEED S2 HAVE S3 ˋˋˋ If segment IDs are globally content-addressed, existing segments never need retransmission. Then: ˋˋˋtext UPDATE_REF ˋˋˋ subject to atomic ref validation. --- # 23. Better yet: advertise segment presence before uploading bytes A client could send: ˋˋˋtext PUSH { ref-update... offered-segments: A B C D } ˋˋˋ Server immediately responds: ˋˋˋtext need: C D ˋˋˋ Client starts them in independent streams. With HTTP/3 you could even optimistically begin sending likely-new segments before the metadata response comes back. If the server already has one, it cancels that stream. The cost is a few redundant bytes instead of another RTT. That is usually the correct trade on high-latency networks. --- # 24. Commit graph and object data should be separate planes I'd explicitly separate: ˋˋˋtext CONTROL PLANE refs commit graph reachability summaries manifests receipts capabilities DATA PLANE immutable object segments ˋˋˋ Control messages might be kilobytes. Data plane might be gigabytes. That means: ˋˋˋtext Control: git.example.com Data: cdn1.example.net cdn2.example.net cdn3.example.net ˋˋˋ The data plane requires almost no Git intelligence. Just: ˋˋˋtext GET segment SHA256:X ˋˋˋ --- # 25. You could eventually make the central Git server almost stateless A hosting request might become: ˋˋˋtext FETCH(repo, receipt, ref) ˋˋˋ The smart front-end looks up: ˋˋˋtext ref -> commit receipt -> known manifest epoch ˋˋˋ and produces: ˋˋˋtext difference = segment-set(new) - segment-set(old) ˋˋˋ using bitmaps/set indexes. Then every byte-heavy operation is handled by blob storage/CDNs. The Git server itself doesn't run a giant bespoke compression job. --- # 26. Why reachability bitmaps still matter Git's modern bitmap machinery is actually something I'd preserve. Git already uses bitmap indexes to accelerate reachability operations; the source describes a bitmap index associated with a pack or MIDX object order. ([GitHub][11]) But I'd change what the bitmap indexes. Today roughly: ˋˋˋtext commit ↓ objects ˋˋˋ PACK/4 could maintain: ˋˋˋtext commit ↓ object bitmap ↓ segment bitmap ˋˋˋ Thus: ˋˋˋtext reachable-segments(commit A) - reachable-segments(receipt B) ˋˋˋ becomes mostly bitmap arithmetic. Very fast. --- # 27. Segment manifests can form a Merkle tree For enormous repositories, don't send a million segment identifiers. Use: ˋˋˋtext ROOT / \ A B / \ / \ C D E F ... ... ... ... ˋˋˋ Client knows: ˋˋˋtext old root = X ˋˋˋ Server has: ˋˋˋtext new root = Y ˋˋˋ Diff only differing branches. This allows repository-state comparison proportional to the changed area, rather than repository size. --- # 28. One important change to Git's concept of a "pack" I'd make this explicit: > **A pack should no longer be an archival snapshot. It should be a disposable physical representation of immutable Git objects.** Git already conceptually separates object identity from pack representation, but implementation details make the representation unusually important. PACK/4 should enforce the abstraction much more strongly. Logical world: ˋˋˋtext OID -> canonical object ˋˋˋ Physical world: ˋˋˋtext OID -> SegmentID -> Frame -> Representation ˋˋˋ And physical representation can change without changing: ˋˋˋtext commit IDs tree IDs blob IDs ˋˋˋ --- # 29. Compatibility with existing Git could actually be good You wouldn't need a flag day. Architecture: ˋˋˋtext +----------------------+ old client ---> | Git compatibility | | upload-pack gateway | +----------+-----------+ | v Segment Store ^ | +----------+-----------+ new client ---> | Git/3 / PACK4 | +----------------------+ ˋˋˋ For an old client: ˋˋˋtext segment store ↓ generate legacy PACK2 stream ˋˋˋ For a new client: ˋˋˋtext native segments ˋˋˋ So the repository can have one modern backend while supporting existing Git wire protocols. --- # 30. I would also eliminate pkt-line framing for bulk payloads Git's current pack transmission is multiplexed using pkt-line framing and sideband codes. ([GitHub][12]) That's historical layering we no longer need for the bulk-data path. Don't do: ˋˋˋtext length channel data length channel data length channel data ... ˋˋˋ for gigabytes of payload. Modern transports already provide framing. Use: ˋˋˋtext QUIC stream HTTP body raw binary segment ˋˋˋ Progress belongs in another stream. For example: