We’re currently in the process of upgrading from MySql 5.0 to 5.5.
We have the edge slaves upgraded and the SQL incompatibilities debugged. All that’s left now is upgrading the production masters.
Ideally, we could pick one of the two masters, say B, and upgrade it to 5.5. We would then make it the writer for a while and test to ensure it can handle to load and there are no problems. At that point we would also upgrade A.
The snag is that, to the best of my knowledge, we can’t have B (on 5.5) replicating to A (still on 5.0) because 5.0 won’t be able to handle the 5.5 replication stream. To promote B to “writer” status, we’d have to take A offline and there would be no going back. If MySql 5.5. can’t handle the load for whatever reason, we’re screwed.
I’m hoping someone on this forum went through a similar upgrade and has a better idea!