Errors in journalctl about virtnbdbackup

[abbbi]

this may be related to apparmor rules or alike. I have no idea what problem libvirtd has here. Help needed.

Mar 06 10:32:08 xxxxxxx libvirtd[1903]: Unable to tear down cgroup access on /var/tmp/backup.ZB7PA.vdc
Mar 06 10:32:08 xxxxxxx libvirtd[1903]: Path '/var/tmp/backup.ZB7PA.vdc' is not accessible: No such file or directory
Mar 06 10:32:08 xxxxxxx libvirtd[1903]: internal error: unable to execute QEMU command 'nbd-server-stop': NBD server not running

it seems to me as if libvirtd wants to stop the NBD service that was used for backup, but it is already fanished.
The file reported is the so called "scratch" file that is temporary created during checkpoint creation (and is
usually removed by libvirtd itself)

I dont think these errors are caused by mis-behavior of virtnbdbackup. You could try to attempt to execute the
same libvirt commands for starting/stopping backup operations and see if your libvirtd reports the same issues.

I dont see the errors related to the NBD Service on debian bookworm, however, the Scratch file issue is
reported too. It may make sense to use the --scratchdir option and use a path that has also related apparmor
settings in your libvirtd setup.

See:

https://libvirt.org/formatbackup.html

or use option "-v" during backup to see which libvirt XML is created to operate here.
I dont think i will invest much time in analyzing what the issue is if everything is working as expected.

[abbbi]

i can reproduce these errors with plain libvirt virsh calls:

Mar 07 12:23:10 cefix libvirtd[3971189]: internal error: child reported (status=125): unable to stat: /var/tmp/backup.UGXZ9.sda: No such file or directory
Mar 07 12:23:10 cefix libvirtd[3971189]: unable to stat: /var/tmp/backup.UGXZ9.sda: No such file or directory
Mar 07 12:23:10 cefix libvirtd[3971189]: internal error: child reported (status=125): unable to stat: /var/tmp/backup.UGXZ9.sda: No such file or directory
Mar 07 12:23:10 cefix libvirtd[3971189]: unable to stat: /var/tmp/backup.UGXZ9.sda: No such file or directory
Mar 07 12:23:10 cefix libvirtd[3971189]: Unable to restore security label on /var/tmp/backup.UGXZ9.sda

consider having the following backup.xml:

<domainbackup mode="pull">
  <server transport="unix" socket="/var/tmp/virtnbdbackup.718571"/>
  <disks>
    <disk name="sda">
      <scratch file="/var/tmp/backup.UGXZ9.sda"/>
    </disk>
  </disks>
</domainbackup>

now start an backup task using virsh:

sudo virsh backup-begin --backupxml backup.xml vm1
Backup started

verify the task is active:

virsh # domjobinfo vm1
Job type:         Unbounded   
Operation:        Backup  

and now stop the task using domjobAbort:

virsh # domjobabort vm1

So for me, this looks like some kind of race condition in libvirtd or libvirtd's apparmor profiles trying to mess with
the scratch file even if its not yet existant.

I dont see a way to workaround these errors in virtnbdbackup, for me it works as expected.