ˋˋˋtext Stream 0: RPC metadata Stream 4: segment A bytes Stream 8: segment B bytes Stream 12: progress/events ˋˋˋ Transport multiplexing should do the multiplexing. --- # 31. The new protocol I'd call it something like: ˋˋˋtext Git Transfer Protocol 3 GTP/3 ˋˋˋ Basic request: ˋˋˋtext FETCH { repository: want { ref: "refs/heads/main" } base { receipt: } filter { blobs: all } capabilities { segment-format: PACK4 compression: zstd parallel-streams: 16 } } ˋˋˋ Response begins immediately: ˋˋˋtext FETCH_RESULT { ref { refs/heads/main: 3c817... } manifest: a741... required-segments: [ 12a..., 348..., 9fe... ] new-receipt: ... } ˋˋˋ followed concurrently by: ˋˋˋtext SEGMENT 12a... SEGMENT 348... SEGMENT 9fe... ˋˋˋ --- # 32. New clone ˋˋˋtext CLIENT CLONE { branch = "main" filter = full max_parallel = 16 } ˋˋˋ One request. Server: ˋˋˋtext metadata + segment streams ˋˋˋ No explicit: ˋˋˋtext discover choose negotiate ACK negotiate ACK done build transfer ˋˋˋ pipeline. --- # 33. Normal fetch After the initial clone: ˋˋˋtext FETCH { ref = main receipt = R329 } ˋˋˋ Server: ˋˋˋtext R329 corresponds to manifest 713 main now corresponds to manifest 719 needed: S91 S92 S95 ˋˋˋ Transmit three segments. Done. --- # 34. No-op fetch becomes extremely cheap Current conceptual fetch still involves protocol work. GTP/3: ˋˋˋtext FETCH { ref = main receipt = ABC } ˋˋˋ response: ˋˋˋtext UNCHANGED { receipt = ABC } ˋˋˋ Potentially a few hundred bytes. --- # 35. Force pushes become straightforward too A receipt shouldn't mean: ˋˋˋtext history must be ancestor ˋˋˋ It means: ˋˋˋtext server knows the exact synchronization state associated with this client token. ˋˋˋ If: ˋˋˋtext A-B-C-D becomes A-B-X-Y ˋˋˋ server computes required segments for `Y`. Objects for C/D don't somehow become invalid locally. Git's immutable-object semantics continue working naturally. --- # 36. Partial clones improve dramatically Current partial clone tracks promisor objects and may dynamically fetch missing objects; Git documents the fallback mechanism for retrieving missing promisor objects. ([Git][13]) PACK/4 can make lazy object acquisition first-class. For example: ˋˋˋtext clone --metadata-only ˋˋˋ gets: ˋˋˋtext commits trees small blobs perhaps ˋˋˋ Opening: ˋˋˋtext src/video/test-data.bin ˋˋˋ causes: ˋˋˋtext GET_OBJECT ˋˋˋ which maps immediately: ˋˋˋtext OID ↓ segment ↓ CDN ˋˋˋ No `upload-pack` traversal is necessary. --- # 37. Checkout could fetch data in priority order This creates a very useful capability. Suppose checkout requires 20 GB. Instead of waiting for everything: ˋˋˋtext metadata first source files second small blobs third large assets fourth old history later ˋˋˋ The client can begin constructing the worktree almost immediately. For instance: ˋˋˋtext Priority 0: commits + trees Priority 1: README build files source files Priority 2: normal assets Priority 3: huge binaries Priority 4: historical objects ˋˋˋ Current pack streaming makes this substantially harder because the pack's compression layout heavily influences transfer order. --- # 38. Clone should become progressive Eventually: ˋˋˋtext git clone URL repo ˋˋˋ could behave like: ˋˋˋtext 0.1 sec refs known 0.2 sec trees available 0.4 sec worktree construction begins 0.7 sec most source code available ... historical/archive segments continue in parallel ˋˋˋ instead of thinking of cloning as: ˋˋˋtext receive opaque giant pack index pack resolve deltas checkout ˋˋˋ --- # 39. There is a cost This architecture will usually sacrifice some maximum compression efficiency. Current Git packing is extraordinarily good at exploiting arbitrary delta relationships. PACK/4 says: ˋˋˋtext do not optimize only for minimum bytes ˋˋˋ because another 3% compression improvement isn't necessarily worth: ˋˋˋtext server CPU RTT serialization dependency chains loss of CDN reuse loss of parallelism poor resumability repacking costs ˋˋˋ The new metric should be: ˋˋˋtext time-to-usable-repository ˋˋˋ not: ˋˋˋtext smallest .pack possible ˋˋˋ --- # 40. The distinction is something like this Classic Git optimized for: ˋˋˋtext storage ▲ | compression | delta chains ˋˋˋ I'd optimize Git/3 for: ˋˋˋtext latency ▲ | parallel transfer | CDN reuse -----+----- CPU | resumability | storage ˋˋˋ It's a different optimum. --- # 41. My PACK/4 rules If I were freezing a specification, my strongest invariants would be: 1. **Git object identity remains independent of physical representation.