[Dovecot] Poor pop3 over nfs performance

[Mark Zealey]

Hi,

About a week ago I upgraded our reasonably heavily loaded mail servers
from a pretty recent courier version to dovecot-1.1rc10. IMAP
performance on dovecot is amazing, however POP3 performance is worse
than courier :-( I have uploaded some munin graphs taken today to
http://linweb.atlas.pipex.net/dovecot/; the dovecot server is handling
about 2000 pop logins/sec and 300 imap sessions (but these are
negligible in terms of nfs ops). The courier server is handling a
similar number of pop logins/sec but no imap (slightly less traffic
overall but a similar number of logins). As you can see, dovecot seems
to issue an awful lot more read requests than courier. The big spike at
10am is where I tried setting the :INDEX=MEMORY flag on dovecot as I
assumed that's what courier is doing. I've also attached graphs of the
relative network usage - as you can see, dovecot is using up to 100mbps
whereas courier is using less than 10mbps; I believe this is due to the
read. I initially assumed this high read load was due to courier
building up its index files, but this has now been happening for about a
week with little variation.

The servers are both centos 5.0 (perhaps 5.1 actually, but with the
latest centosplus kernel which fixes the nfs lookup bug in the standard
kernels); and are not running any other nfs programs. The NFS server is
a high performance NAS unit - no issues there. We're only using
Maildirs; relevant config options are:

mmap_disable: yes
dotlock_use_excl: no
mail_nfs_storage: yes
mail_nfs_index: yes
lock_method: dotlock
maildir_copy_preserve_filename: yes

Thanks,

Mark