On 2017-07-07 16:30, Pavel Butsykin wrote:
> This patch add shrinking of the image file for qcow2. As a result, this allows
> us to reduce the virtual image size and free up space on the disk without
> copying the image. Image can be fragmented and shrink is done by punching holes
> in the image file.
> 
> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
> ---
>  block/qcow2-cluster.c  |  40 +++++++++++++++++++
>  block/qcow2-refcount.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++
>  block/qcow2.c          |  42 +++++++++++++++-----
>  block/qcow2.h          |  14 +++++++
>  qapi/block-core.json   |   3 +-
>  5 files changed, 191 insertions(+), 11 deletions(-)

[...]

> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 0141c9cbe7..443fca6a98 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c

[...]

> @@ -2945,3 +2946,105 @@ done:

[...]

> +            ret = qcow2_cache_get(bs, s->refcount_block_cache, refblock_offs,
> +                                  &refblock);
> +            if (ret < 0) {
> +                goto out;
> +            }
> +
> +            if (s->get_refcount(refblock, block_index) != 1) {
> +                qcow2_cache_put(bs, s->refcount_block_cache, &refblock);
> +                ret = -EINVAL;

Optional: You could mark the image corrupt here.

Anyway:

Reviewed-by: Max Reitz <mreitz@redhat.com>

> +                goto out;
> +            }
> +            s->set_refcount(refblock, block_index, 0);