We have a hot slave at all times for all instances. In the event of a failure the second slave will kick in. All applications are already connected to both servers via the StackExchange.Redis (https://github.com/StackExchange/StackExchange.Redis) library. The mechanisms for failover, etc. are built in there. We use this library via the dashboard (pictured at the bottom of the post) in Opserver (https://github.com/opserver/Opserver) to do quick swapping of master/slave, etc. We can do this during the day without anyone noticing.

Slaves are not just for backups - they also serve as pub/sub mechanisms. Every publish propagates to slaves. Since we use pub/sub for things like web sockets, we can easily move that entire concurrent connection load to another data center with a simple DNS change. Yep, we've tested this - it worked well.

It is of course noting: redis just doesn't fail. We had one out of memory fail for the Q&A web sites when forking and that's it. It has been rock solid here.