[Dovecot] Multiple locations, 2 servers - planning questions...

[list at airstreamcomm.net]

On Mon, 27 Feb 2012 13:38:39 -0500, Charles Marcus
<CMarcus at Media-Brokers.com> wrote:
> On 2012-02-27 1:34 PM, Sven Hartge <sven at svenhartge.de> wrote:
>> Charles Marcus<CMarcus at media-brokers.com>  wrote:
>>> Each location is an entire floor of a 6 story building. The remote
>>> location has the capacity for about 60 users, the new location about
>>> 100. We only allow IMAP access to email, so if everyone is using email
>>> at the same time, that would be a lot of traffic over a single Gb link
>>> I think...
> 
>> Naa, most clients download mails only once and then keep them cached
>> locally (at least Thunderbird and Outlook do).
>>
>> Looking at the used bandwidth of the mailserver of my small university
>> (10.000 users, about 1000 concurrently active during the daytime)
>> shows a steady amount of roughly 5MBit/s with peaks to 10MBit/s in and
>> out.
> 
> Interesting - thanks for the numbers...
> 
> But, again, my main reason for 2 servers is not performance, it is for 
> redundancy...

I too have been tasked with multisite redundancy, and have been
experimenting with GlusterFS
(http://www.gluster.org/community/documentation/index.php/Main_Page), which
is a distributed file system.  In our network we have a dedicated 10GB link
between two datacenters 100 miles apart, and I have a GlusterFS node at
each site setup in Distriubted Replicated mode with 2 replicas which means
the servers are mirrored.  The file writes are done to all the replica
servers (2 servers in this case), so depending on network latency the
writes could potentially slow down.  GlusterFS has it's own file serving
protocol that allows automatic and immediate failover in the case that a
storage node disappears, but there are some caveats to restoring a failed
storage node (takes forever to resync the data).  I have not put this
experiment into production, but I can say that it's extremely simple to
manage, and performance testing has shown that it could handle mail traffic
just fine.  You could also look at GPFS
(http://www-03.ibm.com/systems/software/gpfs/), which is not open source
but it's apparently rock solid and I believe supports multisite clustering.