[libvirt-users] Can I move the disk image of the guest while it is running?

[Eric Blake]

On 02/05/2014 02:54 PM, Gergely Horváth wrote:
> Thank you Eric,
> 
> On 2014-02-05 17:23, Eric Blake wrote:
>> Yes, live storage migration is possible; although at the moment, qemu is
>> lacking a way to restart the operation if it fails midstream, so libvirt
>> only allows the operation if you are willing to temporarily make your
>> guest transient.
> 
> What does this mean? Will I loose anything if - for example - there is
> not enough space on the target device? Or it will still use the original
> disk image?

If blockcopy fails before mirroring is in sync, such as for insufficient
space on the destination, then the source is still valid, with no data
loss.  If it reaches sync, then both source and destination are copies
of each other until you quit the operation (the --wait flag tells virsh
to automatically wait for sync, and the --pivot flag says to quit the
operation as soon as sync is reached, rather than doing an indefinite
mirror).  As long as the guest doesn't quit in the meantime, you haven't
lost anything.  If the guest powers itself off in the middle of the
blockcopy, the blockcopy will fail, but the original is still intact at
the state it was when the guest shut down.

> 
> AFAIK, a transient guest only means it will disappear after the
> virtualisation session ends.

Correct, which is why you can then re-define the guest with the XML you
saved earlier, so that it is once again persistent and can be safely
powered off.  If blockcopy --pivot succeeded, then the guest uses only
the destination, and you have successfully gotten rid of the need to
refer to the source.

> 
>> virsh dumpxml $dom > file
>> virsh undefine $dom
>> virsh blockcopy $dom /ssd/image.raw /hdd/image.raw \
>>   --wait --verbose --pivot
>> virsh define file

Oh, I just realized a caveat - before redefining the file, you'll need
to edit it to reflect a successful blockcopy (otherwise, if the guest
stops and then is rebooted, the reboot will still use the original
source, rather than the destination); a safer set of steps might be:

virsh dumpxml $dom > file
virsh undefine $dom
if virsh blockcopy $dom /ssd/image.raw /hdd/image.raw \
   --wait --verbose --pivot; then
  virsh dumpxml $dom > file # refresh the xml to reflect new disk source
fi
virsh define file

so that file restores you to the original disk location if blockcopy
fails, but tracks the new location if blockcopy succeeds.

[In my development work, I've been testing on throwaway disks, where I
don't care if I clobber things by starting over again - but in a
production environment, it pays to be more careful, and possibly to
experiment on a safe domain with throwaway disk before doing it on your
real domain]

> 
> I could not find anything about "pivot" or "pivoting"? What does --pivot
> do in this case?

--pivot says to break mirroring by using just the destination (in other
words, pivot away from the source).  The alternative is to use --finish,
which says break mirroring by using just the source (in other words, the
destination is now a point-in-time snapshot at the time you broke
mirroring - useful for backup purposes).

And if you are terrified of the possible consequences of a transient
domain, there is another possibility for moving your data to a new disk,
except that it forces you to convert to qcow2:

virsh snapshot-create-as $dom --no-metadata --disk-only \
  --diskspec /ssd/image.raw,file=/hdd/image.qcow2
virsh blockpull $dom --wait --verbose

and also has the drawback that if the guest quits in the middle of the
blockpull, then you have to track both the original file and the
snapshot wrapper - but at least blockpull is restartable from where it
left off, for the next time the guest is running, whereas blockcopy has
to start from scratch.

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

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