Re: [lvs-users] question about load balancing smtp

To: " users mailing list." <lvs-users@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [lvs-users] question about load balancing smtp
From: Michiel van Es <mve@xxxxxxxxxxxxxxxxx>
Date: Mon, 26 Apr 2010 12:48:31 +0200

-------- Original Message --------
Subject: Re: [lvs-users] question about load balancing smtp
From: Bruce Richardson <itsbruce@xxxxxxxxxxx>
To: lvs-users@xxxxxxxxxxxxxxxxxxxxxx
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
>> features.
> 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?

  if an
> 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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