Op 25-10-14 om 02:21 schreef Timo Sirainen: > On 22 Oct 2014, at 06:15, Frido Otten <fr...@dds.nl> wrote: > >> In the past we've had 4 loadbalanced servers configured with courier >> pop3 and dovecot handling respectively POP3 and IMAP connections. These >> 4 servers have a shared NFS storage on a Netapp FAS3220 controller with >> SAS disks. This setup is handling 50 POP3 sessions per second without >> any problems. >> >> Now we're installing 6 new loadbalanced servers to do the same job on >> the same storage, but using Dovecot to handle the POP3 sessions instead >> of Courier. The IMAP sessions are already handled by those 6 new >> servers, but we're encountering high iowait issues when using POP3 on >> those servers. >> >> I've applied the hints from http://wiki2.dovecot.org/NFS but this had no >> improvement. > See also http://wiki2.dovecot.org/POP3Server - especially > pop3_fast_size_lookups may be useful. I've applied the hints on this page and also set pop3_fast_size_lookups to yes. The same for pop3_no_flag_updates. >> mail_nfs_index = yes >> mail_nfs_storage = yes > These slow things down a bit, ideally get rid of these and use Dovecot > directors instead. Can these make such a big difference? I've commented these out. When switching to the Dovecot as POP3 server the iowait immediately skyrockets to 80-90% as well as the overall load of the machine. >> protocol pop3 { >> pop3_uidl_format = %v.%u >> } > I don't think your Courier installation was using this UIDL format? If you > switched to Dovecot the UIDLs would likely differ and clients would > redownload mails. Try to preserve the UIDLs as described in > http://wiki2.dovecot.org/Migration/Courier - and even then I'd recommend > setting pop3_uidl_format to something else, for example %f
This was indeed an issue. I've changed this to %f and checked with our current running courier and now the output of UIDL are the same on Courier and Dovecot. This change unfortunately had no effect on the high iowait.