There's a lot in this patch series. I'm still studying it, but here are some notes and questions. I'll start with direct responses to the RFC here and follow up in a second email with specific questions and comments to keep this from being too long). On 9/15/2017 4:43 PM, Jonathan Tan wrote:
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
I like that git-clone saves the partial clone settings in the .git/config. This should make it easier for subsequent commands to default to the right settings. Do we allow a partial-fetch following a regular clone (such that git-fetch would create these config file settings)? This seems like a reasonable upgrade path for a user with an existing repo to take advantage of partial fetches going forward. Or do we require that git-fetch only be allowed to do partial-fetches after a partial-clone (so that only clone creates these settings) and fetch always does partial-fetches thereafter? It might be useful to allow fetch to do a full fetch on top of a partial-clone, but there are probably thin-pack issues to work out first. Also, there is an assumption here that the user will want to keep using the same filtering settings on subsequent fetches. That's probably fine for now and until we get a chance to try it out for real.
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).
Do we allow EXACTLY ONE promisor-remote? That is, can we later add another remote as a promisor-remote? And then do partial-fetches from either? Do we need to disallow removing or altering a remote that is listed as a promisor-remote? I think for now, one promisor-remote is fine. Just asking. Changing a remote's URL might be fine, but deleting the promisor-remote would put the user in a weird state. We don't need to worry about it now though.
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.
I struggled with the terms here a little when looking at the source. () A remote responding to a partial-clone is termed a "promisor-remote". () Packfiles received from a promisor-remote are marked with "<name>.promisor" like "<name>.keep" names. () An object actually contained in such packfiles is called a "promisor-object". () An object not-present but referenced by one of the above promisor-objects is called a "promised-object" (aka a "missing-object"). I think the similarity of the words "promisOR" and "promisED" threw me here and as I was looking at the code. The code in is_promised() [1] looked like it was adding all promisor- and promised-objects to the "promised" OIDSET, but I could be mistaken. [1] https://github.com/jonathantanmy/git/commit/7a9c2d9b6e2fce293817b595dee29a7eede0dddd#diff-5d5d5dc185ef37dc30bb7d9a7ae0c4e8R1960 The contents of the "<name>.promisor" file are described as arbitrary? Should we write the name of the remote (or some other structured data) into this file so that later fetches can find the server? This is more a question for when we have multiple promisor-remotes and may need to decide who can supply a missing object. Not urgent now though.
(In the future, we might need to add ".promisor" support to loose objects.)
I assume you mean that a dynamic fetch of a single tree object would be unpacked upon receipt (rather than creating a one object packfile) and that we may need to mark it as promisor-object so that missing objects that *IT* references would still follow the promised-object rules.
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.
So is the assumption that as soon as you touch a promisOR-object you might as well stop scanning, because anything it references might be missing? The code in rev-list.c and revision.c in [2] looks like it will continue thru PROMISOR-objects and stop at (still-missing) PROMISED-objects. [2] https://github.com/jonathantanmy/git/commit/2d7ae2bc780dd056552643e4f5061a0ca7b5b1e5
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.)
Seems like we could combine promisor-packfiles -- so, for example, the net result of a complete repack might be one normal- and one promisor-packfile. Question: if the initial clone is a partial-clone, we will ONLY have promisor-packfiles, right? (Unless they later add a second regular remote.) So fsck and GC will basically stop scanning immediately, right? Likewise, repack will basically be disabled until we support repacking promisor-packfiles, right? So performance of the resulting repo will deteriorate over time as subsequent partial-fetches pull in more and more 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.
I don't understand this last statement. A missing-object is assumed to be a promised-object, but we don't check that. By this do you mean that is_promised() [1] is only used by commands like fsck and not elsewhere?
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".
The dynamic object fetching in fetch_object() [3] only fetches 1 object from the server. This will spawn a transport process, network connection (and authentication), a packfile download, and index-pack or unpack for each object. Perhaps this is just preliminary code, but I don't see how you can adapt that model to be efficient. Even using a long-running background process to fetch individual blobs, it will still be fetching blobs individually. We do need to have dynamic object fetching, but we should try to minimize its use; that is, to try have higher-level mechanisms in place to predict and bulk prefetch missing objects. I guess it depends on how many missing-objects you expect the client to have. My concern here is that we're limiting the design to the "occasional" big file problem, rather than the more general scale problem. Also, relying on dynamic object fetching also means that operations, like checkout, may now require a network connection. Because we don't know we need an object until we actually need it (and are half-way thru some operation that the user thought was safe to do off-line). I'll talk about this more in my next response to this thread. [3] https://github.com/jonathantanmy/git/commit/d23be53681549d9ac3c2ecb7b2be9f93d355457b#diff-5c4ccaa3282a2113fc8cafa156b30941R7
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".
Thanks, Jeff