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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 12:08:16 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/13/2017 11:58 AM, Eric Blake wrote:

>>> 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.

And having the qcow2 driverautomatically be able to open a qcow v1 image
read-only might also be nice - although v1 is lacking so many features
that we'd be insane to let a read-write request of the qcow2 driver
downgrade to qcow.

Another observation: is there any official documentation for the qcow
(v1) format?  I know it's an outdated format that we no longer recommend
for new image creation, but it's sad when I have to resort to a google
search to find old posts like this:


rather than being able to see the documentation of v1 alongside v2 in
the qemu.git repository.

At any rate, since qcow and qcow2 share the same 4-byte magic number and
version field, it explains why both drivers are able to identify (but
fail to open) the files of the other version number.

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

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