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
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