| Message ID | 20230614192541.1599256-1-christian.couder@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Repack objects into separate packfiles based on a filter | expand |
Christian Couder <christian.couder@gmail.com> writes: > In some discussions, it was mentioned that such a feature, or a > similar feature in `git gc`, or in a new standalone command (perhaps > called `git prune-filtered`), should put the filtered out objects into > a new packfile instead of deleting them. > > Recently there were internal discussions at GitLab about either moving > blobs from inactive repos onto cheaper storage, or moving large blobs > onto cheaper storage. This lead us to rethink at repacking using a > filter, but moving the filtered out objects into a separate packfile > instead of deleting them. > > So here is a new patch series doing that while implementing the > `--filter=<filter-spec>` option in `git repack`. Very interesting idea, indeed, and would be very useful. Thanks.
Junio C Hamano <gitster@pobox.com> writes: > Christian Couder <christian.couder@gmail.com> writes: > >> In some discussions, it was mentioned that such a feature, or a >> similar feature in `git gc`, or in a new standalone command (perhaps >> called `git prune-filtered`), should put the filtered out objects into >> a new packfile instead of deleting them. >> >> Recently there were internal discussions at GitLab about either moving >> blobs from inactive repos onto cheaper storage, or moving large blobs >> onto cheaper storage. This lead us to rethink at repacking using a >> filter, but moving the filtered out objects into a separate packfile >> instead of deleting them. >> >> So here is a new patch series doing that while implementing the >> `--filter=<filter-spec>` option in `git repack`. > > Very interesting idea, indeed, and would be very useful. > Thanks. Overall, I have a split feeling on the series. One side of my brain thinks that the series does a very good job to address the needs of those who want to partition their objects into two classes, and the problem I saw in the series was mostly the way it was sold (in other words, if it did not mention unbloating lazily cloned repositories at all, I would have said "Yes! It is an excellent series.", and if it said "this mechanism is not meant to be used to unbloat a lazily cloned repository, because the mechanism does not distinguish objects that are only locally available and objects that are retrievable from the promisor remotes, among those that match the filter", it would have been even better) To the other side of my brain, it smells as if the series wanted to address the unbloating issue, but ended up with an unsatisfactory solution, and used "partitioning objects in a full repository on the server side " as an excuse for the resulting mechanism to still exist, even though it is not usable for the original purpose. Ideally, it would be great to have a mechanism that can be used for both. The "partitioning" can be treated as a degenerate case where the repository does not have its upstream promisor (hence, any object that match the filtering criteria can be excluded from the primary pack because there are no "not available (yet) in our promisor" objects), while the "unbloat" case can know who its promisors are and ask the promisors what objects, among those that match the filtering criteria, are still available from them to exclude only those objects from the primary pack. In the second ideal world, we may not be ready to tackle the unbloating issue, but "partitioning" alone may still be a useful feature. In that case, perhaps the series can be salvaged by updating how the feature is sold, with some comments indicating the future direction to extend the mechanism later. Thanks.
