(I did matching versions [5.x] don’t know how diff versions will work)
Setting up a slave with a newer version of MySQL is quite a common setup. It has a couple of advantages:
– it lests you test a new version before deploying on the master (to test that everything goes smoothly)
– it lets you test new major versions (e.g. 5.0) before they are released GA (helps find bugs that may affect your setup).
I know at least one customer generally has a slave runnin the latest BK tree – just to be sure that nothing is going to even potentially break for them. Kudos to them :)
Having a slave that you use for backups is a great idea. No extra load on the master (i.e. you can safely stop the db on the slave and back things up quickly – without having locks held on your master!).
Also, if your master suffers a meltdown, you have a recent live backup system ready to take its place!