[Engine-devel] host cpu feature

[Itamar Heim]

On 12/05/2012 09:15 PM, snmishra at linux.vnet.ibm.com wrote:
>
> Quoting Doron Fediuck <dfediuck at redhat.com>:
>
>> ----- Original Message -----
>>> From: "Laszlo Hornyak" <lhornyak at redhat.com>
>>> To: "Doron Fediuck" <dfediuck at redhat.com>
>>> Cc: "engine-devel" <engine-devel at ovirt.org>
>>> Sent: Wednesday, December 5, 2012 7:14:46 PM
>>> Subject: Re: [Engine-devel] host cpu feature
>>>
>>>
>>>
>>> ----- Original Message -----
>>> > From: "Doron Fediuck" <dfediuck at redhat.com>
>>> > To: "Yaniv Kaul" <ykaul at redhat.com>
>>> > Cc: "Laszlo Hornyak" <lhornyak at redhat.com>, "engine-devel"
>>> > <engine-devel at ovirt.org>
>>> > Sent: Wednesday, December 5, 2012 6:10:55 PM
>>> > Subject: Re: [Engine-devel] host cpu feature
>>> >
>>> > > Alternative idea, inspired by "Thus, if you hit any bugs, you are
>>> > > on
>>> > > your own" (http://libvirt.org/formatdomain.html#elementsCPU wrt
>>> > > 'host-passthrough'):
>>> > > A config option to determine if we use host-model or
>>> > > host-passthrough.
>>> > > Y.
>>> > >
>>> >
>>> > I do not think the engine should go to this level.
>>> > ie- it can ask for passthrough as a feature, and the
>>> > actual implementation is handled by vdsm.
>>> >
>>>
>>> If vdsm decides over host-passthrough or host-model, then how will
>>> the engine user know what exactly his VM gets. I think vdsm must be
>>> told exactly what to do.
>>>
>>
>> VDSM maintains some level of independence. This is why it the engine
>> should be able to ask for passthrough as a feature. Otherwize vdsm will
>> handle it. So for now I suggest we stick to passthrough only, and if
>> we get an RFE for advanced mode we'll support the host model.
>
> What are we gaining by using passthrough over host-model? Looking at
> libvirt documentation, it seems that both modes give host CPU
> capabilities to guest VM. Whereas the downside of passthrough is that it
> limits migration. Whereas host-model will migrate to other hardware and
> if the destination hardware is better than source then the guest VM
> performance can be improved by rebooting guest.
>
> As a stretch goal, ovirt can keep track of host capabilities and inform
> the user after migrating to a better host, that a reboot may improve
> guest performance.

pass-through may give better performance.
host-model would be relevant when we can support live migration inside 
the cluster for some of the nodes, which will be relevant when the 
scheduler is more pluggable/extendable than today.