[ClusterLabs] Colocation and ordering with live migration

Klaus Wenninger kwenning at redhat.com
Mon Oct 10 09:32:58 EDT 2016

On 10/10/2016 02:36 PM, Pavel Levshin wrote:
> 10.10.2016 15:11, Klaus Wenninger:
>> On 10/10/2016 02:00 PM, Pavel Levshin wrote:
>>> 10.10.2016 14:32, Klaus Wenninger:
>>>> Why are the order-constraints between libvirt & vms optional?
>>> If they were mandatory, then all the virtual machines would be
>>> restarted when libvirtd restarts. This is not desired nor needed. When
>>> this happens, the node is fenced because it is unable to restart VM in
>>> absence of working libvirtd.
>> Was guessing something like that ...
>> So let me reformulate my question:
>>    Why does libvirtd have to be restarted?
>> If it is because of config-changes making it reloadable might be a
>> solution ...
> Right, config changes come to my mind first of all. But sometimes a
> service, including libvirtd, may fail unexpectedly. In this case I
> would prefer to restart it without disturbing VirtualDomains, which
> will fail eternally.

And it happens... I remember a project suffering from libvirtd
hanging once in a while.
At this time we tried to tackle the issue with a modification
of the libvirt-monitor so that it would check connectivity to
libvirtd and do a recovery if it wasn't available and just answer
the monitor negatively if this recovery wasn't successful.
Like this you can have hard order constraints with libvirt and
reduce the chance that it is unavailable when it is really needed.

I don't know a way to do this restart not within the RA but
triggered by pacemaker.
But there is a certain relation to a recent and still ongoing
discussion here on the list regarding restarts, final fails, counting
of failing and all this stuff.

> The question is, why the cluster does not obey optional constraint,
> when both libvirtd and VM stop in a single transition?
> In my eyes, these services are bound by a HARD obvious colocation
> constraint: VirtualDomain should never ever be touched in absence of
> working libvirtd. Unfortunately, I cannot figure out a way to reflect
> this constraint in the cluster.
> -- 
> Pavel Levshin
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