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>
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 4b5fc8c..a677ba9 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1797,8 +1797,15 @@
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;
}
