Re: [Qemu-devel] [RFC] qemu snapshot enchancement

[Stefan Hajnoczi]

On Tue, Jan 29, 2013 at 2:32 PM, Paolo Bonzini <address@hidden> wrote:
> Il 29/01/2013 14:27, Stefan Hajnoczi ha scritto:
>>> > and in step 5, may need export the delta data, not the whole disk
>>> > data.
>> NBD doesn't have a way to perform bdrv_is_allocated().  Either we need
>> to enhance the protocol or we need to add a QMP command to read
>> the allocation bitmap for an image.  I'm a little hesitant about sending
>> the bitmap or allocation extent information over QMP (JSON) but it might
>> be doable.
>
> I'm planning to add offline mirroring to qemu-img.  If you use an NBD
> server as the destination, it can be used to send only the delta between
> two snapshots via NBD.
>
> I think this is the opposite of what you suggested, which is to run
> qemu-nbd on the image and query the server.

Yep, exactly.  The nice thing about your approach is that we don't
need an explicit API for dirty block tracking, it's implicit in the
writes that we send to the backup application's NBD server.

Stefan