Re: [PATCH for-5.1 1/2] qcow2: Implement v2 zero writes with discard if possible

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2020-07-20 at 15:18 +0200, Kevin Wolf wrote:
> qcow2 version 2 images don't support the zero flag for clusters, so for
> write_zeroes requests, we return -ENOTSUP and get explicit zero buffer
> writes. If the image doesn't have a backing file, we can do better: Just
> discard the respective clusters.
> 
> This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has
> to assume that the existing target image may contain any data, so it has
> to write zeroes. Without this patch, this results in a fully allocated
> target image, even if the source image was empty.
> 
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/qcow2-cluster.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 4b5fc8c4a7..a677ba9f5c 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
>      assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
>             end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
>  
> -    /* The zero flag is only supported by version 3 and newer */
> +    /*
> +     * The zero flag is only supported by version 3 and newer. However, if we
> +     * have no backing file, we can resort to discard in version 2.
> +     */
>      if (s->qcow_version < 3) {
> +        if (!bs->backing) {
> +            return qcow2_cluster_discard(bs, offset, bytes,
> +                                         QCOW2_DISCARD_REQUEST, false);
> +        }
>          return -ENOTSUP;
>      }
>  

From my knowelege of nvme, I remember that discard doesn't have to zero the blocks.
There is special namespace capability the indicates the contents of the discarded block.
(Deallocate Logical Block Features)

If and only if the discard behavier flag indicates that discarded areas are zero,
then the write-zero command can have special 'deallocate' flag that hints the controller
to discard the sectors.

So woudn't discarding the clusters have theoretical risk of introducing garbage there?

Best regards,
	Maxim Levitsky