Re: [Qemu-devel] [PATCH] spec/qcow2: bitmaps: zero bitmap table offset

[John Snow <jsnow@redhat.com>]

On 06/29/2016 08:22 AM, Vladimir Sementsov-Ogievskiy wrote:
> This allows effectively free in_use bitmap clusters including bitmap
> table without loss of meaningful data.
> 
> Now it is possible only to free end-point clusters and zero-out (not
> free) bitmap table
> 

Same comment as Max, the commit message needs to be a little better.

> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> Hi all!
> 
> Here is one small but significant addition to specification of bitmaps in qcow2.
> 
> Can we apply it just like this or I'll have to inroduce new incompatible feature flag?
> 
> If there is existing implementation of the format, it may break image, saved by
> software, using extended spec. But is there are any implementations except not
> finished my one?
> 

In pure, actual fact... your WIP reference implementation is almost
certainly the only implementation in existence.

I think this change is probably fine, as a zeroed entry here before
would have meant literally the qcow2 header itself which is quite
obviously wrong/invalid.

So it's probably a fairly easy extension to identify
dynamically/on-the-fly and won't collide with prior versions.

> 
>  docs/specs/qcow2.txt | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 80cdfd0..dd07a82 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -435,6 +435,8 @@ Structure of a bitmap directory entry:
>                      Offset into the image file at which the bitmap table
>                      (described below) for the bitmap starts. Must be aligned to
>                      a cluster boundary.
> +                    Zero value means that bitmap table is not allocated and the
> +                    bitmap should be considered as empty (all bits are zero).
>  
>           8 - 11:    bitmap_table_size
>                      Number of entries in the bitmap table of the bitmap.
>