Hi.

today I did master switch for en.wikipedia.org to mysql5.1, we have been running s2 (twenty other major languages) for few days on fully-5.1 setup.

Some higher level description of the branch is at http://wikitech.wikimedia.org/view/MySQL_5.1_(fork) Actual tree is at https://code.launchpad.net/~wikimedia/sakila-server/mysql-5.1-wm

Actual upgrade happened with no downtime, and there're no horror stories to tell about the process, was way too smooth. Rest of cluster wil follow soon.

Cheers, Domas