[libvirt-users] Using virsh blockcopy -- what's it supposed to accomplish?

[Gary R Hook]

On 12/23/14 6:17 AM, Kashyap Chamarthy wrote:
> On Mon, Dec 22, 2014 at 03:50:58PM -0700, Eric Blake wrote:
>> On 12/22/2014 03:27 PM, Gary R Hook wrote:
>>> I am experimenting with the blockcopy command, and after figuring out
>>> how to integrate qemu-nbd, nbd-client and
>>> dumpxml/undefine/blockcopy/define/et. al. I have one remaining question:
>>>
>>> What's the point?
>>
>> Among other uses, live storage migration.
>>
>> Let's say you are running on a cluster, where your VM is running locally
>> but was booted from network-accessed storage.  You don't want any guest
>> downtime, but you want to have the faster performance made possible by
>> accessing local storage instead of the network-accessed storage.  virsh
>> blockcopy can be used to change qemu's notion of where the active layer
>> of the disk lives without any guest time, by copying then pivoting to a
>> local file.
>
> To add to Eric's explanation, I recently wrote a small example about it
> here (this was tested with libvirt 1.2.6 & QEMU 2.1):
>
>      http://kashyapc.com/2014/07/06/live-disk-migration-with-libvirt-blockcopy/

I read that article.

Now shut down the domain (post-pivot) which is using the new disk file, 
and start it up, without using a block device. This is the part that no 
one seems to write about, nor do I see that in your example. But thank 
you very much for your help and your articles; very much appreciated.

-- 
Gary R Hook
Senior Kernel Engineer
NIMBOXX, Inc