Hi!
...if it has v4 slaves
I know how v4 slave will break. It will break entirely. Binlogs are not compatible.
Perhaps someone that wants to be able to switch the master back to a mysql4.
Such steps are one direction, usually :) And why would anyone, except me, want to run on mysql4? :-)
By the way, we're having huge stalls now, buffer pool dirty page flushing sometimes starves when is not able to find any dirty pages to flush.
Was that a wild idea that if there's no 'upgraded to mysql 5.1' entry in the log, the old mysql4 master was kept as a slave?
Well, it is useless pretty soon, as applying binlogs is pain, and it gets old very very very fast.
mysql replicates the sql statements, so I wouldn't be surprised if a mysql5 which only received mysql4 compatible statements worked as master for mysql4s.
Yes, it is possible, if there's binlog conversion going on, or external replication agent is used (such as trainwreck).
Domas