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Wensong,
I agree, but... :-)
1) You can configure most mail programs to start refusing connections when
load rises above a certain limit. The protocol itself has built-in
redundancy and error-recovery. Connections will automatically fail-over to
the secondary server when the primary refuses connections. Mail will
_automatically_ spool on the sender's side if the server experiences
temporary outage.
2) Mail service is a special case. The protocol/RFC itself specified
application-level load balancing, no extra software required.
3) Central load balancer adds complexity/layers that can fail.
I maintain that mail serving (smtp only, pop/imap is another case entirely)
is a special case that does not need the extra complexity of LVS. Basic
Queuing theory aside, the protocol itself specifies load-balancing,
failover, and error-recovery which has been proven with years of real-world
use.
LVS is great for protocols that do not have the built-in protocol-level
load-balancing and error recovery that SMTP inherently has (HTTP being a
great example). All I am saying is use the right tool for the job.
--
Michael Brown
-----Original Message-----
From: Wensong Zhang [mailto:wensong@xxxxxxxxxxxx]
Sent: Tuesday, May 15, 2001 9:40 AM
To: lvs-users@xxxxxxxxxxxxxxxxxxxxxx
Cc: Joseph Mack
Subject: Re: mail farm?
On Mon, 14 May 2001, Michael E Brown wrote:
>
>
> On Mon, 14 May 2001, Joseph Mack wrote:
>
> > Michael E Brown wrote:
> >
> > > > As far as I am aware DNS should respond with all MX records for a
> > > > request. The MTA should then choose one at random from the same
> > > > piority.
> >
> > > The above statement is correct.
> >
> > And if the machine for that MX record happens to be down, the MTA
> > will pick another MX record and try to deliver it there? (This has
> > to be true, I'm sure I read this in the Bat book).
>
> Yes. (As long as there _are_ multiple MX records :-)
>
I think that central load balancing is more efficient in resource
utilization than randomly picking up servers by clients, basic queuing
theory can prove this. For example, if there are two mail servers grouped
by multiple DNS MX records, it is quite possible that a mail server of
load near to 1 still receiving new connections (QoS is bad here), in the
mean while the other mail server just has load 0.1. If the central load
balancing can keep the load of two server around 0.7 respectively, the
resource utilization and QoS is better than that of the above case. :)
Regards,
Wensong
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