multiple messages per second to a single mailbox

[Andrzej A. Filip]

Chad M Stewart <cms at balius.com> wrote:
> Dovecot 2.2.18 on CentOS 6
>
> I have a pair of servers setup with MySQL, Postfix, and Dovecot.  Replication is setup and working between the two dovecot instances. 
>
> The problem I'm running into is that a single mailbox receives a lot
> of messages, at times the rate is multiple messages per
> second. Delivery from Postfix to Dovecot is via tcp based LMTP.  When
> I do 'ps -aef|grep lmtp|wc -l' I get 62 and does not appear to go
> higher than that. At the moment I have 4500 and 8300 messages queued
> on two Postfix instances waiting to deliver via LMTP to the same
> dovecot instance.  Deliveries only happen via LMTP and only one of the
> two nodes actually gets the deliveries.
>
> What I'm seeing is very high load on the system (40) and queues
> building on the Postfix side.  Replication is keeping up.  Looking at
> the logs now I see anywhere from 4-7 messages per second delivered to
> this single mailbox.  I would like to increase that rate a lot.
>
> These machines are VMs hosted on Xenserver 6.x.  I have them setup
> with 8 vCPUs (2 sockets with 4 cores per socket), the dom0 machines
> have dual HBA connectors back to a SAN and have 128 CPUs and 256GB of
> RAM and are not taxed.  I added a 2nd virtual disk that is used for
> storing mailbox data.  It is ext4 and has noatime set during
> mount. /var is also mounted with noatime.
>
> The performance graphs in XenCenter show nearly all 8 vCPUs at about
> 50%, and the writes on the mailbox data disk are about 20%.  iostat is
> showing mostly <5 for await times for the disks, though I do see a 10
> now and again.
>
>
> I'm guessing that maybe I'm hitting a mailbox locking issue and not sure how
> to reduce the contention and thereby increase the delivery rate to this
> mailbox.

Could you provide the following info:
a) mailbox type (maildir/mbox/dbox/...)
[mail_location in dovecot's config]
b) file system type (ext2/ext3/ext4/fat32/...)
[provided by "df -T" command on my system]

-- 
A. Filip