[Linux-cluster] A single server backing up three servers

[Josef Bacik]

On Mon, Jun 11, 2007 at 01:21:26PM -0500, brad at bradandkim.net wrote:
> Hi all,
> 
> I have just begun playing around with redhat cluster and I have a question
> that I am not finding a straight answer to.   I would like to have one
> server be a backup to 3 primary servers.   In other words I guess I would
> have 3 services (db1, db2, and db3) and then a 4th server (db4) that is
> available to take over for any of the 3 primary servers.   Obviously if 2
> primary servers fail I am in trouble, but the odds of that are extremely
> slim.  Is this possible?  Are there caveats?
> 

Yeah you can do this with priority failover domains.  I assume from your
description that you will want 3 failover domains, one for each application.
You will add two servers for each domain, the primary server which will have the
higher priority and then db4.  Then if anything fails on the main servers they
will failover to db4, and then when they come back up the service will failback.
You will want the "restricted" and "ordered" flag set.  The only caveat you have
already specified, if db4 is down and something fails everything hangs since you
will have lost 2 nodes.  You may want to look into qdisk if you want to change
this behavior slightly.

Josef