[Linux-cluster] 2 node vs 3 node cluster

[David Teigland]

On Wed, Aug 25, 2004 at 05:33:34PM +1000, Adam Cassar wrote:
> Hi Guys,
> 
> What are the benefits of running a 3 node cluster, as only one node can
> fail before bringing the entire cluster down?
> 
> It appears that a single node cannot be a member of a cluster if the
> other hosts are missing. I take it that is to prevent single nodes
> splitting and making themselves independent clusters? 

You're right.  Both 2 and 3 node clusters can tolerate the failure of 1
node.  If 2 nodes fail, a 2 node cluster would obviously be out of
commission, while the single remaining node in a 3 node cluster would be
stalled.  So, there's no advantage to a 3 node cluster in that sense.
This is assuming all nodes have the default 1 vote -- probably the most
sensible configuration.

In another sense, having one remaining node in a 3 node cluster would make
bringing things back up nicer after the failures.  All you'd need is one
failed node to join the cluster again to make the cluster quorate and
allow the stalled node to continue running.

Another option for the expert user:  if you know the two failed nodes have
been reset (or detached from storage) you could manually reduce expected
votes to allow the stalled node to continue running.  This is dangerous,
of course, unless you know what you're doing.

-- 
Dave Teigland  <teigland at redhat.com>