[Dovecot] dovecot 2.1.5 performance

[Angel L. Mateo]

Hello,

	I'm migrating from 1.1.16 running in 4 debian lenny servers virtualized 
with xenserver and 1 core and 5GB of RAM to 2.1.5 running in 4 ubuntu 
12.04 servers with 6 cpu cores and 16GB of RAM virtualized with VMWare, 
but I'm having lots a performance problems. I don't think that 
virtualization platform could be the problem, because the new servers 
running in xenserver has the same problems than running in vmware.

	I have about 70000 user accounts, most of them without real activity 
(they are students who doesn't read his email or have its account 
redirected to other provider). I have about 700-1000 concurrent imap 
connections.

	I have storage in nfs (nfsv3, the nfs server is a celerra), but indexes 
are in local filesystems (each server has its own index fs). Mailboxes 
are in maildir format.

	Old servers and actual director servers are load balanced with an 
radware appdirector load balancer (the new backend servers don't need to 
be balanced because I'm using a director farm)

	In the old platform I have scenario number 2 described at 
http://wiki2.dovecot.org/NFS, but in the new ones I have a director 
proxy directing all connections from each user to the same server (I 
don't specify any server for the user, director selects it according to 
the hash algorithm it has).

	Some doubts I have for the recommended in that url:

* 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?
* 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.
* 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 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. 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.

	With the old servers, I have performance problems, access to mail is 
slow, but it works. But with the new ones it doesn't work at all.

	Any idea?

-- 
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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