A couple of days ago we had a discussion on Admission Control and DPM internally at VMware. One of our customers had enabled DPM on a HA cluster. During the evening 4 out of 5 hosts where placed into standby mode because of this.
This customer, as many of our customers have these days, had vCenter running virtual. This of course led to the question; what happens if this one host fails and our virtual vCenter server is running on it?
That’s an easy one; nothing. It might not be the answer you are looking for but when the host fails that runs vCenter there’s no host or service left to get these hosts out of maintenance mode or restart your VMs.
Now maybe even more important; what causes this behavior?
This behavior is caused by the fact that admission control is disabled. If you disable admission control DPM will put hosts into standby mode even if it violates failover requirements. This means that if you have virtualized your vCenter server this is definitely something to be aware of.
For more info/background: http://kb.vmware.com/kb/1007006
I guess another way to prevent this behavior would be to pick 2 hosts to ALWAYS stay up. Set their DPM policy to manual.
CARLO