Re: [Qemu-devel] [PATCH v2] spec/qcow2: bitmaps: zero bitmap table offset

["Denis V. Lunev" <den@openvz.org>]

On 06/30/2016 10:34 AM, Vladimir Sementsov-Ogievskiy wrote:
> After loading bitmap from image and setting IN_USE flag in it's header,
> corresponding data (bitmap table and data clusters) becomes inconsistent
> and is no longer needed. It is better to free bitmap table and
> corresponding clusters from the image immediately after loading the
> bitmap than free them when the bitmap is saved, or deleted or set
> non-persistent.
>
> For now it is impossible to store only bitmap header without bitmap
> table, as specification requires it. Storing zeroed bitmap table (one or
> more clusters) is the only option to implement the behaviour similar to
> specified above.
>
> The same problem is for just storing empty bitmaps.
>
> This patch allows storing only bitmap header for empty bitmaps.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>
> Additional note. Should we also allow here bitmap_table_offset = 1, like
> in bitmap table, for the bitmap with all bits set? I am not sure that it
> is needed at all, but just to keep the company..
>
>   docs/specs/qcow2.txt | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 80cdfd0..9826222 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -435,9 +435,12 @@ 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.
> +                    Number of entries in the bitmap table of the bitmap. It
> +                    must be zero if bitmap_table_offset is zero.
>   
>           12 - 15:    flags
>                       Bit
NACK

no guys, we can not make this change at the moment.
We do have QEMU available in the field which is working
with the currently specified format.

Den