[Dovecot] High level of pop3 popping causing server to become unresponsive

[Timo Sirainen]

On Fri, 2012-05-18 at 09:21 -0400, Root Kev wrote:
> During the last time that the load went up, it became unable to login / su
> to root for the entire period that dovecot was running, we had to kill
> dovecot and go back to Popa3d until the mailq was cleared up.  We are
> running CentOS 5.6 server.  Based on TOP running at the time the CPU usage
> was running under 10%.  Once Dovecot was killed, we were then able to log
> in /su again.

Like Kelsey said, a very high disk IO might explain this, although
normally the login should still eventually succeed. Another thing I'm
wondering is if some process limit reached. How does the login/su fail,
does it just hang or immediately fail with some error?

> We were under the impression that checking to shadow directly should be the
> fastest and least amount of overhead, is any of the other ways to connect
> have less load on authentication to PAM?

Your passwords really are in /etc/shadow file, not LDAP/something else?
I don't think the problem is with authentication. Reading /etc/shadow is
pretty fast (unless maybe if it's a huge file) and it anyway can't block
login/su from working.