[libvirt-users] Backup a VM (using live external snapshot and blockcommit)

[Kashyap Chamarthy]

[. . .]

On Fri, Sep 11, 2015 at 10:45:34AM -0600, Eric Blake wrote:
> On 09/11/2015 10:18 AM, Jérôme wrote:
> 
> >> Yep, that about covers it.  Note that the --quiesce step in snapshot
> >> creation requires qemu-guest-agent running in the guest, and that you
> >> trust interaction with your guest.
> > 
> > Yes, I think I get this. I don't really figure out what these cases
> > could be. We're using Debian Jessie and I installed qemu-guest-agent.
> > Other VM could use other systems, but most likely Linux based.
> 
> qga with support for quiesce has also been ported to Windows guests.
> 
> > 
> > Do you mean that, in cases where you shouldn't trust the guest, using
> > '--quiesce' might end up being worse than nothing? Or just useless?
> 
> If the agent is not running, using --quiesce will fail the entire
> command; you'd learn pretty quickly to retry without --quiesce for
> guests that don't know how to handle it. . But if the guest is
> malicious, it can pretend to be a guest agent, but intentionally refuse
> to reply to the --quiesce request, and leave libvirt hung waiting for a
> reply.  So it boils down to whether you trust your guests to be
> reasonable with their guest agent connection (fine if it is your own
> guests, not so much if you are hosting a cloud for other people's guests).
> 
> >  
> >>> -> Anything wrong about my snapshot-create-as and blockcommit command
> >>> lines? May I remove the snapshot with only a rm command?
> >>
> >> Looks correct to me, and matches my recent KVM Forum slides:
> >> http://events.linuxfoundation.org/sites/events/files/slides/2015-qcow2-expanded.pdf
> > 
> > I'll have a look at these, thanks.

Yes, I highly recommend it.  This talk gives an excellent under-the-hood
details of virtual machine disk image backing chain management.
Associated video:

    https://www.youtube.com/watch?v=etIGp12RHRE

> The libvirt commands were towards the end, in part 3; but the first two
> parts might give a better understanding of the overall operations of
> what is happening.
> 
> 
> >> virsh snapshot-delete --metadata $dom $badname
> >>
> >> to remove $badname snapshot that no longer exists because you changed
> >> things behind the scenes.
> > 
> > Before removing the .xml file, I tried the command indicated in the wiki
> > [1] with no success.
> > 
> > "NOTE-2: Optionally, you can also supply '--no-metadata' option to tell
> > libvirt to not track the snapshot metadata -- this is useful currently
> > as at a later point when you merge snapshot files, then you have to
> > explicitly clean the libvirt metadata (by invoking: virsh
> > snapshot-delete vm1 --delete --current -- repeat this as needed.)"
> > 
> > Shouldn't the 
> > 
> >     virsh snapshot-delete vm1 --delete --current
> > 
> > be rephrased as 
> > 
> >     virsh snapshot-delete vm1 --metadata --current 
> 
> Yep, sounds like a bug in the wiki, so I fixed it.

Indeed, it was a typo.  I didn't even notice it until now as I just type
these commands from muscle memory.

Thanks, Eric, for fixing it (and for all the detailed responses).

-- 
/kashyap