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

[Andrew Martin]

----- Original Message -----
> From: "Eric Blake" <eblake at redhat.com>
> To: "Andrew Martin" <amartin at xes-inc.com>, libvirt-users at 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_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.
> 
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