On May 20, 2010, at 5:31 PM, Bryan O'Neal wrote:
> Personally I vote for RRDNS so that your domain name has multiple IP's
> associated with it. DynDNS polls every few minuets for availability
> and will automatically remove dead servers. That is what clustering is
> all about :)
Keith originally mentioned wanting servers in 2 distinct locations, a
primary site and a backup site, with the backup site able to take over
automatically if the primary becomes unavailable. To me, this sounds
like a concern for availability, not performance, and my comments have
been made in that frame of mind.
'clustering' can mean a lot of things. Increasing performance, as in
high-performance computing, is not the same as increasing
availability. The kind of load-balancing you can achieve through
RRDNS does not necessarily increase availability. You have to
consider how well your current infrastructure is matched to your
current workload.
What I mean is this : If you have 2 servers doing an identical job
(like 2 web servers serving up the same website, to the same users,
etc), you can load-balance through RRDNS, or through a dedicated load-
balancer, or whatever. That doesn't automatically do anything to
increase your site's availability. If your site's traffic load
requires both servers to be functional in order to get decent
performance for the user, then losing 1 server means your site is
effectively unavailable. By contrast, if the 2 servers are in an
active/passive configuration, you aren't load-balancing at all, but
you do have high-availability. If the primary server dies, the old
secondary can become primary and users should never know the difference.
I think it's really important to keep these goals distinct, assign
relative importances to each, and act accordingly. High performance
is not high availability. High availability is not backup. Decide
which you need. Hint: You ALWAYS need backup. :)
I often hear people bemoan the fact that in a typical HA setup,
they've spent money on all this backup hardware, and it's not 'doing
anything'. It's just sitting there waiting for the primary server to
fail. Yes, this is true. You just have to keep in mind that if it
were 'doing something', you'd miss it if it died, and that's not
really high-availability.
alex
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