[ovirt-users] Automatically migrate VM between hosts in the same cluster

[Alex Crow]

On 17/09/15 15:44, Luca Bertoncello wrote:
> Hello Alex
>
>> You can't live migrate on a host failure - as the host has gone down and all the
>> running VMs on it have as well! It would require clairvoyance to enable live
>> migration in that situation.
> Is it possible to enable that? How?

No, it's physically impossible. If your host running VM "foo" has 
unexpectedly gone down (ie the PSU failed), VM "foo" has also gone down. 
Therefore it is not possible to live migrate the VM "foo" since it is 
not longer running!

Even if it was a network failure and the host was still up, how would 
you live migrate a VM from a host you can't even talk to?

The only way you could do it was if you somehow magically knew far 
enough in advance that the host was about to fail (!) and that gave 
enough time to migrate the machines off. But how would you ever know 
that "machine quux.bar.net is going to fail in 7 minutes"?

>
>> However you can enable HA in VMs. If the host they are running on fails they
>> will be restarted automatically on another host. NB. This
>> *requires* power management so the failed host can be fenced.
> Well, we don't have a Power-Management, right now...
> We have Dell hardware, but this PC (right now I just experiment, so I don't use "real servers") does not have any PM...
> Maybe there is an emulator or other solution to check how does it work?

You can use "Manual Fencing" but I don't even know if that is enabled in 
Ovirt. However it would need your intervention (Ie you are told that a 
host has failed, and asked, if it is not powered off, to go and power it 
off by hand. Once you have confirmed that is done, the VMs will start on 
the other host.

The reason you have to fence the failed machine is to make sure you 
don't end up with two of the same VM running, which would completely 
corrupt the image on the shared storage. When using plain old libvirt, I 
have more than once accidentally started the same VM on two hosts, and 
the VM was utterly unrecoverable afterwards.

>
> On the Servers we have Drac6, but oVirt has just Drac5 or Drac7... It does not work with Drac6?

I don't know. Maybe just try both, and run the test on the PM config tab?

Otherwise how about picking up a second-hand APC network switched PDU 
off ebay? That is actually one of the best ways to enable fencing.

Cheers

Alex


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