[Dovecot] Investigating suspected cache corruption

[Timo Sirainen]

On Wed, 2009-10-14 at 16:17 -0400, David Warden wrote:
> To date, the only problem we have experienced is a seemingly random  
> but significant slowdown in Squirrelmail's loading of the messages in  
> a user's Inbox. Each time, I have been able to fix the problem by  
> deleting the user's dovecot.index, dovecot.index.cache and  
> dovecot.index.log files in the user's INDEX directory for their  
> dovecot Inbox.

First thing to try next time: Does it help if you only delete
dovecot.index.cache file? How large is the cache file? And take a copy
of the dovecot.index* files before doing that, I can then ask a few more
questions about them.

> I tried having users experiencing this problem log in to a test  
> Squirrelmail box with mail_debug on, but I didn't see anything out of  
> the ordinary and they report the same slowness on the test server. I  
> don't see anything but normal login/logout IMAP lines in the  
> production dovecot logs.

mail_debug doesn't really help with performance debugging.

> My first idea when I get  
> another one of these is to do a telnet IMAP session to Dovecot on one  
> of the Squirrelmail boxes and see if I can get it to slow down when  
> retrieving messages from the Inbox.

That sounds like a good idea to try. And use the exact same IMAP
commands as Squirrelmail to do it. strace -tt output of of the slow imap
process could also show something interesting.
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