[Pacemaker] Quorum in pacemaker

[Andrew Beekhof]

On 27 Jun 2014, at 10:22 am, Vijay B <os.vbvs at gmail.com> wrote:

> Hi,
> 
> I'm trying to set up a three node cluster using pacemaker+corosync, and I installed the required packages on each node, checked for their network connectivity so they can see each other, added the required startup scripts and edited the cluster.conf file as well so it includes all three nodes.
> 
> Now, when on the first node, I attempt to start up cman using service cman start, it times out thus:
> 
> vagrant at precise64-pmk1:~$ sudo service cman start
> Starting cluster: 
>    Checking if cluster has been disabled at boot... [  OK  ]
>    Checking Network Manager... [  OK  ]
>    Global setup... [  OK  ]
>    Loading kernel modules... [  OK  ]
>    Mounting configfs... [  OK  ]
>    Starting cman... [  OK  ]
>    Waiting for quorum... Timed-out waiting for cluster
> [FAILED]
> vagrant at precise64-pmk1:~$
> 
> Why is this? Is it because I have three nodes to begin with in my /etc/cluster/cluster.conf, and so this node expects that the cluster quorum is 2, and so it should be able to talk to at least one other node?

Yes. CMAN refuses to start unless it can see enough other nodes to form quorum.
Pacemaker doesn't need quorum to behave sanely though, so try:

echo "CMAN_QUORUM_TIMEOUT=0" >> /etc/sysconfig/cman

> At this point, I haven't started the cman or pacemaker services on the other nodes.
> 
> If this is the case, what will happen when two nodes of the three die?

Unrelated. This only affects startup.
What happens in this case depends on how you configured no-quorum-policy.

> In case cluster.conf changes accordingly to reflect the new cluster membership, what if all three nodes are simply powered off and one rebooted? The cluster will be down, won't it?
> 
> What is the best way to get around this? I don't want to set CMAN_QUORUM_TIMEOUT=0, since as I understand it, the node would then go ahead and start itself as a cluster without waiting for the other nodes, and if this causes my service to start up

Pacemaker won't start things without quorum - unless you specifically tell it to.

> and it is already started/running on another node, it could cause issues.
> 
> Now, I don't know how to configure quorum disks for pacemaker - is it possible to do this with pacemaker?

Pacemaker doesn't care. Thats a cman/quorum detail.  Pacemaker only cares "do we have quorum".

> How does it work? What are the recommended ways to address the above problem? I infer that if this disk is configured, the node that grabs the disk first becomes the president of the pacemaker cluster. In this context, I have another question - does corosync have its own cluster membership state distributed across all cluster nodes? If so, I guess quorum is configured at the corosync level rather than at the pacemaker level?
> 
> Apologies in advance if my queries above are addressed in the documentation already - I felt it would be quicker and more accurate to ask the community for reliable info.
> 
> Thanks!
> Regards,
> Vijay
> 
> 
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