On Mon, Jul 20, 2020 at 4:18 PM Kevin Wolf <[email protected]> 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 <[email protected]> > Signed-off-by: Kevin Wolf <[email protected]> > --- > 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; > }
Looks good to me. > > -- > 2.25.4 >
