Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

On 25.01.2016 14:09, Kevin Wolf wrote:
> Am 25.01.2016 um 11:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> On 19.01.2016 20:29, Kevin Wolf wrote:
>>> Am 19.01.2016 um 09:57 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>>> On 19.01.2016 00:16, Eric Blake wrote:
>>>>> preserving semantics of those extra_data bytes).  We
>>>>> have enough room for future extension, and that's good e
>>>> Ok, so, what should go to the spec? Current wording is ok? Just
>>>> delete "Type-specific":
>>>>
>>>> +
>>>> +        20 - 23:    extra_data_size
>>>> +                    Size of type-specific extra data.
>>>> +
>>>> +                    For now, as no extra data is defined, extra_data_size is
>>>> +                    reserved and must be zero.
>>>> +
>>>> +        variable:   Extra data for the bitmap.
>>> Please be explicit that if extra_data_size is non-zero, the bitmap must
>>> not be used (i.e. specify the incompatible-feature-bit-like behaviour).
>> It is not enough. If there are some unknown extra data, then just
>> ignoring this bitmap may lead to its inconsistency. So, if it is
>> non-zero, the whole image should not be written. (real
>> incompatible-feature-bit behavior).
> Don't we generally ignore all bitmaps until a user actively tries to
> make use of it? Of course, with the 'auto' flag set, just ignoring the
> bitmap isn't possible, but I think in all other cases it should be.
>
> If we ever add another type of bitmaps that doesn't have the 'auto' flag
> set, but is still automatically used, so that the image as a whole must
> become read-only without the bitmap, we can still add a normal
> incompatible feature flag. But I think it's more likely that we add
> extra_data that doesn't prevent use of the image, so we should have a
> way to express that.
>
> Kevin

let's add one flag:

    2: extra_data_compatible
        This flag is meaningful when extra data is unknown for the software.
        If it is set, the bitmap may be used as usual, extra data must 
be left as is.
        If it is unset, the bitmap must not be used, but left as is with 
extra data.

Ok? And, if there will be incompatible features we will use normal 
incompatible feature flag.

-- 
Best regards,
Vladimir
* now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.