Merge request diffs storage (CORE ONLY)

Introduced in GitLab 11.8.

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.

Using external storage

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.

To enable external storage of merge request diffs, follow the instructions below.

In Omnibus installations:

Edit /etc/gitlab/gitlab.rb and add the following line:
```
gitlab_rails['external_diffs_enabled'] = true
```
The external diffs will be stored in 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.

In installations from source:

Edit /home/git/gitlab/config/gitlab.yml and add or amend the following lines:
```
external_diffs:
  enabled: true
```
The external diffs will be 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.

Using object storage

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.

Object Storage Settings

For source installations, these settings are nested under external_diffs: and then object_store:. On Omnibus installations, they are prefixed by external_diffs_object_store_.

Setting	Description	Default
`enabled`	Enable/disable object storage	`false`
`remote_directory`	The bucket name where external diffs will be stored
`direct_upload`	Set to true to enable direct upload of external diffs without the need of local shared storage. Option may be removed once we decide to support only single storage for all files.	`false`
`background_upload`	Set to false to disable automatic upload. Option may be removed once upload is direct to S3	`true`
`proxy_download`	Set to true to enable proxying all files served. Option allows to reduce egress traffic as this allows clients to download directly from remote storage instead of proxying all data	`false`
`connection`	Various connection options described below

S3 compatible connection settings

The connection settings match those provided by Fog, and are as follows:

Setting	Description	Default
`provider`	Always `AWS` for compatible hosts	AWS
`aws_access_key_id`	AWS credentials, or compatible
`aws_secret_access_key`	AWS credentials, or compatible
`aws_signature_version`	AWS signature version to use. 2 or 4 are valid options. Digital Ocean Spaces and other providers may need 2.	4
`enable_signature_v4_streaming`	Set to true to enable HTTP chunked transfers with AWS v4 signatures (https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming.html). Oracle Cloud S3 needs this to be false	true
`region`	AWS region	us-east-1
`host`	S3 compatible host for when not using AWS, e.g. `localhost` or `storage.example.com`	s3.amazonaws.com
`endpoint`	Can be used when configuring an S3 compatible service such as Minio, by entering a URL such as `http://127.0.0.1:9000`	(optional)
`path_style`	Set to true to use `host/bucket_name/object` style paths instead of `bucket_name.host/object`. Leave as false for AWS S3	false
`use_iam_profile`	Set to true to use IAM profile instead of access keys	false

In Omnibus installations:

Edit /etc/gitlab/gitlab.rb and add the following lines by replacing with the values you want:

gitlab_rails['external_diffs_enabled'] = true
gitlab_rails['external_diffs_object_store_enabled'] = true
gitlab_rails['external_diffs_object_store_remote_directory'] = "external-diffs"
gitlab_rails['external_diffs_object_store_connection'] = {
  'provider' => 'AWS',
  'region' => 'eu-central-1',
  'aws_access_key_id' => 'AWS_ACCESS_KEY_ID',
  'aws_secret_access_key' => 'AWS_SECRET_ACCESS_KEY'
}

Note that, if you are using AWS IAM profiles, be sure to omit the AWS access key and secret access key/value pairs. For example:

gitlab_rails['external_diffs_object_store_connection'] = {
  'provider' => 'AWS',
  'region' => 'eu-central-1',
  'use_iam_profile' => true
}

Save the file and reconfigure GitLab for the changes to take effect.

In installations from source:

Edit /home/git/gitlab/config/gitlab.yml and add or amend the following lines:

external_diffs:
  enabled: true
  object_store:
    enabled: true
    remote_directory: "external-diffs" # The bucket name
    connection:
      provider: AWS # Only AWS supported at the moment
      aws_access_key_id: AWS_ACCESS_KEY_ID
      aws_secret_access_key: AWS_SECRET_ACCESS_KEY
      region: eu-central-1

Save the file and restart GitLab for the changes to take effect.

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:

In Omnibus installations:

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.

In installations from 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.

With this feature enabled, diffs will initially stored in the database, rather than externally. They will be moved to external storage once 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.