Re: [Qemu-devel] [PATCH] qmp: add 'drive' to enum NewImageMode

[Stefan Hajnoczi <stefanha@redhat.com>]

On Mon, Jun 17, 2013 at 02:44:46PM +0800, Fam Zheng wrote:
> Introduce a "drive" option to new image mode. With this mode, QMP command
> should (this patch only modified drive-backup to support it, and report invalid
> parameter error for drive-mirror) skip creating the image file or trying to
> open it, it should just reuse the existing BDS by looking for the named drive
> with bdrv_find(). It will be useful to utilize "none" sync mode of drive-backup
> for point-in-time snapshot.
> 
> The example with drive-backup is:
> 
>     -> { "execute": "drive-backup", "arguments": { "device": "ide0-hd0",
>                                                    "mode": "drive",
>                                                    "target": "drive_id_here" } }
>     <- { "return": {} }
> 
> 
> Target bs is not released when block job completes in this case since it's
> still used as a device drive or exported by nbd server.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/backup.c            |  9 ++++++--
>  blockdev.c                | 57 ++++++++++++++++++++++++++++++++---------------
>  include/block/block_int.h |  3 ++-
>  qapi-schema.json          |  4 +++-
>  4 files changed, 51 insertions(+), 22 deletions(-)

Fam and I chatted about this yesterday.  The QMP API change here is that
traditionally 'target' and 'mode' always referred to existing filenames.
Now you can set 'mode': 'drive' and then 'target' becomes a QEMU block
device name.

I think this API extension is reasonable but I think we haven't nailed
down the lifecycle of the snapshot block drive yet...

Stefan