Re: [libvirt-users] Update on-disk XML after virsh snapshot-create-as
----- Original Message -----
From: "Eric Blake" <eblake(a)redhat.com>
To: "Andrew Martin" <amartin(a)xes-inc.com>, libvirt-users(a)redhat.com
Sent: Tuesday, April 14, 2015 10:26:50 AM
Subject: Re: [libvirt-users] Update on-disk XML after virsh snapshot-create-as
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.
I am unaware of a 3rd-party vendor that backports newer releases of libvirt
for Ubuntu - is there one you would recommend? Currently, for 14.04 I am using
the packages from the Ubuntu repository, and for 12.04 I built newer versions
of libvirt myself in this PPA:
https://launchpad.net/~xespackages/+archive/ubuntu/virtualization
Is there a better way to keep up-to-date with libvirt releases on Ubuntu LTS?
>
> 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_...
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.
Thanks for this clarification. It looks like the following would update both
the running and on-disk configs?
virsh desc <domain> --config --live --new-desc "<new description
here>"
Thanks,
Andrew