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Just FYI, if some fancies doing the same for XenAPI, there is some
example code here:
http://docs.vmd.citrix.com/XenServer/4.0.1/api/client-examples/python/watch-all-events.py.html

John

On 12 February 2013 17:36, Daniel P. Berrange <berrange at redhat.com> wrote:
> On Tue, Jan 15, 2013 at 10:52:39AM +0000, Daniel P. Berrange wrote:
>> This email is in relation to the following bug
>>
>>   https://bugs.launchpad.net/nova/+bug/1099761
>>
>> If a guest administrator shuts down their VM from inside the guest
>> OS using 'shutdown -h now' (or equivalent), it will take Nova upto
>> 10 minutes to notice that the VM has stopped running
>>
>> The Nova<->hypervisor state synchronization is done in a periodic
>> task (nova.compute.manager._sync_power_states) which only runs
>> every 10 minutes. It used to be every 60 seconds until this bug
>> was addressed:
>>
>>   https://bugs.launchpad.net/nova/+bug/928910
>>
>> I put most of the blame for that bug's perforance issues in the use
>> of the wrong libvirt APIs, but none the less I don't really want to
>> put the periodic task frequency back down to 60 seconds.
>>
>> IMHO the key problem here is the design, not the implementation. In
>> particular the use of polling by the manager to detect the state
>> changes is inherantly inefficient. Libvirt provides an async event
>> notification mechanism allowing applications to have a callback
>> triggered whenever a guest changes lifecycle state. Other hypervisor
>> APIs like ESX have similar notification mechanisms. Making use of
>> this would allow state changes to be detected pretty much immediately.
>>
>> The question is how to structure the processing of events. I don't
>> think that the hypervisor drivers should be directly processing events.
>> Instead I believe they need to pass along the event notifications to
>> the manager.py class. So my current thought is to introduce a new API
>> to nova.virt.api
>>
>>   register_event_notifier(self, callback)
>>
>> and have nova.compute.manager provide a callback impl to receive the
>> events. Before I start coding on this, I want some kind of confirmation
>> that this is an acceptable direction to go in, since there is no current
>> callback based interactions between nova.compute.manager & nova.virt.api
>>
>> Even with the register_event_notifier() callacks, I figure we'd still
>> run the periodic _sync_power_states() tasks at 10 minute intervals. It
>> should merely not have an work todo when it runs. Alternatively we
>> could disable this periodic tasks if-only-if register_event_notifier()
>> was successful (ie didn't raise NotImplementedException)
>
> Based on the discussions in this thread, I have done the work to
> introduce an event mechanism from the compute drivers to the
> compute manager
>
> http://wiki.openstack.org/ComputeDriverEvents
> https://blueprints.launchpad.net/nova/+spec/compute-driver-events
> https://review.openstack.org/#/q/status:open+project:openstack/nova+branch:master+topic:bp/compute-driver-events,n,z
>
> I have only implemented this for libvirt, and it should be a no-op
> for other drivers.
>
> Regards,
> Daniel
> --
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>
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-- 
John Garbutt MA (Cantab) MBCS
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