Merge request diffs storage

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Merge request diffs are size-limited copies of diffs associated with merge requests. When viewing a merge request, diffs are sourced from these copies wherever possible as a performance optimization.

By default, merge request diffs are stored in the database, in a table named merge_request_diff_files. Larger installations may find this table grows too large, in which case, switching to external storage is recommended.

Merge request diffs can be stored on disk, or in object storage. In general, it is better to store the diffs in the database than on disk. A compromise is available that only stores outdated diffs outside of database.

Using external storage

::Tabs

:::TabTitle Linux package (Omnibus)

Edit /etc/gitlab/gitlab.rb and add the following line:
```
gitlab_rails['external_diffs_enabled'] = true
```
The external diffs are stored in /var/opt/gitlab/gitlab-rails/shared/external-diffs. To change the path, for example, to /mnt/storage/external-diffs, edit /etc/gitlab/gitlab.rb and add the following line:
```
gitlab_rails['external_diffs_storage_path'] = "/mnt/storage/external-diffs"
```
Save the file and reconfigure GitLab for the changes to take effect. GitLab then migrates your existing merge request diffs to external storage.

:::TabTitle Self-compiled (source)

Edit /home/git/gitlab/config/gitlab.yml and add or amend the following lines:
```
external_diffs:
  enabled: true
```
The external diffs are stored in /home/git/gitlab/shared/external-diffs. To change the path, for example, to /mnt/storage/external-diffs, edit /home/git/gitlab/config/gitlab.yml and add or amend the following lines:
```
external_diffs:
  enabled: true
  storage_path: /mnt/storage/external-diffs
```
Save the file and restart GitLab for the changes to take effect. GitLab then migrates your existing merge request diffs to external storage.

::EndTabs

Using object storage

WARNING: Migrating to object storage is not reversible.

Instead of storing the external diffs on disk, we recommended the use of an object store like AWS S3 instead. This configuration relies on valid AWS credentials to be configured already.

::Tabs

:::TabTitle Linux package (Omnibus)

Edit /etc/gitlab/gitlab.rb and add the following line:
```
gitlab_rails['external_diffs_enabled'] = true
```
Set object storage settings.
Save the file and reconfigure GitLab for the changes to take effect. GitLab then migrates your existing merge request diffs to external storage.

:::TabTitle Self-compiled (source)

Edit /home/git/gitlab/config/gitlab.yml and add or amend the following lines:
```
external_diffs:
  enabled: true
```
Set object storage settings.
Save the file and restart GitLab for the changes to take effect. GitLab then migrates your existing merge request diffs to external storage.

::EndTabs

Object Storage Settings

You should use the consolidated object storage settings.

Alternative in-database storage

Enabling external diffs may reduce the performance of merge requests, as they must be retrieved in a separate operation to other data. A compromise may be reached by only storing outdated diffs externally, while keeping current diffs in the database.

To enable this feature, perform the following steps:

::Tabs

:::TabTitle Linux package (Omnibus)

Edit /etc/gitlab/gitlab.rb and add the following line:
```
gitlab_rails['external_diffs_when'] = 'outdated'
```
Save the file and reconfigure GitLab for the changes to take effect.

:::TabTitle Self-compiled (source)

Edit /home/git/gitlab/config/gitlab.yml and add or amend the following lines:
```
external_diffs:
  enabled: true
  when: outdated
```
Save the file and restart GitLab for the changes to take effect.

::EndTabs

With this feature enabled, diffs are initially stored in the database, rather than externally. They are moved to external storage after any of these conditions become true:

A newer version of the merge request diff exists
The merge request was merged more than seven days ago
The merge request was closed more than seven day ago

These rules strike a balance between space and performance by only storing frequently-accessed diffs in the database. Diffs that are less likely to be accessed are moved to external storage instead.

Switching from external storage to object storage

Automatic migration moves diffs stored in the database, but it does not move diffs between storage types. To switch from external storage to object storage:

Move files stored on local or NFS storage to object storage manually.

Run this Rake task to change their location in the database.

For Linux package installations:

sudo gitlab-rake gitlab:external_diffs:force_object_storage

For self-compiled installations:

sudo -u git -H bundle exec rake gitlab:external_diffs:force_object_storage RAILS_ENV=production

By default, sudo does not preserve existing environment variables. You should append them, rather than prefix them, like this:

sudo gitlab-rake gitlab:external_diffs:force_object_storage START_ID=59946109 END_ID=59946109 UPDATE_DELAY=5

These environment variables modify the behavior of the Rake task:

Name	Default value	Purpose
`ANSI`	`true`	Use ANSI escape codes to make output more understandable.
`BATCH_SIZE`	`1000`	Iterate through the table in batches of this size.
`START_ID`	`nil`	If set, begin scanning at this ID.
`END_ID`	`nil`	If set, stop scanning at this ID.
`UPDATE_DELAY`	`1`	Number of seconds to sleep between updates.

START_ID and END_ID can be used to run the update in parallel, by assigning different processes to different parts of the table.
BATCH and UPDATE_DELAY enable the speed of the migration to be traded off against concurrent access to the table.
ANSI should be set to false if your terminal does not support ANSI escape codes.