On 26.10.19 23:25, Alberto Garcia wrote:
> The bdrv_co_pwrite_zeroes() call here fills complete clusters with
> zeroes, but it can happen that some subclusters are not part of the
> write request or the copy-on-write. This patch makes sure that only
> the affected subclusters are overwritten.
> 
> A potential improvement would be to also fill with zeroes the other
> subclusters if we can guarantee that we are not overwriting existing
> data. However this would waste more disk space, so we should first
> evaluate if it's really worth doing.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 0261e87709..01322ca449 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2304,6 +2304,9 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
>  
>      for (m = l2meta; m != NULL; m = m->next) {
>          int ret;
> +        uint64_t start_offset = m->alloc_offset + m->cow_start.offset;
> +        uint64_t nb_bytes = m->cow_end.offset + m->cow_end.nb_bytes -
> +            m->cow_start.offset;

It might be more honest to make nb_bytes an unsigned.  (There shouldn’t
be any overflows here, because the total size is limited to INT64_MAX by
handle_alloc().)

Max

>          if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
>              continue;