[lvs-users] question about load balancing smtp
Michiel van Es
mve at pcintelligence.nl
Mon Apr 26 11:48:31 BST 2010
-------- Original Message --------
Subject: Re: [lvs-users] question about load balancing smtp
From: Bruce Richardson <itsbruce at workshy.org>
To: lvs-users at linuxvirtualserver.org
Date: 04/26/2010 12:27 PM
> On Mon, Apr 26, 2010 at 12:01:17PM +0200, Michiel van Es wrote:
>> I know that with DNS and the SMTP protocol load balancing and failover
>> can be established.
>> But I want to experience load balancing through a software load balance
>> setup, a load balancer can balance the load on several algorithms which
>> a smtp proxy or dns setup doesn't have.
> Do you actually need this for incoming mail, though? Given that any
> external SMTP host that's trying to send you mail will keep trying MX
> hosts until it finds one that's listening? Given the resilience of the
> protocol, a combination of DNS load-balancing (multiple A records for
> the same MX record) and MX load-balancing (multiple MX records with the
> same priority) is perfectly good for managing mail load until you reach
> very high volumees of mail.
I don't need it but I want to know the basics and try it features.
What about outgoing smtp services?
>> What about outgoing smtp servers for mailing list servers etc?
> Can you not put a minimal SMTP service on those servers and make
> configure that service to be aware of multiple mail gateways within your
> datacentre? If your mailing list server is a *nix box, that's trivial.
I know Postfix and ASSP can route it to several mailservers with a
simple failover scenario.
>> I understood that LVS is capable of showing the source ip to the real
>> servers so there are no problems with the protocol's own resilience
> I don't see what that has to do with it. The reason why load-balancing
> can cause problems is because when you use LVS to cluster SMTP services
> then multiple hosts appear to the outside world as one host;
The outgoing mailservers will not relay through the LVS load balancer
but directly to the internet (SPF and PTR correctly setup).
That is why the direct routing setup exists right?
> external SMTP host has a problem with the particular realserver it
> connects to, it will then back off and not try to connect to any of the
> other realservers (because it thinks there's only one host there).
> There are several ways in which this can delay mail delivery in ways
> which would not happen if you used simple DNS and MX load-balancing, as
> described above. I can talk you through them if you like.
What is the diffirence between a connection forwarded through the load
balancer to one of the 2 broken mx hosts or a round robin setup where
one of the 2 is broken and stops accepting mails and the mail bounces?
If one of the mailservers is broken, I want to directly disable a host
in the load balancer not through DNS which has a nasty caching TTL...
> Unless you have very high mail volumes, you gain nothing from
> TCP/IP-level load-balancing and you actually create unnecessary delays.
See my motivation above + I want to learn and use that feature.
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