[libvirt-users] Update on-disk XML after virsh snapshot-create-as

[Eric Blake]

On 04/14/2015 09:17 AM, Andrew Martin wrote:
> Hello,
> 
> I am using "virsh snapshot-create-as" to create external snapshots of running
> VMs using libvirt 1.0.2 on Ubuntu 12.04 and libvirt 1.2.2 on Ubuntu 14.04. This
> works well for creating the snapshots, but I notice that the on-disk XML file
> in /etc/libvirt/qemu is not updated with the new disk image file after the
> snapshot is created. This appears to be very similar to
> https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1403841, but occurs on
> both 12.04 and 14.04 so I am not sure it is the same bug.

There have been some bug fixes in newer libvirt; for example, commit
9f97485 (v1.2.12) is relevant.  I'm not sure why your vendor is not
backporting these fixes to the build shipped in Ubuntu, but you can
manually run a newer libvirt to work around the problem.

> 
> This presents a problem if the virtual machine host is suddenly powered off
> since a subsequent boot will find the VM using an older image file to boot
> rather than the most recent snapshot. Is it safe for me to use an external
> wrapper script that does the following to ensure the on-disk XML is up-to-date:
> virsh snapshot-create-as ....  
> virsh dumpxml > /etc/libvirt/qemu/<domain>.xml

No.  That is not safe.

> 
> Is it safe for me to overwrite the contents of /etc/libvirt/qemu/<domain>.xml
> manually while the VM is running?

Not at all.
http://wiki.libvirt.org/page/FAQ#Where_are_VM_config_files_stored.3F_How_do_I_edit_a_VM.27s_XML_config.3F

Instead, if you are still suffering from a build of libvirt that fails
to update the files in /etc when taking a snapshot, do some other action
that DOES update /etc (such as changing the domain description), or at a
bare minimum use 'virsh define' instead of manually writing into /etc
yourself.

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

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