[Pacemaker] Split-site cluster in two locations

[Robert van Leeuwen]

-----Original message-----
To:	The Pacemaker cluster resource manager <pacemaker at oss.clusterlabs.org>; 
From:	Christoph Herrmann <C.Herrmann at science-computing.de>
Sent:	Tue 11-01-2011 10:24
Subject:	Re: [Pacemaker] Split-site cluster in two locations
 
> As long as you have only two computing centers it doesn't matter if you run a 
> corosync
> only piece or whatever  on a physikal or a virtual machine. The question is: 
> How to
> configure a four node (or six node, an even number bigger then two) 
> corosync/pacemaker
> cluster to continue services if you have a blackout in one computing center 
> (you will
> always loose (at least) one half of your nodes), but to shutdown everything if 
> you have
> less then half of the node available. Are there any best practices on how to 
> deal with
> clusters in two computing centers? Anything like an external quorum node or a 
> quorum
> partition? I'd like to set the expected-quorum-votes to "3" but this is not 
> possible
> (with corosync-1.2.6 and pacemaker-1.1.2 on SLES11 SP1) Does anybody know why?
> Currently, the only way I can figure out is to run the cluster with 
> no-quorum-policy="ignore". But I don't like that. Any suggestions?


Apart from the number of nodes in de datacenter: with 2 datacentre's you have another issue:
How do you know which DC is reachable (from you're clients point of view) when the communication between DC fails?
Best fix for this would be a node at a third DC but you still run into problems with the fencing devices.
I doubt you can remotely power-off the non-responding DC :-)
So a split brain situation is likely to happen sometime. 

So for 100% data integrity I think it is best to let the cluster freeze itself...

Best Regards,
Robert van Leeuwen