203 lines
8.8 KiB
Markdown
203 lines
8.8 KiB
Markdown
# Repository Storage Types
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> [Introduced][ce-28283] in GitLab 10.0.
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Two different storage layouts can be used
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to store the repositories on disk and their characteristics.
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GitLab can be configured to use one or multiple repository shard locations
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that can be:
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- Mounted to the local disk
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- Exposed as an NFS shared volume
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- Acessed via [gitaly] on its own machine.
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In GitLab, this is configured in `/etc/gitlab/gitlab.rb` by the `git_data_dirs({})`
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configuration hash. The storage layouts discussed here will apply to any shard
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defined in it.
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The `default` repository shard that is available in any installations
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that haven't customized it, points to the local folder: `/var/opt/gitlab/git-data`.
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Anything discussed below is expected to be part of that folder.
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## Legacy Storage
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Legacy Storage is the storage behavior prior to version 10.0. For historical
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reasons, GitLab replicated the same mapping structure from the projects URLs:
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- Project's repository: `#{namespace}/#{project_name}.git`
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- Project's wiki: `#{namespace}/#{project_name}.wiki.git`
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This structure made it simple to migrate from existing solutions to GitLab and
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easy for Administrators to find where the repository is stored.
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On the other hand this has some drawbacks:
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Storage location will concentrate huge amount of top-level namespaces. The
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impact can be reduced by the introduction of [multiple storage
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paths][storage-paths].
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Because backups are a snapshot of the same URL mapping, if you try to recover a
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very old backup, you need to verify whether any project has taken the place of
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an old removed or renamed project sharing the same URL. This means that
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`mygroup/myproject` from your backup may not be the same original project that
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is at that same URL today.
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Any change in the URL will need to be reflected on disk (when groups / users or
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projects are renamed). This can add a lot of load in big installations,
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especially if using any type of network based filesystem.
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## Hashed Storage
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CAUTION: **Important:**
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Geo requires Hashed Storage since 12.0. If you haven't migrated yet,
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check the [migration instructions](#how-to-migrate-to-hashed-storage) ASAP.
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Hashed Storage is the new storage behavior we rolled out with 10.0. Instead
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of coupling project URL and the folder structure where the repository will be
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stored on disk, we are coupling a hash, based on the project's ID. This makes
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the folder structure immutable, and therefore eliminates any requirement to
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synchronize state from URLs to disk structure. This means that renaming a group,
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user, or project will cost only the database transaction, and will take effect
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immediately.
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The hash also helps to spread the repositories more evenly on the disk, so the
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top-level directory will contain less folders than the total amount of top-level
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namespaces.
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The hash format is based on the hexadecimal representation of SHA256:
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`SHA256(project.id)`. The top-level folder uses the first 2 characters, followed
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by another folder with the next 2 characters. They are both stored in a special
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`@hashed` folder, to be able to co-exist with existing Legacy Storage projects:
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```ruby
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# Project's repository:
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"@hashed/#{hash[0..1]}/#{hash[2..3]}/#{hash}.git"
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# Wiki's repository:
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"@hashed/#{hash[0..1]}/#{hash[2..3]}/#{hash}.wiki.git"
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```
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### Hashed object pools
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> [Introduced](https://gitlab.com/gitlab-org/gitaly/issues/1606) in GitLab 12.1.
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Forks of public projects are deduplicated by creating a third repository, the object pool, containing the objects from the source project. Using `objects/info/alternates`, the source project and forks use the object pool for shared objects. Objects are moved from the source project to the object pool when housekeeping is run on the source project.
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```ruby
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# object pool paths
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"@pools/#{hash[0..1]}/#{hash[2..3]}/#{hash}.git"
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```
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Object pools can be disabled using the `object_pools` feature flag, and can be
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disabled for individual projects by executing
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`Feature.disable(:object_pools, Project.find(<id>))`. Disabling object pools
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will not change existing deduplicated forks, but will prevent new forks from
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being deduplicated.
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DANGER: **Danger:**
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Do not run `git prune` or `git gc` in pool repositories! This can
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cause data loss in "real" repositories that depend on the pool in
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question.
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### How to migrate to Hashed Storage
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To start a migration, enable Hashed Storage for new projects:
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1. Go to **Admin > Settings > Repository** and expand the **Repository Storage** section.
