[Linux-cluster] Optimal number of nodes?

[Tarun Reddy]

On Feb 8, 2007, at 1:13 PM, David Teigland wrote:

> 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
>

Thank you Dave, that explains a lot and yet so simple. :-)

Just another question. In my cage I actually have a fifth machine,  
also running RedHat 4 but purposed for usage outside of the cluster.  
Actually adding it to the cluster, but limiting  services only to the  
four "real" nodes would actually significant improve the tolerance of  
my environment by allowing any two nodes to fail or be powered off,  
correct?

Tarun