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