Re: [Qemu-devel] [PATCH v4 0/7] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Nov 22, 2013 at 01:24:47PM +0800, Fam Zheng wrote:
> This series adds for point-in-time snapshot NBD exporting based on
> blockdev-backup (variant of drive-backup with existing device as target).
> 
> We get a thin point-in-time snapshot by COW mechanism of drive-backup, and
> export it through built in NBD server. The steps are as below:
> 
>  1. (SHELL) qemu-img create -f qcow2 BACKUP.qcow2 <source size here>
> 
>     (Alternatively we can use -o backing_file=RUNNING-VM.img to omit explicitly
>     providing the size by ourselves, but it's risky because RUNNING-VM.qcow2 is
>     used r/w by guest. Whether or not setting backing file in the image file
>     doesn't matter, as we are going to override the backing hd in the next
>     step)
> 
>  2. (QMP) blockdev-add backing=source-drive file.driver=file file.filename=BACKUP.qcow2 id=target0 if=none driver=qcow2
> 
>     (where ide0-hd0 is the running BlockDriverState name for
>     RUNNING-VM.img. This patch implements "backing=" option to override
>     backing_hd for added drive)
> 
>  3. (QMP) blockdev-backup device=source-drive sync=none target=target0
> 
>     (this is the QMP command introduced by this series, which use a named
>     device as target of drive-backup)
> 
>  4. (QMP) nbd-server-add device=target0
> 
> When image fleecing done:
> 
>  1. (QMP) block-job-complete device=ide0-hd0
> 
>  2. (HMP) drive_del target0
> 
>  3. (SHELL) rm BACKUP.qcow2

Interesting implementation, it looks pretty good.  I'll need to review
it a second time to track all the operation block/unblocks.  It wasn't
immediately clear to me whether these patches will restrict something
that used to work.

Stefan