Re: [Qemu-devel] [6907] Introducing qcow2 extensions (Uri Lublin)

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
>> We should introduce a notion of compatible vs. incompatible extensions.
>>
>> A compatible extension my be ignored by the qcow2 code if it does not 
>> understand the magic number.  An incompatible extension causes an 
>> abort.  This allows both more flexibility in how we can change the 
>> file format.  I believe ext* does the same thing.
>
> I was assuming that all extensions would be compatible.  I don't like 
> the idea of having qcow2 files floating around that require specific 
> version of QEMU.
>
> For that, we should just bump to qcow3 (that's what versioning is for, 
> right? :-).

The problem is that you would need to bump the version each time you 
made an incompatible change.  We'd end up with qcow4351 very quickly.

One option is to declare qcow3 as qcow2 + all incompatible extensions 
just before 0.11.  Another is to require explicit user action to enable 
an incompatible option.  If you did that, you could expect only to 
upgrade qemu, not degrade, but that's a reasonable assumption in many 
scenarios.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.