[Qemu-block] QCOW2 runtime options and json: backing file strings

[Alberto Garcia]

Hi,

I was away for a few days but I just came back. There's a ton of
patches that I'll try to review these days but I also have a question.

Let's create a QCOW2 image and pass a runtime option such as
l2-cache-size:

   $ qemu -drive if=virtio,file=hd0.qcow2,l2-cache-size=10M

Now we make a snapshot:

   (qemu) snapshot_blkdev virtio0 hd1.qcow2

And here's the backing file string of hd1.qcow2:

   $ qemu-img info hd1.qcow2 | grep backing
   backing file: json:{"driver": "qcow2", "l2-cache-size": "10M", "file": 
{"driver": "file", "filename": "hd0.qcow2"}}
   backing file format: qcow2

The problem is that this QCOW2 image cannot be opened with older
versions of QEMU (because of the "json:" syntax), although there's
nothing in the image itself that would prevent that.

Is this something we should deal with?

Berto