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author | Jacob Vosmaer <jacob@gitlab.com> | 2019-06-12 10:12:15 +0300 |
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committer | Achilleas Pipinellis <axil@gitlab.com> | 2019-06-12 10:12:15 +0300 |
commit | 45c5c2aad628242b88e0938e21a40d30299e3d81 (patch) | |
tree | 84a0a35d1edadd3038aac189433c1c355633e3ea /doc/development | |
parent | 9f35bf3a5c87fcc68194b2fe343741110a958600 (diff) |
Update git object deduplication overview
Diffstat (limited to 'doc/development')
-rw-r--r-- | doc/development/git_object_deduplication.md | 109 |
1 files changed, 33 insertions, 76 deletions
diff --git a/doc/development/git_object_deduplication.md b/doc/development/git_object_deduplication.md index 81c5f69c7b8..b512d7611d3 100644 --- a/doc/development/git_object_deduplication.md +++ b/doc/development/git_object_deduplication.md @@ -10,10 +10,10 @@ GitLab implements Git object deduplication. ## Enabling Git object deduplication via feature flags -As of GitLab 11.9, Git object deduplication in GitLab is in beta. In this -document, you can read about the caveats of enabling the feature. Also, -note that Git object deduplication is limited to forks of public -projects on hashed repository storage. +As of GitLab 12.0, Git object deduplication in GitLab is still behind a +feature flag. In this document, you can read about the effects of +enabling the feature. Also, note that Git object deduplication is +limited to forks of public projects on hashed repository storage. You can enable deduplication globally by setting the `object_pools` feature flag to `true`: @@ -51,6 +51,15 @@ configuration. Objects in A that are not in B will 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. +DANGER: **Danger:** +Do not run `git prune` or `git gc` in pool repositories! This can +cause data loss in "real" repositories that depend on the pool in +question. + +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 @@ -80,43 +89,10 @@ across a collection of GitLab project repositories at the Git level: The effectiveness of Git object deduplication in GitLab depends on the amount of overlap between the pool repository and each of its -participants. As of GitLab 11.9, we have a somewhat optimistic system. -The only data that will be deduplicated is the data in the source -project repository at the time the pool repository is created. That is, -the data in the source project at the time of the first fork *after* the -deduplication feature has been enabled. - -When we enable the object deduplication feature for -gitlab.com/gitlab-org/gitlab-ce, which is about 1GB at the time of -writing, all new forks of that project would be 1GB smaller than they -would have been without Git object deduplication. So even in its current -optimistic form, we expect Git object deduplication in GitLab to make a -difference. - -However, if a lot of Git objects get added to the project repositories -in a pool after the pool repository was created these new Git objects -will currently (GitLab 11.9) not get deduplicated. Over time, the -deduplication factor of the pool will get worse and worse. - -As an extreme example, if we create an empty repository A, and fork that -to repository B, behind the scenes we get an object pool P with no -objects in it at all. If we then push 1GB of Git data to A, and push the -same Git data to B, it will not get deduplicated, because that data was -not in A at the time P was created. - -This also matters in less extreme examples. Consider a pool P with -source project A and 500 active forks B1, B2,...,B500. Suppose, -optimistically, that the forks are fully deduplicated at the start of -our scenario. Now some time passes and 200MB of new Git data gets added -to project A. Because of the forking workflow, this data makes also its way -into the forks B1, ..., B500. That means we would now have 100GB of Git -data sitting around (500 \* 200MB) across the forks, that could have -been deduplicated. But because of the way we do deduplication this new -data will not be deduplicated. - -> TODO Add periodic maintenance of object pools to prevent gradual loss -> of deduplication over time. -> https://gitlab.com/groups/gitlab-org/-/epics/524 +participants. Each time garbage collection runs on the source project, +Git objects from the source project will get migrated to the pool +repository. One by one, as garbage collection runs, other member +projects will benefit from the new objects that got added to the pool. ## SQL model @@ -136,6 +112,9 @@ are as follows: - a `PoolRepository` has exactly one "source `Project`" (`pool.source_project`) +> TODO Fix invalid SQL data for pools created prior to GitLab 11.11 +> https://gitlab.com/gitlab-org/gitaly/issues/1653. + ### Assumptions - All repositories in a pool must use [hashed @@ -146,10 +125,6 @@ are as follows: 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. -- All project repositories in a pool must have "Public" visibility in - GitLab at the time they join. There are gotchas around visibility of - Git objects across alternates links. This restriction is a defense - against accidentally leaking private Git data. - 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. @@ -187,17 +162,14 @@ are as follows: ### Consequences - If a normal Project participating in a pool gets moved to another - Gitaly storage shard, its "belongs to PoolRepository" relation must + Gitaly storage shard, its "belongs to PoolRepository" relation will be broken. Because of the way moving repositories between shard is implemented, we will automatically 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, we may have to break the "PoolRepository has - one source Project" relation? - -> TODO What happens, or should happen, if a source project changes -> visibility, is deleted, or moves to another storage shard? -> https://gitlab.com/gitlab-org/gitaly/issues/1488 + shard or is deleted the "source project" relation is not broken. + However, as of GitLab 12.0 a pool will not fetch from a source + unless the source is on the same Gitaly shard. ## Consistency between the SQL pool relation and Gitaly @@ -209,16 +181,8 @@ repository and a pool. ### Pool existence If GitLab thinks a pool repository exists (i.e. it exists according to -SQL), but it does not on the Gitaly server, then certain RPC calls that -take the object pool as an argument will fail. - -> TODO What happens if SQL says the pool repo exists but Gitaly says it -> does not? https://gitlab.com/gitlab-org/gitaly/issues/1533 - -If GitLab thinks a pool does not exist, while it does exist on disk, -that has no direct consequences on its own. However, if other -repositories on disk borrow objects from this unknown pool repository -then we risk data loss, see below. +SQL), but it does not on the Gitaly server, then it will be created on +the fly by Gitaly. ### Pool relation existence @@ -226,26 +190,19 @@ There are three different things that can go wrong here. #### 1. SQL says repo 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 RPC's on A itself -will function fine. As long as Git can find all its objects, it does not -matter exactly where those objects are. +In this case, we miss out on disk space savings but all RPC's on A +itself will 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 repo A belongs to pool P1 but Gitaly says A has alternate objects in pool P2 -If we are not careful, this situation can lead to data loss. During some -operations (repository maintenance), GitLab will try to re-link A to its -pool P1. If this clobbers the existing link to P2, then A will loose Git -objects and become invalid. - -Also, keep in mind that if GitLab's database got messed up, it may not -even know that P2 exists. - -> TODO Ensure that Gitaly will not clobber existing, unexpected -> alternates links. https://gitlab.com/gitlab-org/gitaly/issues/1534 +In this case `Projects::GitDeduplicationService` will throw an exception. #### 3. SQL says repo A does not belong to any pool but Gitaly says A belongs to P -This has the same data loss possibility as scenario 2 above. +In this case `Projects::GitDeduplicationService` will try to +"re-duplicate" the repository A using the DisconnectGitAlternates RPC. ## Git object deduplication and GitLab Geo |