[Pacemaker] Using "avoids" location constraint

[Florian Crouzat]

Le 08/07/2013 09:49, Andrew Morgan a écrit :
> I'm attempting to implement a 3 node cluster where only 2 nodes are
> there to actually run the services and the 3rd is there to form a quorum
> (so that the cluster stays up when one of the 2 'workload' nodes fails).
>
> To this end, I added a location avoids contraint so that the services
> (including drbd) don't get placed on the 3rd node (drbd3)...
>
> pcs constraint location ms_drbd avoids drbd3.localdomain
>
> the problem is that this constraint doesn't appear to be enforced and I
> see failed actions where Pacemaker has attempted to start the services
> on drbd3. In most cases I can just ignore the error but if I attempt to
> migrate the services using "pcs move" then it causes a fatal startup
> loop for drbd. If I migrate by adding an extra location contraint
> preferring the other workload node then I can migrate ok.
>
> I'm using Oracle Linux 6.4; drbd83-utils 8.3.11; corosync 1.4.1; cman
> 3.0.12.1; Pacemaker 1.1.8 & pcs 1.1.8
>

I'm no quorum-node expert but I believe your initial design isn't optimal.
You could probably even run with only two nodes (real nodes) and 
no-quorum-policy=ignore + fencing (for data integrity) [1]
This is what most (all?) people with two nodes clusters do.

But if you really believe you need to be quorate, then I think you need 
to define your third node as quorum-node in corosync/cman (not sure how 
since EL6.4 and CMAN) and I cannot find a valid link. IIRC with such 
definition, you won't need the location constraints.


[1] 
http://clusterlabs.org/doc/en-US/Pacemaker/1.1-plugin/html/Clusters_from_Scratch/_perform_a_failover.html#_quorum_and_two_node_clusters



-- 
Cheers,
Florian Crouzat