Repository Storage Types

Introduced in GitLab 10.0.

Two different storage layouts can be used to store the repositories on disk and their characteristics.

GitLab can be configured to use one or multiple repository shard locations that can be:

  • Mounted to the local disk
  • Exposed as an NFS shared volume
  • Acessed via gitaly on its own machine.

In GitLab, this is configured in /etc/gitlab/gitlab.rb by the git_data_dirs({}) configuration hash. The storage layouts discussed here will apply to any shard defined in it.

The default repository shard that is available in any installations that haven’t customized it, points to the local folder: /var/opt/gitlab/git-data. Anything discussed below is expected to be part of that folder.

Legacy Storage

Legacy Storage is the storage behavior prior to version 10.0. For historical reasons, GitLab replicated the same mapping structure from the projects URLs:

  • Project’s repository: #{namespace}/#{project_name}.git
  • Project’s wiki: #{namespace}/#{project_name}.wiki.git

This structure made it simple to migrate from existing solutions to GitLab and easy for Administrators to find where the repository is stored.

On the other hand this has some drawbacks:

Storage location will concentrate huge amount of top-level namespaces. The impact can be reduced by the introduction of multiple storage paths.

Because backups are a snapshot of the same URL mapping, if you try to recover a very old backup, you need to verify whether any project has taken the place of an old removed or renamed project sharing the same URL. This means that mygroup/myproject from your backup may not be the same original project that is at that same URL today.

Any change in the URL will need to be reflected on disk (when groups / users or projects are renamed). This can add a lot of load in big installations, especially if using any type of network based filesystem.

Caution: For Geo in particular: Geo does work with legacy storage, but in some edge cases due to race conditions it can lead to errors when a project is renamed multiple times in short succession, or a project is deleted and recreated under the same name very quickly. We expect these race events to be rare, and we have not observed a race condition side-effect happening yet. This pattern also exists in other objects stored in GitLab, like issue Attachments, GitLab Pages artifacts, Docker Containers for the integrated Registry, etc. Hashed storage is a requirement for Geo.

Hashed Storage

Hashed Storage is the new storage behavior we rolled out with 10.0. Instead of coupling project URL and the folder structure where the repository will be stored on disk, we are coupling a hash, based on the project’s ID. This makes the folder structure immutable, and therefore eliminates any requirement to synchronize state from URLs to disk structure. This means that renaming a group, user, or project will cost only the database transaction, and will take effect immediately.

The hash also helps to spread the repositories more evenly on the disk, so the top-level directory will contain less folders than the total amount of top-level namespaces.

The hash format is based on the hexadecimal representation of SHA256: SHA256(project.id). The top-level folder uses the first 2 characters, followed by another folder with the next 2 characters. They are both stored in a special @hashed folder, to be able to co-exist with existing Legacy Storage projects:

# Project's repository:
"@hashed/#{hash[0..1]}/#{hash[2..3]}/#{hash}.git"

# Wiki's repository:
"@hashed/#{hash[0..1]}/#{hash[2..3]}/#{hash}.wiki.git"

Hashed object pools

Beta: Hashed objects pools are considered beta, and are not ready for production use. Follow gitaly#1548 for updates.

For deduplication of public forks and their parent repository, objects are pooled in an object pool. These object pools are a third repository where shared objects are stored.

# object pool paths
"@pools/#{hash[0..1]}/#{hash[2..3]}/#{hash}.git"

The object pool feature is behind the object_pools feature flag, and can be enabled for individual projects by executing Feature.enable(:object_pools, Project.find(<id>)). Note that the project has to be on hashed storage, should not be a fork itself, and hashed storage should be enabled for all new projects.

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.

How to migrate to Hashed Storage

To start a migration, enable Hashed Storage for new projects:

  1. Go to Admin > Settings > Repository and expand the Repository Storage section.
  2. Select the Use hashed storage paths for newly created and renamed projects checkbox.

Check if the change breaks any existing integration you may have that either runs on the same machine as your repositories are located, or may login to that machine to access data (for example, a remote backup solution).

To schedule a complete rollout, see the rake task documentation for storage migration for instructions.

If you do have any existing integration, you may want to do a small rollout first, to validate. You can do so by specifying a range with the operation.

This is an example of how to limit the rollout to Project IDs 50 to 100, running in an Omnibus Gitlab installation:

sudo gitlab-rake gitlab:storage:migrate_to_hashed ID_FROM=50 ID_TO=100

Check the documentation for additional information and instructions for source-based installation.

Rollback

Similar to the migration, to disable Hashed Storage for new projects:

  1. Go to Admin > Settings > Repository and expand the Repository Storage section.
  2. Uncheck the Use hashed storage paths for newly created and renamed projects checkbox.

To schedule a complete rollback, see the rake task documentation for storage rollback for instructions.

The rollback task also supports specifying a range of Project IDs. Here is an example of limiting the rollout to Project IDs 50 to 100, in an Omnibus Gitlab installation:

sudo gitlab-rake gitlab:storage:rollback_to_legacy ID_FROM=50 ID_TO=100

If you have a Geo setup, please note that the rollback will not be reflected automatically on the secondary node. You may need to wait for a backfill operation to kick-in and remove the remaining repositories from the special @hashed/ folder manually.

Hashed Storage coverage

We are incrementally moving every storable object in GitLab to the Hashed Storage pattern. You can check the current coverage status below (and also see the issue).

Note that things stored in an S3 compatible endpoint will not have the downsides mentioned earlier, if they are not prefixed with #{namespace}/#{project_name}, which is true for CI Cache and LFS Objects.

Storable Object Legacy Storage Hashed Storage S3 Compatible GitLab Version
Repository Yes Yes - 10.0
Attachments Yes Yes - 10.2
Avatars Yes No - -
Pages Yes No - -
Docker Registry Yes No - -
CI Build Logs No No - -
CI Artifacts No No Yes 9.4 / 10.6
CI Cache No No Yes -
LFS Objects Yes Similar Yes 10.0 / 10.7
Repository pools No Yes - 11.6

Implementation Details

Avatars

Each file is stored in a folder with its id from the database. The filename is always avatar.png for user avatars. When avatar is replaced, Upload model is destroyed and a new one takes place with different id.

CI Artifacts

CI Artifacts are S3 compatible since 9.4 (GitLab Premium), and available in GitLab Core since 10.6.

LFS Objects

LFS Objects implements a similar storage pattern using 2 chars, 2 level folders, following git own implementation:

"shared/lfs-objects/#{oid[0..1}/#{oid[2..3]}/#{oid[4..-1]}"

# Based on object `oid`: `8909029eb962194cfb326259411b22ae3f4a814b5be4f80651735aeef9f3229c`, path will be:
"shared/lfs-objects/89/09/029eb962194cfb326259411b22ae3f4a814b5be4f80651735aeef9f3229c"

They are also S3 compatible since 10.0 (GitLab Premium), and available in GitLab Core since 10.7.