Re: Sparse clones (Was: Re: [PATCH 1/2] upload-pack: support subtree packing)

> 1) A user controls sparseness by passing rev-list arguments to clone.
>
> This allows a user to control sparseness both in terms of span of
> content (files/directories) and depth of history.  It can also be used
> to limit to a subset of refs (cloning just one or two branches instead
> of all branches and tags).  For example,
>  $ git clone ssh://repo.git dst -- Documentation/

>  $ git clone ssh://repo.git dst master~6..master
>  $ git clone ssh://repo.git dst -3
> (Note that the destination argument becomes mandatory for those doing
> a sparse clone in order to disambiguate it from rev-list options.)
>
> This method also means users don't need much training to learn how to
> use sparse clones -- they just use syntax they've already learned with
> log, and clone will pass this info on to upload-pack.
>
> There is a slight question as to whether users should have to specify
> "--all HEAD" with all sparse clones or whether it should be assumed
> when no other refs are listed.

> 4) All revision-walking operations automatically use these limiting args.
>
> This should be a simple code change, and would enable rev-list, log,
> etc. to avoid missing blobs/trees and thus enable them to work with
> sparse clones.  fsck would take a bit more work, since it doesn't use
> the setup_revisions() and revision.h walking machinery, but shouldn't
> be too bad (I hope).
>
> There are also performance ramifications: There should be no
> measurable performance overhead for non-sparse clones (something that
> might be a problem with a different implementation that did
> does-this-exist check each time it references a blob).  It should also
> be a significant performance boost for those using it, as operations
> will only need to deal with the subset of the repository they specify
> (faster downloads, stats, logs, etc.)

> 5) "Densifying" a sparse clone can be done
>
> One can fetch a new pack and replace the limiting rev-list args with
> the new choice.  The sparse checkout information needs to be updated
> too.
>
> (So users probably would want to densify a sparse clone with "pull"
> rather than "fetch", as manually updating sparse checkouts may be a
> bit of a hassle.)