# Intro Last year, John Cai sent 2 versions of a patch series to implement `git repack --filter=<filter-spec>` and later I sent 4 versions of a patch series trying to do it a bit differently: - https://lore.kernel.org/git/pull.1206.git.git.1643248180.gitgitgadget@gmail.com/ - https://lore.kernel.org/git/20221012135114.294680-1-christian.couder@gmail.com/ In these patch series, the `--filter=<filter-spec>` removed the filtered out objects altogether which was considered very dangerous even though we implemented different safety checks in some of the latter series. In some discussions, it was mentioned that such a feature, or a similar feature in `git gc`, or in a new standalone command (perhaps called `git prune-filtered`), should put the filtered out objects into a new packfile instead of deleting them. Recently there were internal discussions at GitLab about either moving blobs from inactive repos onto cheaper storage, or moving large blobs onto cheaper storage. This lead us to rethink at repacking using a filter, but moving the filtered out objects into a separate packfile instead of deleting them. So here is a new patch series doing that while implementing the `--filter=<filter-spec>` option in `git repack`. This could be useful for the following purposes: - As a way for servers to save storage costs by for example moving large blobs, or blobs in inactive repos, to separate storage (while still making them accessible using for example the alternates mechanism). - As a way to use partial clone on a Git server to offload large blobs to, for example, an http server, while using multiple promisor remotes (to be able to access everything) on the client side. (In this case the packfile that contains the filtered out object can be manualy removed after checking that all the objects it contains are available through the promisor remote.) - As a way for clients to reclaim some space when they cloned with a filter to save disk space but then fetched a lot of unwanted objects (for example when checking out old branches) and now want to remove these unwanted objects. (In this case they can first move the packfile that contains filtered out objects to a separate directory or storage, then check that everything works well, and then manually remove the packfile after some time.) As the features and the code are quite different from those in the previous series, I decided to start a new series instead of continuing a previous one. # Commit overview * 1/9 pack-objects: allow `--filter` without `--stdout` This patch is the same as the first patch in the previous series. To be able to later repack with a filter we need `git pack-objects` to write packfiles when it's filtering instead of just writing the pack without the filtered out objects to stdout. * 2/9 pack-objects: add `--print-filtered` to print omitted objects We need a way to know the objects that are filtered out of the packfile generated by `git pack-objects --filter=<filter-spec>`. The simplest way is to teach pack-objects to print their oids to stdout. * 3/9 t/helper: add 'find-pack' test-tool For testing `git repack --filter=...` that we are going to implement, it's useful to have a test helper that can tell which packfiles contain a specific object. * - 4/9 repack: refactor piping an oid to a command - 5/9 repack: refactor finishing pack-objects command These are small refactorings so that `git repack --filter=...` will be able to reuse useful existing functions. * 6/9 repack: add `--filter=<filter-spec>` option This actually adds the `--filter=<filter-spec>` option. It uses one `git pack-objects` process with both the `--filter` and the `--print-filtered` options. From this process it reads the oids of the filtered out objects and pass them to a separate `git pack-objects` process which will pack these objects into a separate packfile. * 7/9 gc: add `gc.repackFilter` config option This is a gc config option so that `git gc` can also repack using a filter and put the filtered out objects into a separate packfile. * 8/9 repack: implement `--filter-to` for storing filtered out objects For some use cases, it's interesting to create the packfile that contains the filtered out objects into a separate location. This is similar to the --expire-to option for cruft packfiles. * 9/9 gc: add `gc.repackFilterTo` config option This allows specifying the location of the packfile that contains the filtered out objects when using `gc.repackFilter`. Christian Couder (9): pack-objects: allow `--filter` without `--stdout` pack-objects: add `--print-filtered` to print omitted objects t/helper: add 'find-pack' test-tool repack: refactor piping an oid to a command repack: refactor finishing pack-objects command repack: add `--filter=<filter-spec>` option gc: add `gc.repackFilter` config option repack: implement `--filter-to` for storing filtered out objects gc: add `gc.repackFilterTo` config option Documentation/config/gc.txt | 11 ++ Documentation/git-pack-objects.txt | 14 ++- Documentation/git-repack.txt | 11 ++ Makefile | 1 + builtin/gc.c | 10 ++ builtin/pack-objects.c | 55 ++++++-- builtin/repack.c | 166 ++++++++++++++++++------- t/helper/test-find-pack.c | 35 ++++++ t/helper/test-tool.c | 1 + t/helper/test-tool.h | 1 + t/t5317-pack-objects-filter-objects.sh | 27 ++++ t/t6500-gc.sh | 23 ++++ t/t7700-repack.sh | 43 +++++++ 13 files changed, 345 insertions(+), 53 deletions(-) create mode 100644 t/helper/test-find-pack.c