** 2. **Every segment is immutable and byte-content-addressed.** 3. **Every segment is independently verifiable.** 4. **Every transport segment is independently decodable.** 5. **No delta base outside its segment.** 6. **Segment indexes occur before or independently from bulk payloads.** 7. **Compressed payload consists of independently decodable frames.** 8. **Large Git blobs may have chunked physical representations.** 9. **Partial segment download cannot corrupt repository state.** 10. **Repository state is described by immutable manifests.** 11. **Clients receive reusable synchronization receipts.** 12. **Normal fetch requires no iterative HAVE/ACK negotiation.** 13. **Object/segment transfer is inherently parallel.** 14. **The server need not construct a bespoke pack for ordinary fetches.** 15. **Legacy PACK2 can be generated as a compatibility view.** Those fifteen rules are the heart of my redesign. --- # 42. Architecture diagram I would turn Git: ˋˋˋtext CURRENT GIT refs | commit graph | objects | pack-objects | request-specific PACK | network | index-pack | local packs ˋˋˋ into: ˋˋˋtext GIT/3 refs | commit graph | objects | Segment Builder | +-------------+-------------+ | | | Segment A Segment B Segment C | | | +-------------+-------------+ | Merkle Manifest | +-----------+-----------+ | | Metadata/API CDN | | ref/receipt immutable segments | | +-----------+-----------+ | client ˋˋˋ --- # 43. The interesting irony The initial 2005 Git repository you linked starts from an exceptionally clean idea: > a content-addressed collection of objects which can refer to other objects. ([GitHub][1]) That abstraction aged extremely well. I'd argue that **packfiles aged less well than Git objects did**. Modern Git has spent years successfully mitigating packfile consequences with: ˋˋˋtext bitmaps MIDX reverse indexes delta islands partial clones promisor packs bundle URIs packfile URIs geometric repacking skipping negotiation ˋˋˋ Those are all useful engineering solutions. ([Git][7]) But taken together, they also suggest the cleaner next step: > **Stop making the pack the fundamental transfer unit. Make the immutable object segment the transfer unit.** That would give Git something much closer to **zero sequential negotiation RTT for the normal case, static CDN-compatible payloads, massively parallel decoding and transfer, trivial resume, and far less server-side pack generation**, while preserving the part of Git that was brilliant from day one: its immutable content-addressed object graph. [1]: https://github.com/git/git/commit/e83c5163316 "Initial revision of \"git\", the information manager from hell · git/git@e83c516 · GitHub" [2]: https://git-scm.com/docs/gitprotocol-v2?utm_source=chatgpt.com "Git - gitprotocol-v2 Documentation" [3]: https://github.com/git/git/blob/master/Documentation/gitprotocol-v2.adoc?utm_source=chatgpt.com "git/Documentation/gitprotocol-v2.adoc at master · git/git · GitHub" [4]: https://git-scm.com/docs/git-fetch?utm_source=chatgpt.com "Git - git-fetch Documentation" [5]: https://git-scm.com/docs/pack-format?utm_source=chatgpt.com "Git - pack-format Documentation" [6]: https://git-scm.com/docs/git-index-pack?utm_source=chatgpt.com "Git - git-index-pack Documentation" [7]: https://git-scm.com/docs/git-pack-objects?utm_source=chatgpt.com "Git - git-pack-objects Documentation" [8]: https://git-scm.com/docs/protocol-v2?utm_source=chatgpt.com "Git - protocol-v2 Documentation" [9]: https://git-scm.com/docs/git-multi-pack-index?utm_source=chatgpt.com "Git - git-multi-pack-index Documentation" [10]: https://github.com/git/git/blob/master/Documentation/technical/pack-heuristics.adoc?utm_source=chatgpt.com "git/Documentation/technical/pack-heuristics.adoc at master · git/git · GitHub" [11]: https://github.com/git/git/blob/master/pack-bitmap.c?utm_source=chatgpt.com "git/pack-bitmap.c at master · git/git · GitHub" [12]: https://github.com/git/git/blob/master/Documentation/gitprotocol-pack.adoc?utm_source=chatgpt.com "git/Documentation/gitprotocol-pack.adoc at master · git/git · GitHub" [13]: https://git-scm.com/docs/partial-clone?utm_source=chatgpt.com "Git - partial-clone Documentation" Bye for now, Skybuck Flying. ```