Bring back Rugged implementation of find_commit

This brings back some of the changes in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20339.

For users using Gitaly on top of NFS, accessing the Git data directly
via Rugged is more performant than Gitaly. This merge request introduces
the feature flag `rugged_find_commit` to activate Rugged paths.

There are also Rake tasks `gitlab:features:enable_rugged` and
`gitlab:features:disable_rugged` to enable/disable these feature
flags altogether.

Part of four Rugged changes identified in
https://gitlab.com/gitlab-org/gitlab-ce/issues/57317.

2 files changed, 60 insertions, 0 deletions

diff --git a/doc/administration/high_availability/nfs.md b/doc/administration/high_availability/nfs.md
index 74b0e2c8184..78ebf8a083b 100644
--- a/doc/administration/high_availability/nfs.md
+++ b/doc/administration/high_availability/nfs.md
@@ -37,6 +37,28 @@ options:
   circumstances it could lead to data loss if a failure occurs before data has
   synced.
 
+### Improving NFS performance with GitLab
+
+If you are using NFS to share Git data, we recommend that you enable a
+number of feature flags that will allow GitLab application processes to
+access Git data directly instead of going through the [Gitaly
+service](../gitaly/index.md). Depending on your workload and disk
+performance, these flags may help improve performance. See [the
+issue](https://gitlab.com/gitlab-org/gitlab-ce/issues/57317) for more
+details.
+
+To do this, run the Rake task:
+
+```sh
+gitlab-rake gitlab:features:enable_rugged
+```
+
+If you need to undo this setting for some reason, run:
+
+```sh
+gitlab-rake gitlab:features:disable_rugged
+```
+
 ### Known issues
 
 On some customer systems, we have seen NFS clients slow precipitously due to
diff --git a/doc/development/gitaly.md b/doc/development/gitaly.md
index d5fc403bf8b..d52920b9a5d 100644
--- a/doc/development/gitaly.md
+++ b/doc/development/gitaly.md
@@ -56,6 +56,44 @@ If your test-suite is failing with Gitaly issues, as a first step, try running:
 rm -rf tmp/tests/gitaly
 ```
 
+## Legacy Rugged code
+
+While Gitaly can handle all Git access, many of GitLab customers still
+run Gitaly atop NFS. The legacy Rugged implementation for Git calls may
+be faster than the Gitaly RPC due to N+1 Gitaly calls and other
+reasons. See [the
+issue](https://gitlab.com/gitlab-org/gitlab-ce/issues/57317) for more
+details.
+
+Until GitLab has eliminated most of these inefficiencies or the use of
+NFS is discontinued for Git data, Rugged implementations of some of the
+most commonly-used RPCs can be enabled via feature flags:
+
+* `rugged_find_commit`
+* `rugged_get_tree_entries`
+* `rugged_tree_entry`
+* `rugged_commit_is_ancestor`
+
+A convenience Rake task can be used to enable or disable these flags
+all together. To enable:
+
+```sh
+bundle exec rake gitlab:features:enable_rugged
+```
+
+To disable:
+
+```sh
+bundle exec rake gitlab:features:disable_rugged
+```
+
+Most of this code exists in the `lib/gitlab/git/rugged_impl` directory.
+
+NOTE: **Note:** You should NOT need to add or modify code related to
+Rugged unless explicitly discussed with the [Gitaly
+Team](https://gitlab.com/groups/gl-gitaly/group_members). This code will
+NOT work on GitLab.com or other GitLab instances that do not use NFS.
+
 ## `TooManyInvocationsError` errors
 
 During development and testing, you may experience `Gitlab::GitalyClient::TooManyInvocationsError` failures.