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

[Eric Blake]

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.

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.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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