Shortly after the MySQL 5.5 upgrade the whole cluster was upgraded with extra ram. This was a nice test to see how differently 5.1 and 5.5 behave when they A) innodb bufferpool is too small and B) when the innodb bufferpool has enough room to fit everything in memory.
The MySQL 5.5 had just the same pattern in terms of disk utilization as the other nodes before (around 30% to 40%) and after the upgrade (4% to 5%), so not much difference at all. However the number of free pages within the bufferpool is significantly lower (about 10%) than on the other nodes. This definitely needs some further investigation.
Apart from that the machine is stable and it seems we will proceed with the upgrade on the whole cluster soon.
A sidenote: I’m happy to announce that I was selected as a speaker at the Percona Live MySQL Conference & Expo in San Francisco, April 2012. I’ll be talking about Spil Games (the company I work for) and how our new architecture will solve or ease up the majority of our database issues.