[Dovecot] So, what about clustering and load balancing?

Stan Hoeppner stan at hardwarefreak.com
Sun Feb 14 05:39:55 EET 2010

Eric Rostetter put forth on 2/13/2010 8:39 PM:

> This ignores the delivery of mail to the user (again, not so bad for
> maildir
> but a killer for mbox).  If the delivery is on a separate box than dovecot
> your can have lock contention...

You attach the inbound MTA to the FC switch, export the LUN with the GFS2
filesystem and drop new mail to the appropirate folder(s).  The dovecot cluster
machines pick it up just as if it were on a local filesystem.  This can be done
very easily with with mbox or maildir and there's no more potential for lock
contention than the imap files.

> Also there may be other things to cause lock contention like backups,
> admin cron jobs to check things, etc.

You have all these things with a non clustered filesystem and have to deal with
them there anyway, so there's really no difference is there?  Is this much
different than an IBM P595 with 64 Power6 5 GHz cores, 1TB of RAM, and 100TB of
FasTt FC disk arrays, running a local inbound MTA (postfix), tens of thousands
of imap processes handling concurrent users for a few million imap mailboxen?
The only difference is lock data travels the wire outside the box in a cluster
setup instead of through shared memory with the big SMP.  You'll still have
locking contention during backup etc, and probably more of it given the scale of
this example.  GigE is plenty fast enough to carry the small extra load of the
cluster fs metadata.  In fact, if the load balancing is implemented well, there
will be very little locking contention at all, even during backup.

> Anyway, I run dovecot in a cluster without any issue, but that is because
> of my client base and performance expectations (and some real nice
> hardware).

Tell us more about your hardware setup if you don't mind.   General info, a
brief few lines would be fine:  load balancer, number/config of server nodes,
SAN switch, storage arrays, software setup.  Also, when do you plan to move to GFS2?



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