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

[Kevin Wolf <kwolf@redhat.com>]

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