On 22/06/16 02:03 AM, Indunil Jayasooriya wrote: > On Wed, Jun 22, 2016 at 11:08 AM, Barak Korren <bkorren at redhat.com> wrote: > >>> >>> My question is: Is this even possible? All the documentation for HA that >> I've found appears to not >>> do this. Am I missing something? >> >> You can use oVirt for that (www.ovirt.org). >> > > When an UNCLEAN SHUDWON happens or ifdown eth0 in node1 , can OVIRT > migrate VMs from node1 to node2? > > in that case, Is power management such as ILO needed? I can't speak to ovirt (it's more of a cloud platform than an HA one), but in HA in general, this is how it works... Say node1 is hosting vm-A. Node1 stops responding for some reason (maybe it's hung, maybe it's running by lost net, maybe it's a pile of flaming rubble, you don't know). Within a moment, the other cluster node(s) will declare it lost and initiate fencing. Typically "fencing" means "shut the target off over IPMI (iRMC, iLO, DRAC, RSA, etc). However, lets assume that the node lost all power (we've seen this with voltage regulators failing on the mainboard, shorted cable harnesses, etc). In that case, the IPMI BMC will fail as well so this method of fencing will fail. The cluster can't assume that "no response from fence device A == dead node". All you know is that you still don't know what state the peer is in. To make assumption and boot vm-A now would be potentially disastrous. So instead, the cluster blocks and retries the fencing indefinitely, leaving things hung. The logic being that, as bad as it is to hang, it is better than risking a split-brain/corruption. What we do to mitigate this, and pacemaker supports this just fine, is add a second layer for fencing. We do this with a pair of switched PDUs. So say that node1's first PSU is plugged into PDU 1, Outlet 1 and its second PSU is plugged into PDU 2, Outlet 1. Now, instead of blocking after IPMI fails, it instead moves on and turns off the power to those two outlets. Being that the PDUs are totally external, they should be up. So in this case, now we can say "yes, node1 is gone" and safely boot vm-A on node2. Make sense? -- Digimer Papers and Projects: https://alteeve.ca/w/ What if the cure for cancer is trapped in the mind of a person without access to education?