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

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

On 29.06.2016 15:22, 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
>
> 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?
>
>
>   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.

+               bitmap_table_size must be zero if bitmap_table_size is zero.



-- 
Best regards,
Vladimir