Add documentation on how to configure Redis Sentinel by persistent class

doc/administration/high_availability/redis.md

@@ -650,6 +650,47 @@ gitlab_rails['redis_sentinels'] = [
 Omnibus GitLab configures some things behind the curtains to make the sysadmins'
 lives easier. If you want to know what happens underneath keep reading.
 
+### Running multiple Redis clusters
+
+GitLab supports running [separate Redis clusters for different persistent
+classes](https://docs.gitlab.com/omnibus/settings/redis.html#running-with-multiple-redis-instances):
+cache, queues, and shared_state. To make this work with Sentinel:
+
+1. Set the appropriate variable in `/etc/gitlab/gitlab.rb` for each instance you are using:
+
+    ```ruby
+    gitlab_rails['redis_cache_instance'] = REDIS_CACHE_URL
+    gitlab_rails['redis_queues_instance'] = REDIS_QUEUES_URL
+    gitlab_rails['redis_shared_state_instance'] = REDIS_SHARED_STATE_URL
+    ```
+    **Note**: Redis URLs should be in the format: `redis://:PASSWORD@SENTINEL_MASTER_NAME`
+
+    1. PASSWORD is the plaintext password for the Redis instance
+    2. SENTINEL_MASTER_NAME is the Sentinel master name (e.g. `gitlab-redis-cache`)
+1. Include an array of hashes with host/port combinations, such as the following:
+
+    ```ruby
+    gitlab_rails['redis_cache_sentinels'] = [
+      { host: REDIS_CACHE_SENTINEL_HOST, port: PORT1 },
+      { host: REDIS_CACHE_SENTINEL_HOST2, port: PORT2 }
+    ]
+    gitlab_rails['redis_queues_sentinels'] = [
+      { host: REDIS_QUEUES_SENTINEL_HOST, port: PORT1 },
+      { host: REDIS_QUEUES_SENTINEL_HOST2, port: PORT2 }
+    ]
+    gitlab_rails['redis_shared_state_sentinels'] = [
+      { host: SHARED_STATE_SENTINEL_HOST, port: PORT1 },
+      { host: SHARED_STATE_SENTINEL_HOST2, port: PORT2 }
+    ]
+    ```
+1. Note that for each persistence class, GitLab will default to using the
+   configuration specified in `gitlab_rails['redis_sentinels']` unless
+   overriden by the settings above.
+1. Be sure to include BOTH configuration options for each persistent classes. For example,
+   if you choose to configure a cache instance, you must specify both `gitlab_rails['redis_cache_instance']`
+   and `gitlab_rails['redis_cache_sentinels']` for GitLab to generate the proper configuration files.
+1. Run `gitlab-ctl reconfigure`
+
 ### Control running services
 
 In the previous example, we've used `redis_sentinel_role` and