Troubleshooting Redis

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There are a lot of moving parts that must be taken care carefully in order for the HA setup to work as expected.

Before proceeding with the troubleshooting below, check your firewall rules:

• Redis machines
  • Accept TCP connection in 6379
  • Connect to the other Redis machines via TCP in 6379
• Sentinel machines
  • Accept TCP connection in 26379
  • Connect to other Sentinel machines via TCP in 26379
  • Connect to the Redis machines via TCP in 6379

Basic Redis activity check

Start Redis troubleshooting with a basic Redis activity check:

1. Open a terminal on your GitLab server.
2. Run gitlab-redis-cli --stat and observe the output while it runs.
3. Go to your GitLab UI and browse to a handful of pages. Any page works, such as group or project overviews, issues, or files in repositories.
4. Check the stat output again and verify that the values for keys, clients, requests, and connections increases as you browse. If the numbers go up, basic Redis functionality is working and GitLab can connect to it.

Troubleshooting Redis replication

You can check if everything is correct by connecting to each server using redis-cli application, and sending the info replication command as below.

/opt/gitlab/embedded/bin/redis-cli -h <redis-host-or-ip> -a '<redis-password>' info replication

When connected to a Primary Redis, you see the number of connected replicas, and a list of each with connection details:

# Replication
role:master
connected_replicas:1
replica0:ip=10.133.5.21,port=6379,state=online,offset=208037514,lag=1
master_repl_offset:208037658
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:206989083
repl_backlog_histlen:1048576

When it's a replica, you see details of the primary connection and if its up or down:

# Replication
role:replica
master_host:10.133.1.58
master_port:6379
master_link_status:up
master_last_io_seconds_ago:1
master_sync_in_progress:0
replica_repl_offset:208096498
replica_priority:100
replica_read_only:1
connected_replicas:0
master_repl_offset:0
repl_backlog_active:0
repl_backlog_size:1048576
repl_backlog_first_byte_offset:0
repl_backlog_histlen:0

High CPU usage on Redis instance

By default, GitLab uses over 600 Sidekiq queues, each stored as a Redis list. Each Sidekiq thread issues a BRPOP command with all the queues listed in a long string. Redis CPU utilization grows as the number of queues and the rate of BRPOP calls increases. If your GitLab instance has many Sidekiq processes, this can cause Redis CPU utilization to approach 100%. High CPU utilization degrades GitLab performance significantly.

To reduce CPU usage on Redis caused by Sidekiq you can both:

• Use routing rules to reduce the number of Sidekiq queues.
• If you are using GitLab 16.6 and earlier, increase the SIDEKIQ_SEMI_RELIABLE_FETCH_TIMEOUT environment variable to improve CPU usage on Redis. On GitLab 16.7 and later, the default value is 5, which should be sufficient.

The SIDEKIQ_SEMI_RELIABLE_FETCH_TIMEOUT option reduces the overhead that tearing down and connecting causes, but increase the shutdown delay of Sidekiq.

Troubleshooting Sentinel

If you get an error like: Redis::CannotConnectError: No sentinels available., there may be something wrong with your configuration files or it can be related to this issue.

You must make sure you are defining the same value in redis['master_name'] and redis['master_password'] as you defined for your sentinel node.

The way the Redis connector redis-rb works with sentinel is a bit non-intuitive. We try to hide the complexity in the Linux package, but it still requires a few extra configurations.

To make sure your configuration is correct:

1. SSH into your GitLab application server

2. Enter the Rails console:

   # For Omnibus installations
   sudo gitlab-rails console
   
   # For source installations
   sudo -u git rails console -e production

3. Run in the console:

   redis = Gitlab::Redis::SharedState.redis
   redis.info

   Keep this screen open, and proceed to trigger a failover as described below.

4. To trigger a failover on the primary Redis, SSH into the Redis server and run:

   # port must match your primary redis port, and the sleep time must be a few seconds bigger than defined one
    redis-cli -h localhost -p 6379 DEBUG sleep 20

   WARNING: This action affects services, and takes the instance down for up to 20 seconds. If successful, it should recover after that.

5. Then back in the Rails console from the first step, run:

   redis.info

   You should see a different port after a few seconds delay (the failover/reconnect time).

Troubleshooting a non-bundled Redis with a self-compiled installation

If you get an error in GitLab like Redis::CannotConnectError: No sentinels available., there may be something wrong with your configuration files or it can be related to this upstream issue.

You must make sure that resque.yml and sentinel.conf are configured correctly, otherwise redis-rb does not work properly.

The master-group-name (gitlab-redis) defined in (sentinel.conf) must be used as the hostname in GitLab (resque.yml):

# sentinel.conf:
sentinel monitor gitlab-redis 10.0.0.1 6379 2
sentinel down-after-milliseconds gitlab-redis 10000
sentinel config-epoch gitlab-redis 0
sentinel leader-epoch gitlab-redis 0

# resque.yaml
production:
  url: redis://:myredispassword@gitlab-redis/
  sentinels:
    -
      host: 10.0.0.1
      port: 26379  # point to sentinel, not to redis port
    -
      host: 10.0.0.2
      port: 26379  # point to sentinel, not to redis port
    -
      host: 10.0.0.3
      port: 26379  # point to sentinel, not to redis port

When in doubt, read the Redis Sentinel documentation.