Reference architectures: Modify suggestion to co-locate Redis + Sentinel
I noticed we now have 'Redis (Cache | Persistent) + Sentinel' on many of our reference architectures at https://docs.gitlab.com/ee/administration/high_availability/.
In the past we have generally recommended that customers not run Redis data nodes and Sentinel together. If you lose one node then your quorum is at risk. And if the loss of one Redis data node happens to increase load on the remaining nodes you could lock up one of your remaining Sentinel nodes which can lead to loss of quorum.
I would recommend we update our reference architectures to split those services.
While we're at it, it might be worth discussing whether it always makes sense to split the persistent and cache Redis functions. For example, I see we recommend that for the 2,000 user arch. I'm personally not sure that's necessary at that scale, and it would allow us to cut down on nodes by co-locating Consul and Sentinel and have only one set of Redis data nodes, too.