[libvirt-users] ballooning option xml

[Matthias Bolte]

2010/5/30 udo1 <udo1 at udo.hu>:
> Hi Matthias,
>
> In Ubuntu lucid using kvm, the balloning is disabled by default, so I would
> need to enable it.

How do you detect that it's disabled by default, because I cannot
reproduce that here with Ubuntu 10.04.

libvirt enables ballooning by default. Okay, it only does this when it
detects that your QEMU/KVM version supports it, but Ubuntu 10.04 has
QEMU 0.12.3 and that version has balloon support.

If libvirt detects that your QEMU supports the -device flag then it adds

  -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3

to the command line in order to enable ballooning. This what's added
to the command line for me here.

If your QEMU doesn't supports the -device flag, but supports the
-balloon flag then

  -balloon virtio

is added to the command line in order to enable ballooning.

If your QEMU doesn't support non of these flags that you don't get ballooning.

You can check the used command line in
/var/log/libvirt/qemu/<guest>.log for qemu:///system connections or in
~/.libvirt/log/qemu/<guest>.log for qemu:///session connections.

....

At this point I realize that Ubuntu 10.04 comes with libvirt 0.7.5 and
now I understand your problem.

A comment in the current QEMU driver code says (since libvirt 0.7.6,
git commit 7a90f216):

    /* QEMU changed its default behavior to not include the virtio balloon
     * device.  Explicitly request it to ensure it will be present. */

So, libvirt 0.7.5 still assumes that QEMU automatically adds the
virtio balloon device. The comment and commit message don't say in
which QEMU version this was changed, but I assume that QEMU 0.12.3 (as
in Ubuntu 10.04) doesn't enable ballooing by default anymore.

So you're suffering from a version mismatch between your libvirt and
QEMU version: your libvirt is too old for your QEMU, or QEMU is too
new for libvirt.

I couldn't reproduce your problem at first because I don't have the
Ubuntu libvirt packages installed, but compile bleeding-edge libvirt
from source.

> (I currently have written a wrapper named as kvm adding this kvm option but
> this should not be the right way)

So you can fix this version mismatch with that script, or upgrade
libvirt to a version >= 0.7.6. There is a PPA on Launchpad that offers
more up-to-date libvirt packages for Ubuntu.

  https://launchpad.net/~dnjl/+archive/virtualization

Matthias

> br
> Udo Attila Fischer
>
>
> on 2010.05.26. 14:21, Matthias Bolte wrote:
>>
>> 2010/5/13 udo1<udo1 at udo.hu>:
>>
>>>
>>> Hi everybody,
>>>
>>> I need the ballooning option with kvm using libvirt xml file and virsh
>>> (virsh create domain.xml)
>>> kvm line must be appended with the -balloon virtio switch to enable
>>> ballooning. (can be disabled if there no option or with -balloon none )
>>>
>>> I currently using Ubuntu Lucid
>>>
>>> How can I define this in the domain xml file, or what need I to do to get
>>> this option?
>>> is there any option like
>>> enabling with
>>> <balloon>virtio</balloon>
>>> or
>>> disabling with
>>> <balloon>none</balloon>
>>>
>>>
>>> Thanks in Advance
>>> Udo Attila Fischer
>>>
>>>
>>
>> Ballooning is enabled by default and cannot be disabled via the domain
>> XML.
>>
>> Do you have any specific problem that makes you think there is a
>> problem with ballooning?
>>
>> Matthias
>>
>>
>
>