[libvirt-users] Create and revert to snapshot fails silently after device update on running domain

Eric Blake eblake at redhat.com
Thu Oct 4 16:44:27 UTC 2012

On 09/26/2012 04:19 AM, Jasper Spit wrote:
> Hi list,
> I'm having an issue with snapshot creation. Scenario:
> qemu 1.1
> libvirt 0.9.12
> I create a domain, and start it. The domain has 1 IDE cdrom device
> defined (see below).
> When started, I want to mount an ISO file to it. So I do
> updateDeviceFlags in libvirt-python or update-device in virsh (both have
> the same problem).

Both update approaches go through the same libvirt API, so it the action
of a live device hot-plug that is messing things up here.

> This works fine, the ISO image becomes available to the domain. Now I
> create a snapshot on the still running domain using snapshotCreateXML in
> libvirt-python or snapshot-create in virsh. The command returns
> immediately without error (a normal snapshot takes several seconds to
> complete).

That sounds like a bug, where libvirt should have given an error about
being unable to create a snapshot.

> If I revert to this snapshot this command also returns
> immediately without error, but the snapshot is not actually reverted to,

Probably fallout from the first bug - if the snapshot was never created
in the first place, but libvirt went ahead and updated its metadata to
claim that the snapshot exists, then reverting will have nothing to
return to.

> the domain remains running in the same state as if nothing had happened
> (I test this by verifying console output and checking if a testfile is
> present on the domain). If I do not use the update device commands prior
> to creating a snapshot, all is well. If I remove the source from the
> cdrom device using update device, the snapshots work properly again.
> Any idea what causes this?

It sounds like qemu's 'savevm' command is not very good about handling
saves after a hotplug event, and that libvirt isn't properly recognizing
this as a situation in which it must fail the snapshot.  I'll have to
try and reproduce the setup to see if I can come up with a libvirt
patch, but your steps look pretty detailed.  Would you mind opening this
as a BZ, so it doesn't get lost?

> Steps to reproduce using virsh:
> virsh # start 4d5d722b-864c-657e-0f39-55d1bafc760e
> Domain 4d5d722b-864c-657e-0f39-55d1bafc760e started
> virsh # snapshot-create 4d5d722b-864c-657e-0f39-55d1bafc760e
> Domain snapshot 1348653920 created
> virsh # snapshot-revert 4d5d722b-864c-657e-0f39-55d1bafc760e 1348653920
> All is good, the snapshot is reverted to properly. Now I update the
> cdrom device:
> virsh # update-device 4d5d722b-864c-657e-0f39-55d1bafc760e deb.xml
> Device updated successfully
> virsh # snapshot-create 4d5d722b-864c-657e-0f39-55d1bafc760e
> Domain snapshot 1348654116 created
> Command returns instantly.

> virsh # dumpxml 4d5d722b-864c-657e-0f39-55d1bafc760e
> <domain type='kvm' id='135'>

>   <devices>
>     <emulator>/usr/bin/kvm</emulator>
>     <disk type='file' device='cdrom'>
>       <driver name='qemu' type='raw'/>
>       <source file='/data/images/debian-live-6.0.4-amd64-standard.iso'/>
>       <target dev='hdc' bus='ide'/>
>       <readonly/>
>       <alias name='ide0-1-0'/>
>       <address type='drive' controller='0' bus='1' target='0' unit='0'/>
>     </disk>
>     <disk type='file' device='disk'>
>       <driver name='qemu' type='qcow2'/>
>       <source file='/data/domains/2f8baacd-563c-b747-b621-c0ddb4aa84bd'/>
>       <target dev='vda' bus='virtio'/>
>       <alias name='virtio-disk0'/>
>       <address type='pci' domain='0x0000' bus='0x00' slot='0x05'
> function='0x0'/>
>     </disk>

> </domain>
> deb.xml:
> <disk type='file' device='cdrom'>
>   <driver name='qemu' type='raw'/>
>   <source file='/data/images/debian-live-6.0.4-amd64-standard.iso'/>
>   <target dev='hdc' bus='ide'/>
> </disk>
> deb-off.xml:
> <disk type='file' device='cdrom'>
>   <driver name='qemu' type='raw'/>
>   <target dev='hdc' bus='ide'/>
> </disk>
> Thanks much,

Hopefully we can get this resolved before the next libvirt release.

Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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