[vidarh]

Regarding your point about MySQL:

We used haproxy to do close to zero downtime migration of our postgres servers (and other stuff) from one data centre to another recently when we moved out of our old data centre: Set up a slave in the new dc, set up haproxy with the slave as backup, point all the clients to haproxy. Shut down the master, and as soon as the master stopped responding, haproxy would shove clients to the slave in the new data centre instead. Then touch the trigger file to make the slave recover and switch to master mode.

It meant we were in read-only mode because requests where going to the slaves for about 2-3 seconds (and unlike MySQL Postgres does not allow writes to a slave until it's been promoted) before the slave started handling updates - other than that everything just chugged along.