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

Eric Jon Rostetter eric.rostetter at physics.utexas.edu
Mon Feb 15 04:02:24 EET 2010

Quoting Stan Hoeppner <stan at hardwarefreak.com>:

> Eric Rostetter put forth on 2/13/2010 11:02 PM:

I'm bowing out of this discussion, as I was using words in a non-precise
way, and it is clear that Stan is using them in a very precise way,
and hence we're not really discussing the same thing...

My fault for not thinking/writing in precise, technical terms...
I was basically introducing things like i/o contention and bandwidth
issues into his thread which was solely on actual lock contention...

Suffice it to say: yes, there can be lock contention, but no, it isn't
really any worse than a SMP machine...  What I tend to think of informally
as being lock contention issues are really i/o contention and bandwidth

> You have the same lock contention if both the MTA and dovecot are on the same
> host.  The only difference is that for the clustered case,  
> notification of the
> lock takes longer [...]

Yeah, which is what I was thinking of as lock contention, but really it
is about i/o or bandwidth, and not lock contention...  I was just being
real sloppy with my language...

> My $deity that is an unnecessarily complicated HA setup.  You could  
> have an FC
> switch, a 14 x 10K rpm 300GB SAN array and FC HBAs for all your  
> hosts for about
> $20K or a little less.  Make it less than $15K for an equivalent iSCSI setup.

My setup, since it used some existing machines with only 4 new machines,
was cheaper.  And it supports iSCSI, I just choose not to do that for the
email as my opinion (right or wrong) was that the DRBD+GFS local to the
mail server would be better performance than iSCSI back to my SAN cluster.

> An inexpensive SAN will outperform this setup by leaps and bounds,  
> and eliminate a boat load of complexity.  You might want to look  
> into it.

Well, essentially I do have an inexpensive SAN (see the description), I
just choose not to use it for the e-mail, which may or may not have been
a good idea, but it was what I decided to do.

I could always switch the email to an iSCSI connection in the future
without any problem other than some very limited downtime to copy the
files off the local disks to the SAN machines...  (Very limited as
I can move the mbox files one by one when they are not in use, using
the routing mentioned to send dovecot to the proper location... That's
actually how I migrated to this system, and I only had an issue with 13
users who ran non-stop imap connections and I had to actually disconnect
them...  All the other users had times they were not connected and I could
migrate them without them ever noticing...  Took a while, but it went
completely unnoticed by all but the last 13 users...)

> --
> Stan

Eric Rostetter
The Department of Physics
The University of Texas at Austin

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