Re: [Qemu-devel] [PATCH 01/19] Specification for qcow2 version 3

[Stefan Hajnoczi]

On Mon, Apr 16, 2012 at 12:34 PM, Kevin Wolf <address@hidden> wrote:
> Am 13.04.2012 12:30, schrieb Stefan Hajnoczi:
>> On Thu, Apr 12, 2012 at 4:01 PM, Kevin Wolf <address@hidden> wrote:
>>>  The remaining space between the end of the header extension area and the 
>>> end of
>>> -the first cluster can be used for other data. Usually, the backing file 
>>> name is
>>> -stored there.
>>> +the first cluster can be used for the backing file name. It is not allowed 
>>> to
>>> +store other data here, so that an implementation can safely modify the 
>>> header
>>> +and add extensions without harming data of compatible features that it
>>> +doesn't support. Compatible features that need space for additional data 
>>> can
>>> +use a header extension.
>>
>> Does this change the spec for qcow2 version 2?  Previously anything
>> could be after the header extension area, now this has been changed so
>> only the backing filename is allowed (for safe modification).  In
>> practice this should be okay but in theory I think this is changes the
>> qcow2 version 2 semantics.
>
> I guess it does. Are you aware of anyone using this? We could in theory
> add the restriction only for version 3+, but I'd rather avoid special
> cases if possible.
>
> I think in practice even the old qcow2 implementation would break images
> if you put arbitrary data there.

I'm not aware of anything that uses it.

Stefan