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"<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.

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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