[libvirt] surprising <backingStore type='file'> setting in domain.xml

[Eric Blake]

On 5/16/19 10:20 AM, Thomas Stein wrote:
> Hello all.
> 
> My currently used versions: libvirt-5.2.0 and qemu-4.0.0.
> 
> Here is my problem. I'm struggeling since a few weeks with a strange
> behaviour by either qemu or libvirt. After a reboot of
> the hardware node the $domain.xml contains suddenly a backingStore
> setting which was not there before reboot.
> Something like that:
> 
>   <devices>
>     <emulator>/usr/bin/qemu-system-x86_64</emulator>
>     <disk type='file' device='disk'>
>       <driver name='qemu' type='qcow2'/>
>       <source
> file='/var/lib/libvirt/shinymail/shinymail_weekly.qcow2-2019-05-15'/>
>       <backingStore type='file'>
>         <format type='qcow2'/>
>         <source file='/var/lib/libvirt/images/shinymail.qcow2'/>

Yes, this matches:

>       </backingStore>
>       <target dev='vda' bus='virtio'/>
>       <address type='pci' domain='0x0000' bus='0x00' slot='0x07'
> function='0x0'/>
>     </disk>
> ...
> 
> This obviousely happens after a backup has been running. The Backup
> Script looks like this:
> 
> <snip>
> virsh snapshot-create-as --domain shinymail weekly --diskspec
> vda,file=/var/lib/libvirt/shinymail/shinymail_weekly.qcow2-$(date
> +%Y-%m-%d) --disk-only --atomic --no-metadata

the effects of this command.

Ultimately, I'm TRYING to get my new 'virsh domain-backup' command
integrated into the next libvirt release, which has the advantage of
performing a backup WITHOUT having to modify the <domain> XML. But until
that happens, any time you use 'virsh snapshot-create-as' as part of a
sequence for performing backups, you ARE modifying the <domain> XML, and
if you want to revert to the external backup, or if...

> 
> cp ...
> 
> virsh blockcommit shinymail vda --active --verbose --pivot
> <snip>

...blockcommit fails for whatever reason to undo the effects of
'snapshot-create-as' in creating a temporary overlay, then yes, you do
have to worry about the temporary overlay being in the way, where you'll
have to manually edit the <domain> definition to match the actual disk
layouts you really want.

> 
> So after that "dmblklist shinymail" does show the right source file but
> after a reboot it tries to use the weekly snapshot
> again which leads to filesystem errors.
> 
> Someone has an idea what could cause such a behaviour?
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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