Hello,
I can try to comment a bit from the RHEL High Availability Add On side of things, but keep in mind that I'm not a MySQL specialist, so we may need someone else to comment from that perspective and say if there's a preferred approach with respect to the database layout.

My suggestion would be to split things out to as fine a granularity as you can get, for the sake of simplified management. In other words, I would set app1, app2, app3, and app4 all up with their own independent services (assuming none of them need to share other resources like file systems or IPs), and then to have MySQL A and MySQL B running in distinct instances each with their own service. This gives you the flexibility to move, start and stop, or otherwise manage the distinct pieces of this puzzle without affecting the others. If you needed to do maintenance on MySQL A, then you'd only have to take an outage on app1 and app2, but could leave MySQL B and app3 and app4 up and running. If they were managed in one instance, this would be more difficult, if not impossible.

As far as performance goes, there wouldn't be any strong considerations from the HA side that would influence the single-versus-multiple instance decision. The cluster resources will each perform status checks periodically and with more resources this means more status checks, but this really won't matter much because:

• The only serious check mysql resources perform during status is to look at the pid file, and
• The interval on these status checks for mysql are a minute apart, which would mean minimal work for the server to do even with 2 instances to check.

Hopefully these points will help in your configuration of the cluster. But as I said, from a MySQL perspective there may be more considerations, so you may want to continue to evaluate if there's any significant difference between the two approaches. I'll see if I can find a specialist on that side to offer more input there. Let us know if you have any questions.

Regards,
John Ruemker, RHCA
Senior Software Maintenance Engineer
Global Support Services
Red Hat, Inc.