diff options
| author | Kevin Wolf <kwolf@redhat.com> | 2020-07-21 15:55:19 +0200 |
|---|---|---|
| committer | Kevin Wolf <kwolf@redhat.com> | 2020-07-21 16:28:57 +0200 |
| commit | 61b3043965fe3552ee2684a97e7cc809ca7a71b3 (patch) | |
| tree | 496e9ee83a0312ebd88803a49761d14b83b18fa8 /block | |
| parent | bae127d4dcf6158c5042e2eee9582430839a9967 (diff) | |
qcow2: Implement v2 zero writes with discard if possible
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>
Message-Id: <20200721135520.72355-2-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'block')
| -rw-r--r-- | block/qcow2-cluster.c | 9 |
1 files changed, 8 insertions, 1 deletions
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; } |