[Dovecot] critical performance problem

[Rene Luria]

I have been using dovecot 1.0alpha4 + deliver/sieve in production for a
few accounts (400'000).

After testing 1.0.rc22 I decided to upgrade one of the imap servers...

And... what a mess !

The server just gets unusable. I don't know why but with new release, it
just overloads the system with I/O.
It turns that with 1.0.rc22 I get 15Mo/s read on disk average, without
any reason.
What did really change ? How come a system (with a few thousand
accounts) works perfectly and then with 1.0.rc22 gets unusable with a
load approaching 300 ?

If you have any ideas...