While doing some testing with converting accounts while simulating incoming mail load (no other pop/imap processes going but 4 processes converting users), we found that we were maxing out the local disk in the server with the index activity. To find out that it was the index activity, I mounted a tmpfs for dovecot to keep indexes on, and the system load dropped from 70 to 3 :)
Anyway, Timo and others, do you have any thoughts about keeping indexes on a tmpfs partition? I realize the obvious issues with this: running out of space, and the fact that the partition is lost on reboot. However, we plan to run a cluster of these servers anyway, where we will keep domains/accounts going to a particular server but fail over to one of the others in case of an outage. Recreating the indexes seems pretty trivial when writing to tmpfs anyway, plus we could copy the indexes to disk (restore after reboot) in the case of planned outages... unplanned outages result in another server rebuilding them all anyway.
So, I'm looking for non-obvious issues we may have with this approach. I need to do more testing to see how much memory we would actually need to feasibly keep the indexes there, but otherwise is there a good reason not to do this?