Am 22.07.2020 um 19:01 hat Maxim Levitsky geschrieben: > 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 <[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; > > } > > > > 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?
No, qcow2_cluster_discard() has a defined behaviour. For v2 images, it unallocates the cluster in the L2 table (this is only safe without a backing file), for v3 images it converts them to zero clusters. Kevin
