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

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):


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

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