| Message ID | 20200721135520.72355-2-kwolf@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | qemu-img convert -n: Keep qcow2 v2 target sparse | expand |
Am 21.07.2020 um 15:55 hat Kevin Wolf geschrieben: > 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> Oops, forgot to apply this from v1: Reviewed-by: Max Reitz <mreitz@redhat.com> The patch is unchanged, so I'll include this when applying. Kevin
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; }
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(-)