How Git object deduplication works in GitLab
When a GitLab user forks a project, GitLab creates a new Project with an associated Git repository that is a copy of the original project at the time of the fork. If a large project gets forked often, this can lead to a quick increase in Git repository storage disk use. To counteract this problem, we are adding Git object deduplication for forks to GitLab. In this document, we describe how GitLab implements Git object deduplication.
Understanding Git alternates
At the Git level, we achieve deduplication by using Git alternates. Git alternates is a mechanism that lets a repository borrow objects from another repository on the same machine.
If we want repository A to borrow from repository B, we first write a
path that resolves to
B.git/objects in the special file
A.git/objects/info/alternates. This establishes the alternates link.
Next, we must perform a Git repack in A. After the repack, any objects
that are duplicated between A and B are deleted from A. Repository
A is now no longer self-contained, but it still has its own refs and
configuration. Objects in A that are not in B remain in A. For this
to work, it is of course critical that no objects ever get deleted from
B because A might need them.
Do not run
git prune or
git gc in object pool repositories, which are
stored in the
@pools directory. This can cause data loss in the regular
repositories that depend on the object pool.
The danger lies in
git prune, and
git gc calls
git prune. The
problem is that
git prune, when running in a pool repository, cannot
reliable decide if an object is no longer needed.
Git alternates in GitLab: pool repositories
GitLab organizes this object borrowing by creating special pool repositories which are hidden from the user. We then use Git alternates to let a collection of project repositories borrow from a single pool repository. We call such a collection of project repositories a pool. Pools form star-shaped networks of repositories that borrow from a single pool, which resemble (but not be identical to) the fork networks that get formed when users fork projects.
At the Git level, pool repositories are created and managed using Gitaly RPC calls. Just like with normal repositories, the authority on which pool repositories exist, and which repositories borrow from them, lies at the Rails application level in SQL.
In conclusion, we need three things for effective object deduplication across a collection of GitLab project repositories at the Git level:
- A pool repository must exist.
- The participating project repositories must be linked to the pool
repository via their respective
- The pool repository must contain Git object data common to the participating project repositories.
The effectiveness of Git object deduplication in GitLab depends on the amount of overlap between the pool repository and each of its participants. Each time garbage collection runs on the source project, Git objects from the source project are migrated to the pool repository. One by one, as garbage collection runs, other member projects benefit from the new objects that got added to the pool.
As of GitLab 11.8, project repositories in GitLab do not have their own
SQL table. They are indirectly identified by columns on the
table. In other words, the only way to look up a project repository is to
first look up its project, and then call
With pool repositories we made a fresh start. These live in their own
pool_repositories SQL table. The relations between these two tables
are as follows:
Projectbelongs to at most one
- as an automatic consequence of the above, a
PoolRepositoryhas exactly one "source
TODO Fix invalid SQL data for pools created prior to GitLab 11.11 https://gitlab.com/gitlab-org/gitaly/-/issues/1653.
- All repositories in a pool must use hashed storage.
This is so that we don't have to ever worry about updating paths in
- All repositories in a pool must be on the same Gitaly storage shard. The Git alternates mechanism relies on direct disk access across multiple repositories, and we can only assume direct disk access to be possible within a Gitaly storage shard.
- The only two ways to remove a member project from a pool are (1) to delete the project or (2) to move the project to another Gitaly storage shard.
Creating pools and pool memberships
When a pool gets created, it must have a source project. The initial contents of the pool repository are a Git clone of the source project repository.
The occasion for creating a pool is when an existing eligible (non-private, hashed storage, non-forked) GitLab project gets forked and this project does not belong to a pool repository yet. The fork parent project becomes the source project of the new pool, and both the fork parent and the fork child project become members of the new pool.
Once project A has become the source project of a pool, all future eligible forks of A become pool members.
If the fork source is itself a fork, the resulting repository will neither join the repository nor is a new pool repository seeded.
Suppose fork A is part of a pool repository, any forks created off of fork A are not a part of the pool repository that fork A is a part of.
Suppose B is a fork of A, and A does not belong to an object pool. Now C gets created as a fork of B. C is not part of a pool repository.
TODO should forks of forks be deduplicated? https://gitlab.com/gitlab-org/gitaly/-/issues/1532
- If a normal Project participating in a pool gets moved to another Gitaly storage shard, its "belongs to PoolRepository" relation will be broken. Because of the way moving repositories between shard is implemented, we get a fresh self-contained copy of the project's repository on the new storage shard.
- If the source project of a pool gets moved to another Gitaly storage shard or is deleted the "source project" relation is not broken. However, as of GitLab 12.0 a pool does not fetch from a source unless the source is on the same Gitaly shard.
Consistency between the SQL pool relation and Gitaly
As far as Gitaly is concerned, the SQL pool relations make two types of claims about the state of affairs on the Gitaly server: pool repository existence, and the existence of an alternates connection between a repository and a pool.
If GitLab thinks a pool repository exists (that is, it exists according to SQL), but it does not on the Gitaly server, then it is created on the fly by Gitaly.
Pool relation existence
There are three different things that can go wrong here.
1. SQL says repository A belongs to pool P but Gitaly says A has no alternate objects
In this case, we miss out on disk space savings but all RPCs on A
itself function fine. The next time garbage collection runs on A,
the alternates connection gets established in Gitaly. This is done by
Projects::GitDeduplicationService in GitLab Rails.
2. SQL says repository A belongs to pool P1 but Gitaly says A has alternate objects in pool P2
In this case
Projects::GitDeduplicationService throws an exception.
3. SQL says repository A does not belong to any pool but Gitaly says A belongs to P
In this case
Projects::GitDeduplicationService tries to
"re-duplicate" the repository A using the DisconnectGitAlternates RPC.
Git object deduplication and GitLab Geo
When a pool repository record is created in SQL on a Geo primary, this eventually triggers an event on the Geo secondary. The Geo secondary then creates the pool repository in Gitaly. This leads to an "eventually consistent" situation because as each pool participant gets synchronized, Geo eventually triggers garbage collection in Gitaly on the secondary, at which stage Git objects are deduplicated.
TODO How do we handle the edge case where at the time the Geo secondary tries to create the pool repository, the source project does not exist? https://gitlab.com/gitlab-org/gitaly/-/issues/1533