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

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> 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.

Well what sort of incompatible extensions are we talking about?  Are 
there things being kicked around?

Regards,

Anthony Liguori