For those interested in partial clones and/or missing objects in repos, I've updated my original partialclone patches to not require an explicit list of promises. Fetch/clone still only permits exclusion of blobs, but the infrastructure is there for a local repo to support missing trees and commits as well. They can be found here: https://github.com/jonathantanmy/git/tree/partialclone2 To make the number of patches more manageable, I have omitted support for a custom fetching hook (but it can be readded in fetch-object.c), and only support promisor packfiles for now (but I don't have any objection to supporting promisor loose objects in the future). Let me know what you think of the overall approach. In particular, I'm still wondering if there is a better approach than to toggle "fetch_if_missing" whenever we need lazy fetching (or need to suppress it). Also, if there any patches that you think might be useful to others, let me know and I'll send them to this mailing list for review. A demo and an overview of the design (also available from that repository's README): Demo ==== Obtain a repository. $ make prefix=$HOME/local install $ cd $HOME/tmp $ git clone https://github.com/git/git Make it advertise the new feature and allow requests for arbitrary blobs. $ git -C git config uploadpack.advertiseblobmaxbytes 1 $ git -C git config uploadpack.allowanysha1inwant 1 Perform the partial clone and check that it is indeed smaller. Specify "file://" in order to test the partial clone mechanism. (If not, Git will perform a local clone, which unselectively copies every object.) $ git clone --blob-max-bytes=100000 "file://$(pwd)/git" git2 $ git clone "file://$(pwd)/git" git3 $ du -sh git2 git3 116M git2 129M git3 Observe that the new repo is automatically configured to fetch missing objects from the original repo. Subsequent fetches will also be partial. $ cat git2/.git/config [core] repositoryformatversion = 1 filemode = true bare = false logallrefupdates = true [remote "origin"] url = [snip] fetch = +refs/heads/*:refs/remotes/origin/* blobmaxbytes = 100000 [extensions] partialclone = origin [branch "master"] remote = origin merge = refs/heads/master Unlike in an older version of this code (see the `partialclone` branch), this also works with the HTTP/HTTPS protocols. Design ====== Local repository layout ----------------------- A repository declares its dependence on a *promisor remote* (a remote that declares that it can serve certain objects when requested) by a repository extension "partialclone". `extensions.partialclone` must be set to the name of the remote ("origin" in the demo above). A packfile can be annotated as originating from the promisor remote by the existence of a "(packfile name).promisor" file with arbitrary contents (similar to the ".keep" file). Whenever a promisor remote sends an object, it declares that it can serve every object directly or indirectly referenced by the sent object. A promisor packfile is a packfile annotated with the ".promisor" file. A promisor object is an object in a promisor packfile. A promised object is an object directly referenced by a promisor object. (In the future, we might need to add ".promisor" support to loose objects.) Connectivity check and gc ------------------------- The object walk done by the connectivity check (as used by fsck and fetch) stops at all promisor objects and promised objects. The object walk done by gc also stops at all promisor objects and promised objects. Only non-promisor packfiles are deleted (if pack deletion is requested); promisor packfiles are left alone. This maintains the distinction between promisor packfiles and non-promisor packfiles. (In the future, we might need to do something more sophisticated with promisor packfiles.) Fetching of promised objects ---------------------------- When `sha1_object_info_extended()` (or similar) is invoked, it will automatically attempt to fetch a missing object from the promisor remote if that object is not in the local repository. For efficiency, no check is made as to whether that object is a promised object or not. This automatic fetching can be toggled on and off by the `fetch_if_missing` global variable, and it is on by default. The actual fetch is done through the fetch-pack/upload-pack protocol. Right now, this uses the fact that upload-pack allows blob and tree "want"s, and this incurs the overhead of the unnecessary ref advertisement. I hope that protocol v2 will allow us to declare that blob and tree "want"s are allowed, and allow the client to declare that it does not want the ref advertisement. All packfiles downloaded in this way are annotated with ".promisor". Fetching with `git fetch` ------------------------- The fetch-pack/upload-pack protocol has also been extended to support omission of blobs above a certain size. The client only allows this when fetching from the promisor remote, and will annotate any packs received from the promisor remote with ".promisor".
