[ClusterLabs] 3 nodes cluster on Centos 7

[Ken Gaillot]

On 07/07/2015 04:19 AM, Nicolas S. wrote:
> Hello everybody,
> 
> I'm posting first time on this mailing list for an advice.
> 
> I try actually trying to build a cluster on Centos 7.
> 
> The cluster has 3 nodes :
> 
> - 1 virtual machine (machine1). This machine is supposed to be high-available
> - 2 physical machines identical (machine2 and 3)
>
> The physical machines are supposed to use DRBD to replicate storage.
> A shared volume is going to be mounted on both machine 2 and 3 in read/write (gfs2).
> 
> The virtual machine is only here for the vote in the qorum, it has no data . 
> 
> I m' following this guide : http://clusterlabs.org/doc/en-US/Pacemaker/1.1-pcs/html/Clusters_from_Scratch/index.html
> 
> My questions are :
> 
> - Is this configuration OK or shoud I better go to a 2 node only cluster ?

Hi Nicolas,

As others mentioned, the VM is useful as a quorum node only if it's not
running on one of the other cluster nodes. If the VM is itself a highly
available resource started by the cluster, then it should not be a
cluster node. (It can potentially be a pacemaker_remote node, but that's
an advanced topic that you don't need at this stage.)

CentOS 7 has corosync 2, which has good support for 2-node clusters. If
you're following Clusters From Scratch, pcs will configure corosync
appropriately for you.

> - Is there a way to "group" the storage machines 2 and 3 and put some ressources (like DRBD and GFS2) only on them ? So that the machine 1 is here , but does nothing, just vote.
> I looked around in the pcs command line option but maybe i missed it.
> Thanks in advance

Yes, "location constraints" do that. See the pcs man page under
"constraint" or the Pacemaker Explained manual for details, but
basically you can tie particular resources to particular nodes either
positively (prefer this node) or negatively (avoid this node).

There are also colocation constraints to say that certain resources
should be kept together, and order constraints to say that certain
resources should be started/stopped sequentially.