[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-block] [Qemu-devel] [PATCH] spec/qcow2: bitmaps: zero bitmap t
Re: [Qemu-block] [Qemu-devel] [PATCH] spec/qcow2: bitmaps: zero bitmap table offset
Wed, 29 Jun 2016 20:33:18 -0400
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1
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 <address@hidden>
> Hi all!
> Here is one small but significant addition to specification of bitmaps in
> 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
> 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
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
> a cluster boundary.
> + Zero value means that bitmap table is not allocated and
> + 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.