On Fri, Aug 14, 2009 at 05:29:12PM +0100, Ian Eiloart wrote:
> > Any sort of clustering or load balancing would be done outside of Exim.
> > high-availability is "built in" to SMTP by the ability to set multiple
> > MX records, and the ability to retry.
Although you would need some 'high availability' mail store that exim could
deliver to ...
As pointed out below: talking about exim is not quite the same as talking about
MS exchange; to get the same thing you would need to add mail storage (or use a MDA)
and something like imap/pop so that the MUAs could get mail. If you really are into
high availabilty then you perhaps want to choose cyrus over squirrel mail,
cyrus will handle the MDA & IMAP serving.
Cyrus is much harder to get going than many other IMAP servers.
> Which is fine for MX servers. Not so good for MSA. MUAs (user clients)
> don't use MX records, and don't retry even if the smtp server address is a
> round robin in the DNS. At least, none of the servers that I've tried.
No, you prob need an aggegator or IP failover system (heartbeat, etc) to handle that.
> We have a cluster of four OSX Server servers with Exim. With three, Apple's
> IP failover was just about managable. With four we switched to using
> Wackamole on Spread. That works quite nicely, but occasionally a failover
> throws up faults with ARP caches, leaving some of the IP addresses
> unavailable.
>
> As a result, we've recently put the MSA servers behind a CoyotePoint
> equaliser. We publish a single IP address for MSA, and the equaliser does
> load balancing and high availability. It regularly checks to see that it
> can get an SMTP greeting from each of the servers - if not, then that
> server doesn't get any traffic till its fixed.
>
> Of course, the equaliser becomes a single point of failure... And so it
> goes...
>
> The equaliser doesn't fail as often as the ARP cache corruptions occur.
--
Alain Williams
Linux/GNU Consultant - Mail systems, Web sites, Networking, Programmer, IT Lecturer.
+44 (0) 787 668 0256
http://www.phcomp.co.uk/
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