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

[Max Reitz]

On 26.04.2016 13:32, Alberto Garcia wrote:
> 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?

Yes, we do.

Yes, the problem is known.

Yes, I have patches for it.

No, I have no clue why I haven't sent them out yet. I literally have a
series lying around, including its cover letter, from last November. But
for some reason I forgot to send it. Thanks for remining me. :-)

Max

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