Dovecot cluster using GlusterFS

[Gordon Grubert]

Hi Filip,

On 12/05/2015 10:42 AM, Filip Pytloun wrote:
> I have recently setup mailserver solution using 2-node master-master
> setup (mainly based on MySQL M-M replication and GlusterFS with 2
> replica volume) on Ubuntu 14.04 (Dovecot 2.2.9).

that's no good idea due to different reasons - see below.

> Anyway because of the above and high possibility of GlusterFS
> split-brains, I have decided to setup Dovecot Director according to the
> docs [1] but I have a couple of questions:
>
> - is custom monitoring still required? Poolmon [2] is 4 year old so I
>    would suppose there's some progress since that?

Using the dovecot director, poolmon is strongly recommended - see the
official dovecot wiki. The tool seems to be old, but the task is the
same since a couple of years ;-)

> - it's not possible to have same backends and directors in Dovecot
>    <2.2.17. I can backport newer Dovecot for Ubuntu Trusty, so this is
>    not an issue, but..

You have to use 2 instances of dovecot when running the director and
the backend on the same server. Otherwise, you have to use 2 systems.

> - documentation states that it still doesn't work for LMTP [3]?
>    Which is probably important for my setup, because both Postfix servers
>    are using dovecot-lmtp for mail delivery so there can be still some
>    issues (but probably less frequent?) when both servers will deliver
>    new mails for one user at once.
>    So do I really have to split directors from backends?

At the moment, I cannot recognize the requirement for using lmtp over
the directors. When using postfix for delivering e-mails to the
backend, do this directly with an corresponding MX record.

> Why is Dovecot behaving so bad when it pretends to be shared storage
> friendly? Are these issues only specific for older Dovecot?
> Or is there something wrong in my architecture design?

IMHO, this is not a problem of dovecot. In general, using cluster
filesystems in such a setup is no good idea. You've already mentioned
all the problems of such a setup. In particular, the performance of
GlusterFS is absolutely not suitable for (bigger) mailserver cluster.

Asking any search engine for "dovecot clustering" shows a lot of
results for good cluster designs. In particular, a shared mailbox
storage should be avoided. Here, the dovecot internal replication
mechanism on the backends seems to be the best solution. The rest of
the setup (e.g., clustered directors or smtp servers) is trivial. Due
to the fact, that MUAs are able to use A records only and the behavior
using round robin A records is catastrophically, a service IP address
is recommended. Here, keepalived is your friend.

HTH.

Best regards,
Gordon
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