DNS load balancing

Vincent Fox vbfox at ucdavis.edu
Tue May 11 12:36:55 EDT 2010

On 5/11/2010 5:35 AM, Andre Nathan wrote:
> Hello
> I'm setting up a two-machine cyrus cluster using OCFS2 over DRBD. The
> setup is working fine, and I'm now considering the load balancing
> options I have.
> I believe the simplest option would be to simply rely on DNS load
> balancing. However, for this to work, I need to consider what happens
> when one of the cluster nodes fails, regarding the clients which access
> the cluster:
>   - Will IMAP and POP3 clients automatically try the second DNS entry if
>     connecting to the first one fails?
>   - Will the LMTP client also do this? I'm using the postfix LMTP client
>     for delivery.
> Does any one have experience with this kind of setup?

DNS rollovers and particularly round-robin DNS solutions should
not be considered IMO in modern HA datacenter practice. May work
OK for a few hundred people  and departmental servers, shows all
it's warts for thousands.

The logjam created when thousands of connections EACH have to
timeout and rollover is not desirable.

If you are doing a failover (active/passive) cluster then bring up
a VIP for the service and have the VIP hostname be the official
client point of contact.  If you are doing active/active in some form
then a proper load-balancer in front of your mail-store can do health
checks and route client connections to servers known to be up.

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