[Engine-devel] [vdsm] reserve virtio-balloon device created by libvirt

[Itamar Heim]

On 04/29/2012 04:19 PM, Dan Kenigsberg wrote:
> On Sun, Apr 29, 2012 at 07:24:52AM -0400, Andrew Cathrow wrote:
>>
>>
>> ----- Original Message -----
>>> From: "Dan Kenigsberg"<danken at redhat.com>
>>> To: "Gal Hammer"<ghammer at redhat.com>
>>> Cc: vdsm-devel at lists.fedorahosted.org
>>> Sent: Sunday, April 29, 2012 7:19:10 AM
>>> Subject: Re: [vdsm] reserve virtio-balloon device created by libvirt
>>>
>>> On Mon, Apr 23, 2012 at 04:00:55PM +0300, Gal Hammer wrote:
>>>> On 23/04/2012 12:26, Mark Wu wrote:
>>>>> Hi guys,
>>>>>
>>>>> I saw that an option to create balloon device was added by Gal in
>>>>> http://gerrit.ovirt.org/1573
>>>>> I have a question about it. Why don we preserve the old default
>>>>> behaviour? I know it's not supported by ovirt-engine now, but I
>>>>> can't
>>>>> figure out what will break if it's not disabled explicitly. So do
>>>>> you
>>>>> think we can just make use of the balloon device added by libvirt?
>>>>
>>>> We didn't change the old behavior.
>>>>
>>>> Libvirt creates by default a memory-balloon device, so vdsm
>>>> defaults
>>>> was to disable it by adding a "none"-type device. This was done
>>>> because vdsm didn't include an option to add such device.
>>>>
>>>> My patch added an option to create a memory-balloon through vdsm.
>>>> If
>>>> the user didn't request to add the device, the behavior is same as
>>>> before, disabling the memory-balloon.
>>>
>>> I feel that it would be best not to flip Vdsm's default at the
>>> moment,
>>> even though it is the opposite of libvirt's. I would consider to flip
>>> them only after your (Mark's) patches are in, tested, and proven
>>> worthwhile for the common case.
>>>
>>> Currently, without any management for the balloon, reserving a guest
>>> PCI
>>> device was deemed wasteful.
>>
>> On the other side of the fence
>> - We know that we do need to do ballooning
>> - In the (next?) release we'll end up adding this support
>> - There's no harm (see next point) in adding the device now in fact it saves a config change on upgrade.
>
> Well, there is a surprise factor, for someone running a guest generated
> in a previous version. Suddenly, after Vdsm upgrade, it would see an
> additional device. At the least, I would like Vdsm to have a
> configurable option to keep the old behavior.

please take into consideration engine has an algorithm testing max 
number of devices and it should be aware of newly introduced devices by 
vdsm or it will overflow.

>
>> - While it takes up a PCI slot it's going to be very, very rare deployments that will ever see the limit, libvirt/virtmanager/virt-install has done this forever without seeing push back.
>
>
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