[ClusterLabs] Colocation and ordering with live migration

Klaus Wenninger kwenning at redhat.com
Mon Oct 10 13:58:37 EDT 2016

On 10/10/2016 06:56 PM, Ken Gaillot wrote:
> On 10/10/2016 10:21 AM, Klaus Wenninger wrote:
>> On 10/10/2016 04:54 PM, Ken Gaillot wrote:
>>> On 10/10/2016 07:36 AM, 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.
>>> I think the mandatory colocation of VMs with libvirtd negates your goal.
>>> If libvirtd stops, the VMs will have to stop anyway because they can't
>>> be colocated with libvirtd. Making the colocation optional should fix that.
>>>> The question is, why the cluster does not obey optional constraint, when
>>>> both libvirtd and VM stop in a single transition?
>>> If it truly is in the same transition, then it should be honored.
>>> You have *mandatory* constraints for DLM -> CLVMd -> cluster-config ->
>>> libvirtd, but only an *optional* constraint for libvirtd -> VMs.
>>> Therefore, libvirtd will generally have to wait longer than the VMs to
>>> be started.
>>> It might help to add mandatory constraints for cluster-config -> VMs.
>>> That way, they have the same requirements as libvirtd, and are more
>>> likely to start in the same transition.
>>> However I'm sure there are still problematic situations. What you want
>>> is a simple idea, but a rather complex specification: "If rsc1 fails,
>>> block any instances of this other RA on the same node."
>>> It might be possible to come up with some node attribute magic to
>>> enforce this. You'd need some custom RAs. I imagine something like one
>>> RA that sets a node attribute, and another RA that checks it.
>>> The setter would be grouped with libvirtd. Anytime that libvirtd starts,
>>> the setter would set a node attribute on the local node. Anytime that
>>> libvirtd stopped or failed, the setter would unset the attribute value.
>>> The checker would simply monitor the attribute, and fail if the
>>> attribute is unset. The group would have on-fail=block. So anytime the
>>> the attribute was unset, the VM would not be started or stopped. (There
>>> would be no constraints between the two groups -- the checker RA would
>>> take the place of constraints.)
>> In how far would that behave differently to just putting libvirtd
>> into this on-fail=block group? (apart from of course the
>> possibility to group the vms into more than one group ...)
> You could stop or restart libvirtd without stopping the VMs. It would
> cause a "failure" of the checker that would need to be cleaned later,
> but the VMs wouldn't stop.

Ah, yes forgot about the manual restart case. Had already
turned that into a reload in my mind ;-)
As long as libvirtd is a systemd-unit ... would a restart
via systemd create a similar behavior?
But forget about it ... with pacemaker enabling to receive
systemd-events we should probably not foster this
use-case ;-)

>>> I haven't thought through all possible scenarios, but it seems feasible
>>> to me.
>>>> 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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