[Dovecot] Poor pop3 over nfs performance

[Timo Sirainen]

On Mon, 2008-06-30 at 17:05 +0100, Mark Zealey wrote:
> Hi,
> 
> About a week ago I upgraded our reasonably heavily loaded mail servers
> from a pretty recent courier version to dovecot-1.1rc10. 

rc11 had this fix which may be relevant:

	- dovecot-uidlist is now recreated if it results in file shrinking
	  over 25%.

> 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.

Dovecot's POP3 performance isn't that great, but it shouldn't be (much)
worse than Courier's since they both use a nearly identical
dovecot-uidlist/courierpop3dsizes files.

> 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.

With v1.1 INDEX=MEMORY shouldn't matter that much anymore, since it
should cache the virtual message size to dovecot-uidlist.

1) Do you have ,W=<size> in maildir filenames? Apparently not?

2) Do you have W=<size> added to dovecot-uidlist?

3) Are your POP3 users download + delete or do they leave mail on the
server?

> 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

You should be able to use dotlock_use_excl=yes.

> mail_nfs_storage: yes
> mail_nfs_index: yes
> lock_method: dotlock

dotlock is the slowest locking method. Did you try how well fcntl would
have worked?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: This is a digitally signed message part
Url : http://dovecot.org/pipermail/dovecot/attachments/20080701/6ddfc596/attachment.bin