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



On Thu, Feb 06, 2014 at 12:02:11PM +0530, Kashyap Chamarthy wrote:
> On Wed, Feb 05, 2014 at 10:54:15PM +0100, 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?
> 
> As I understand it, no, you'll not lose anything.
> 
> Here's some old notes for live backup with 'virsh blockcopy', this was
> after discussing with Eric
> 
>   http://kashyapc.fedorapeople.org/virt/lc-2012/live-backup-with-blockcopy.txt
> 
>   Currently due to QEMU's limitation, libvirt expects a domain to be
>   trasient. Because, blockcopy jobs last forever until canceled, which
>   implies that they should last across domain restarts if the domain
>   were persistent. But qemu doesn't yet provide a way to restart a copy
>   job on domain restart. So the trick is to temporarily make the domain
>   transient
> 
> 
> > Or it will still use the original
> > disk image?
> > 
> > AFAIK, a transient guest only means it will disappear after the
> > virtualisation session ends.
> 
> The disk image stays intact, just it'll disappear from libvirt's view,
> hence you take a copy of your XML file below ('virsh dumpxml') does that
> 
> > 
> > > virsh dumpxml $dom > file
> > > virsh undefine $dom
> > > virsh blockcopy $dom /ssd/image.raw /hdd/image.raw \
> > >   --wait --verbose --pivot
> > > virsh define file
> > 
> > I could not find anything about "pivot" or "pivoting"? What does --pivot
> > do in this case?
> 
> When you "pivot", libvirt will use the new copy (i.e. /hdd/image.raw
> in this case), and this is done while the guest is running.
> 
> Eric, please correct if I said something wrong.

Ah, Eric already explained it more eloquently (I hit send before seeing
his email).

-- 
/kashyap


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