On Sun, Apr 29, 2012 at 07:24:52AM -0400, Andrew Cathrow wrote:
> 
> 
> ----- Original Message -----
> > From: "Dan Kenigsberg" <dan...@redhat.com>
> > To: "Gal Hammer" <gham...@redhat.com>
> > Cc: vdsm-devel@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.

> - 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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