On Thu, Sep 17, 2015 at 6:00 PM, Alex Crow <acrow@integrafin.co.uk> wrote:


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!

- If the PSU failed, your UPS could alert you. If you have one...
- If the machine is going down in an ordinary flow, surely it can be done. 
 

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?

It could be suspended to disk (local) - if the disk is available.
Then the decision if it is to be resumed from local disk or not (as it might be HA'ed and is running elsewhere) need to be taken later, of course.

 

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"?

I completely agree there are situations in which you can't foresee the failure. 
But in many, you can. In those cases, it makes sense for the host to self-initiate 'move to maintenance' mode. The policy of what to do when 'self-moving-to-maintenance-mode' could be pre-fetched from the engine.
Y.




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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