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2. Select the **Use hashed storage paths for newly created and renamed projects** checkbox.
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Check if the change breaks any existing integration you may have that
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either runs on the same machine as your repositories are located, or may login to that machine
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to access data (for example, a remote backup solution).
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To schedule a complete rollout, see the
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[rake task documentation for storage migration][rake/migrate-to-hashed] for instructions.
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If you do have any existing integration, you may want to do a small rollout first,
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to validate. You can do so by specifying a range with the operation.
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This is an example of how to limit the rollout to Project IDs 50 to 100, running in
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an Omnibus Gitlab installation:
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```bash
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sudo gitlab-rake gitlab:storage:migrate_to_hashed ID_FROM=50 ID_TO=100
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```
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Check the [documentation][rake/migrate-to-hashed] for additional information and instructions for
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source-based installation.
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#### Rollback
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Similar to the migration, to disable Hashed Storage for new
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projects:
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1. Go to **Admin > Settings > Repository** and expand the **Repository Storage** section.
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2. Uncheck the **Use hashed storage paths for newly created and renamed projects** checkbox.
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To schedule a complete rollback, see the
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[rake task documentation for storage rollback](raketasks/storage.md#rollback-from-hashed-storage-to-legacy-storage) for instructions.
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The rollback task also supports specifying a range of Project IDs. Here is an example
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of limiting the rollout to Project IDs 50 to 100, in an Omnibus Gitlab installation:
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```bash
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sudo gitlab-rake gitlab:storage:rollback_to_legacy ID_FROM=50 ID_TO=100
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```
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If you have a Geo setup, please note that the rollback will not be reflected automatically
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on the **secondary** node. You may need to wait for a backfill operation to kick-in and remove
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the remaining repositories from the special `@hashed/` folder manually.
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### Hashed Storage coverage
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We are incrementally moving every storable object in GitLab to the Hashed
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Storage pattern. You can check the current coverage status below (and also see
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the [issue][ce-2821]).
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Note that things stored in an S3 compatible endpoint will not have the downsides
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mentioned earlier, if they are not prefixed with `#{namespace}/#{project_name}`,
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which is true for CI Cache and LFS Objects.
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| Storable Object | Legacy Storage | Hashed Storage | S3 Compatible | GitLab Version |
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| --------------- | -------------- | -------------- | ------------- | -------------- |
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| Repository | Yes | Yes | - | 10.0 |
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| Attachments | Yes | Yes | - | 10.2 |
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| Avatars | Yes | No | - | - |
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| Pages | Yes | No | - | - |
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| Docker Registry | Yes | No | - | - |
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| CI Build Logs | No | No | - | - |
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| CI Artifacts | No | No | Yes | 9.4 / 10.6 |
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| CI Cache | No | No | Yes | - |
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| LFS Objects | Yes | Similar | Yes | 10.0 / 10.7 |
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| Repository pools| No | Yes | - | 11.6 |
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#### Implementation Details
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##### Avatars
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Each file is stored in a folder with its `id` from the database. The filename is always `avatar.png` for user avatars.
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When avatar is replaced, `Upload` model is destroyed and a new one takes place with different `id`.
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##### CI Artifacts
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CI Artifacts are S3 compatible since **9.4** (GitLab Premium), and available in GitLab Core since **10.6**.
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##### LFS Objects
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LFS Objects implements a similar storage pattern using 2 chars, 2 level folders, following git own implementation:
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```ruby
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"shared/lfs-objects/#{oid[0..1}/#{oid[2..3]}/#{oid[4..-1]}"
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# Based on object `oid`: `8909029eb962194cfb326259411b22ae3f4a814b5be4f80651735aeef9f3229c`, path will be:
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"shared/lfs-objects/89/09/029eb962194cfb326259411b22ae3f4a814b5be4f80651735aeef9f3229c"
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```
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They are also S3 compatible since **10.0** (GitLab Premium), and available in GitLab Core since **10.7**.
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[ce-2821]: https://gitlab.com/gitlab-com/infrastructure/issues/2821
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[ce-28283]: https://gitlab.com/gitlab-org/gitlab-ce/issues/28283
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[rake/migrate-to-hashed]: raketasks/storage.md#migrate-existing-projects-to-hashed-storage
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[storage-paths]: repository_storage_types.md
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[gitaly]: gitaly/index.md
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