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Re: [Qemu-devel] using "qemu-img convert -O qcow2" to convert qcow v1 to

From: Eric Blake
Subject: Re: [Qemu-devel] using "qemu-img convert -O qcow2" to convert qcow v1 to v2 creates a qcow v3 file?
Date: Mon, 13 Nov 2017 11:58:30 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/13/2017 11:29 AM, Peter Maydell wrote:
> On 13 November 2017 at 17:14, Peter Maydell <address@hidden> wrote:
>> I have a qcow v1 file which I created by mistake (forgetting that
>> you need to tell qemu-img create 'qcow2' and not just 'qcow'),
>> which I want to convert to a v2 file so I can put snapshots into
>> it. But when I try to do this with qemu-img convert it creates a
>> v3 file instead:
>> $ file hda-old.qcow
>> hda-old.qcow: QEMU QCOW Image (v1), 5368709120 bytes
>> $ build/x86/qemu-img convert -O qcow2 hda-old.qcow hda.qcow2
>> $ file hda.qcow2
>> hda.qcow2: QEMU QCOW Image (v3), 5368709120 bytes
>> and if you then try to use that in QEMU it complains:
>> qemu-system-aarch64: -drive if=none,file=hda.qcow2,format=qcow,id=hd:
>> Unsupported qcow version 3
> ah, this means it wants "format=qcow2".

Oh, I should have read this followup before writing my other reply.

> This is pretty confusing, especially the error message, the
> output of "file", and the fact that "format=qcow" can't just
> DTRT if it gets a qcow version 3 (2?), since it can clearly
> identify what it's got.

Indeed, making the qcow driver smart enough to reopen with the qcow2
driver (for both v2 and v3 images) might be an interesting ease-of-use hack.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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