Re: [Qemu-devel] [PATCH v2 0/6] external backup api

[Paolo Bonzini]

On 29/02/2016 09:14, Markus Armbruster wrote:
> I completely agree with you that Get LBA Status cannot just reflect the
> top layer.  But that's not what I meant to propose.  Let me try to
> explain myself more clearly.
> 
> Consider a QCOW2 image D (for delta) with a backing file B (for base).
> If you open it normally, you see "D over B".  Get LBA Status should
> certainly claim the "deallocated" state only for blocks that are
> allocated neither in D nor B.
> 
> However, you can also open D *without* its backing file.  Then you see
> "D over nothing".  Here, get LBA Status should claim "deallocated" state
> for anything not allocated in D.

Ok, this makes more sense.

The question then is whether to implement this NBD server inside QEMU,
or outside it as a separate process to which QEMU "pushes" blocks as in
the existing backup job.  I would prefer the latter, so that it is
possible to implement various APIs (get block status, but also VMware or
Parallels or whatever).

Basically the same points made in
https://lists.gnu.org/archive/html/qemu-devel/2013-03/msg01969.html
still apply.

Paolo