[Linux-cluster] Multipathed quorum disk

[Lon Hohberger]

On Sat, Sep 01, 2007 at 02:57:48AM +0200, Claudio Tassini wrote:
> Hi,
> I recently upgraded a 2-nodes cluster adding two more nodes. I would like a
> single node to remain in cluster even if the other three are out of service,
> so I'm trying to add a quorum disk to the cluster.
> 
> The problem is that the quorum disk is a LUN in a shared storage which has
> not the same device name through all the cluster nodes. Moreover, we use
> device-mapper AND lvm. I could resolve the problem using an lvm logical
> volume, because it would always have the same name and recognize the
> underlying "dm" or "sd" device name even if it changes across a reboot, but
> I've read that it's not advisable to use a logical volume as quorum device.

You can do it, but if the LVM volume is clustered, you can introduce a
circular dependency:

  * need quorum to access CLVM volume
  * need CLVM volume to become quorate...

You can work around this by making qdisk's votes 1 less than the number
of nodes in the cluster.   Ex: 1 vote for qdisk and 1 vote per node,
expected_votes = 3 & two_node = 0 for CMAN.

Then, both nodes can come online before you start qdisk to eliminate the
"chicken and egg" problem.

This might also be related -

There's a bugzilla open against qdisk; as qdisk doesn't work with
devices which do not have a 512 byte sector size.  I should have a fix
for it this week.

https://bugzilla.redhat.com/show_bug.cgi?id=272861

-- 
Lon Hohberger - Software Engineer - Red Hat, Inc.