[Linux-cluster] Optimal number of nodes?

[David Teigland]

On Thu, Feb 08, 2007 at 01:00:59PM -0700, Tarun Reddy wrote:
> I'm currently testing a RHCS 4 cluster with 4 nodes. Is there any  
> benefit from actually reducing this number down to 3? Other  
> clustering systems I've used seem to prefer odd number of nodes and  
> was wondering if RHCS has that same preference....

The odd number of nodes (votes) relates to quorum which requires over half
of the nodes (votes) to be present for any to operate.  So, if you define
three nodes (each with 1 vote) in cluster.conf, you need two in the
cluster to run.  If you define four nodes in cluster.conf, you need three
in the cluster to run.  Both configurations stop operating if two or more
nodes are down.

Dave