Migration and upgrade
I don't know if there is an easy way to get diffs - it must be possible for mysql slave mirrors, but the wiki is small so we can be lazy with our backup mechanisms ;-)
I've been using mysql slaves for years, and it is not only easy to set up, but also a perfect candidate as a readonly mirror of the wiki.
And the slave can be set as master anytime, making the mirror the main wiki when the original one goes down. This ensures zero downtime if set up automatically.
Anyway, this is not a replacement of monthly backup. If a hacker managed to run drop db, the slave will also be destroyed.
Could you elaborate on mysql slave? I would like to try it, and maybe even host readonly replica.