Merge branch 'remove-sidekiq-backup-ar-threads' into 'master'

Don't use backup AR connections for Sidekiq

Closes #27713

See merge request !9001

changelogs/unreleased/remove-sidekiq-backup-ar-threads.yml

+---
+title: Don't use backup Active Record connections for Sidekiq
+merge_request:
+author:

config/initializers/sidekiq.rb

@@ -36,11 +36,9 @@ Sidekiq.configure_server do |config|
 
   Gitlab::SidekiqThrottler.execute!
 
-  # Database pool should be at least `sidekiq_concurrency` + 2
-  # For more info, see: https://github.com/mperham/sidekiq/blob/master/4.0-Upgrade.md
   config = ActiveRecord::Base.configurations[Rails.env] ||
     Rails.application.config.database_configuration[Rails.env]
-  config['pool'] = Sidekiq.options[:concurrency] + 2
+  config['pool'] = Sidekiq.options[:concurrency]
   ActiveRecord::Base.establish_connection(config)
   Rails.logger.debug("Connection Pool size for Sidekiq Server is now: #{ActiveRecord::Base.connection.pool.instance_variable_get('@size')}")