I have experienced this on all versions of Dovecot that Ive used (1.0 - 1.1b6) using IMAP (its difficult to test if its also there with POP3).
What I see is that if there is a peak in disk usage at the time of a specific request that requests stalls. The saturation of disk I/O is momentary but when its done (maybe after one or two seconds) Dovecot still waits for its I/O operation instead of continuing as soon as resources are available.
Now, I would expect Dovecot to only wait until there are resources available and not stall for at long time (sometimes it stalls for like 10 seconds, and sometimes completely).
This could be OS specific (Im using ZFS and Solaris 10 on Sparc) but it may also be due to the way dovecot is programmed. Theres always plenty of RAM and CPU available so thats not whats causing troubles.
Is anyone else familiar with this issue?
Regards, Mikkel