[Dovecot] dovecot 2.1.5 performance

Angel L. Mateo amateo at um.es
Wed Jun 20 13:49:24 EEST 2012

On 20/06/12 12:05, Timo Sirainen wrote:
> On Wed, 2012-06-20 at 11:40 +0200, Angel L. Mateo wrote:
>> * mmap_disable: both single and multi server configurations have
>> mmap_disable=yes but in index file section says that you need it if you
>> have your index files stored in nfs. I have it stored locally. Do I need
>> mmap_disable=yes? What it's the best?
> mmap_disable is used only for index files, so with local indexes use
> "no". (If indexes were on NFS, "no" would probably still work but I'm
> not sure if the performance would be better or worse. Errors would also
> trigger SIGBUS crashes.)
>> * dotlock_use_excl: it is set to no in both configurations, but the
>> comment says that it is needed only in nfsv2. Since I have nfs3, I have
>> it set it to yes.
> "yes" is ok.
>> * mail_nfs_storage: In single server is set to no, but in multi server
>> it set to yes. Since I have a director in front of my backend server,
>> what is the recommended?
> With director you can set this to "no".
	Ok, I'm going to change it.

>> 	With this configuration, when I have a few connections (about 300-400
>> imap connections) everything is working fine, but when I disconnect the
>> old servers and direct all my users' connections to the new servers I
>> have lot of errors.
> Real errors that show up in Dovecot logs? What kind of errors?
	Lot of errors like:

Jun 20 12:42:37 myotis31 dovecot: imap(vlo): Warning: Maildir 
/home/otros/44/016744/Maildir/.INBOX.PRUEBAS: Synchronization took 278 
seconds (0 new msgs, 0 flag change attempts, 0 expunge attempts)
Jun 20 12:42:38 myotis31 dovecot: imap(vlo): Warning: Transaction log 
file /var/indexes/vlo/.INBOX.PRUEBAS/dovecot.index.log was locked for 
279 seconds

	and in the relay server, lots of timeout errors delivering to lmtp:

un 20 12:38:29 xenon14 postfix/lmtp[12004]: D48D55D4F7: to=<dmv at um.es>, 
relay=pop.um.es[]:24, delay=150, delays=0.09/0/0/150, 
dsn=4.4.0, status=deferred (host pop.um.es[] said: 451 
4.4.0 Remote server not answering (timeout while waiting for reply to 
DATA reply) (in reply to end of DATA command))

>> server loads increments to over 300 points, with a
>> very high io wait. With atop, I could see that of my 6 cores, I have one
>> with almost 100% waiting for i/o and the other with almost 100% idle,
>> but load of the server is very, very high.
> Does the server's disk IO usage actually go a lot higher, or is it
> simply waiting without doing much of anything? I wonder if this is
> related to the inotify problems:
> http://dovecot.org/list/dovecot/2012-June/066474.html
	Now we have rollbacked to the old servers, so I don't know. Next time 
we try, I'll check this.

> Another thought: Since indexes are stored locally, is it possible that
> the extra load comes simply from building the indexes on the new
> servers, while they already exist on the old ones?
	I don't think so, because:

* In the old servers, we have no "director like" mechanism. One IP is 
always directed to the same server (during a session timeout, today 
could be one server and tomorrow another different), but mail is 
delivered randomly through one of the server.
* Since last week (when we started migration) all mail is delivered into 
the mailboxes by the new servers, passing through director. So new 
server's indexes should be updated.

>> mail_fsync = always
> v1.1 did the equivalent of mail_fsync=optimized. You could see if that
> makes a difference.
	I'll try this.

>> maildir_stat_dirs = yes
> Do you actually need this? It causes unnecessary disk IO and probably
> not needed in your case.
	My fault. I understood the explanation completely wrong. I thought that 
yes should do what actually does no. I have fixed it.

>> default_process_limit = 1000
> Since you haven't enabled high-performance mode for imap-login processes
> and haven't otherwise changed the service imap-login settings, this
> means that you can have max. 1000 simultaneous IMAP SSL/TLS connections.
	I know it. I have to tune it.

Angel L. Mateo Martínez
Sección de Telemática
Área de Tecnologías de la Información       _o)
y las Comunicaciones Aplicadas (ATICA)      / \\
http://www.um.es/atica                    _(___V
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