[Linux-cluster] Cluster not starting backup after reboot

James Parsons jparsons at redhat.com
Tue Sep 11 14:32:48 UTC 2007

James Wilson wrote:

> I think I have the wrong syntax for fencing xen. Do I add 
> <fence_xvmd/> to the config file or <xvmd/>?

The <fence_xvmd/> tag should be in the cluster.conf file as a child of 
the <cluster> tag, in the dom0 cluster. This  just tells the outside 
cluster that vm fencing is going to be employed, so the fence_xvm daemon 
is started and begins listening for distress from the virtual cluster it 
is hosting.

BTW, I am pretty sure that if you include this tag and you do NOT have a 
virtual cluster set up (yet), then nothing bad happens except that a few 
cpu cycles are stolen from the dom0 machine running the daemon for 
nothing. The inverse, however, is not true. A virtual cluster cannot be 
depended upon without the daemon running on the physical host(s).

I hope this explanation buys you some insight. The simple reason for all 
of this, is that DomU machines do not know they are virtual and they 
cannot call 'vm destroy' on another vm even if they did know. They call 
the fence_xvm fence agent when there is trouble, and this agent calls 
out to the fence_xvm daemon running in the outer physical cluster and 
asks it to please shut a particular VM down.

Perhaps xen kernels should include Kierkagaard and Sarte libraries for 
helping them deal with their isolation, alienation, and dreaded lonliness.


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