libvirt users - Apr 2015 - Update on-disk XML after virsh snapshot-create-as

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.

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

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

Thanks,

Andrew Martin

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

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