[libvirt-users] Snapshot without volumes temporarily

[Hubert Chiang]

Dear Eric,
Yes, I have two <disk> devices, and I want to take a snapshot with one
disk, which the format is qcow2.
And I want to skip the other disk during the creation which the format is
raw.
I will take your suggestions.
Thanks for your detailed explanations and advices.
I will also try the higher version of libvirt and qemu
Thank you!


On Thu, Mar 28, 2013 at 7:54 PM, Eric Blake <eblake at redhat.com> wrote:

> On 03/28/2013 03:46 AM, Hubert Chiang wrote:
> > Hello,
> >
> > I want to do a snapshot which the VM has volumes.
> >
> > But I want to skip the volume check when I do the snapshot. (Which means
> I
> > want to do a snapshot without detaching the volume)
>
> If I understand you, you have a VM with two <disk> devices, but you only
> need a snapshot of the contents of one of those disks, so you want to
> skip the other one during snapshot creation.


>
>
> > So I try to write a snapshot XML refer by
> > http://libvirt.org/formatsnapshot.html as following:
> >
> > vda is my VM basic disk (qcow2)
> > vdb is a volume (raw)
> > ================ t1.xml ===============
> > <domainsnapshot>
> > <disks>
> > <disk name='vda' snapshot='internal'>
> > <source file='/var/lib/libvirt/VM1/disk' />
> > </disk>
>
> Mixing snapshot='internal' and <disk> was not possible until libvirt
> 1.0.1; furthermore, use of snapshot='internal' requires that ALL of your
> <disk>s are qcow2, based on limitations still present in qemu 1.4.  What
> you are asking for is not possible with the current tools, although we
> might be able to support it in future releases.


> > <disk name='vdb' snapshot='no' />
> > </disks>
> > </domainsnapshot>
> > =====================================
> >
> > with this command: # virsh snapshot-create VM1 --xmlfile t1.xml
> >
> > and get the message: # error: argument unsupported: unable to handle disk
> > requests in snapshot
> >
> > My environment is Libvirt 0.9.8, Qemu 1.0.0 on Ubuntu12.04.02
>
> Correct - the version of libvirt that you are using is too old to mix
> <disks> with internal snapshots.  To use <disk name='vdb'
> snapshot='no'/> with your version of libvirt, you _have_ to pass the
> --disk-only flag and take an external snapshot, but then you are no
> longer able to capture the VM memory state (which was not possible until
> libvirt 1.0.1).
>
> >
> > I try to do this action before at libvirt 0.9.2 QEMU 0.14.1 on Ubuntu
> 11.10
> > before with following commands, It's success.
>
> No, what was really happening with libvirt 0.9.2 is that the <disks>
> element of your snapshot was completely ignored.  <disks> support wasn't
> added until 0.9.5.  You were taking an internal snapshot of ALL disks,
> whether you realized it or not, because that old of libvirt didn't know
> to use <disks>.
>
> >
> > Step1. I copy the XML from /etc/libvirt/qemu/VM1.xml to VM1.xml.backup
>
> Dangerous.  Copying files straight from /etc/libvirt is NOT guaranteed
> to be reliable; instead, you should use 'virsh dumpxml $dom' if you want
> to back up a domain's XML.
>
> > Step2. I edit the VM1.xml to remove the disk tag which is create by
> volume
>
> I'm not sure I follow.  It would help if you showed a before-and-after
> of the .xml file that you are editing, and a bit more explanation of
> what you hoped to accomplish with your edit.
>
> > Step3. do virsh command: virsh snapshot-create VM1
> > Step4. Move VM1.xml.backup to VM1.xml
>
> Even more dangerous.  While it is possible to copy files out of /etc
> without too much risk (the worst that can happen is that you copied
> inconsistent state), writing files _into_ /etc goes behind libvirt's
> back and is liable to break things.
>
> >
> > It's success on libvirt 0.9.2. But on 0.9.8 It doesn't work. Because
> > libvirt will check the setups from memory not the XML file in
> > /etc/libvirt/qemu/
> >
> > What could I do with libvirt 0.9.8?
>
> I'm not sure what you are really wanting to do.  Do you want an internal
> snapshot (requires qcow2, but the snapshot lives in the same file as the
> original) or an external snapshot (which creates a second qcow2 around
> your original file which is now frozen in time)?  Do you want this while
> your domain is running or offline?  Are you willing to upgrade to new
> enough libvirt and qemu to get things properly supported (at least
> libvirt 1.0.1 and qemu 1.2 or better)?
>
> It sounds like maybe you have a situation where your disk for vdb is not
> qcow2, so you can't do an internal snapshot while it is attached.  If
> that is the case, then the only supported way of doing an internal
> snapshot of a running domain, while using your current versions of
> libvirt and qemu, is to use 'virsh suspend VM1' to suspend the domain,
> 'virsh detach-disk ...' to hot-unplug the vdb disk, 'virsh
> snapshot-create ...' to create your snapshot, 'virsh attach-disk ...' to
> hot-plug the vdb disk back in, then 'virsh resume VM1' to resume things.
>
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
>
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