Normally, discarded sectors should read back as zero. However, there are
cases in which a sector (or rather cluster) should be discarded as if
they were never written in the first place, that is, reading them should
fall through to the backing file again.
Signed-off-by: Max Reitz <mre...@redhat.com>
---
block/qcow2-cluster.c | 26 ++++++++++++++++----------
block/qcow2-snapshot.c | 2 +-
block/qcow2.c | 2 +-
block/qcow2.h | 2 +-
4 files changed, 19 insertions(+), 13 deletions(-)
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 331ab08..9b73d97 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1343,7 +1343,7 @@ int qcow2_decompress_cluster(BlockDriverState *bs,
uint64_t cluster_offset)
* clusters.
*/
static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
- unsigned int nb_clusters, enum qcow2_discard_type type)
+ unsigned int nb_clusters, enum qcow2_discard_type type, bool full_discard)
{
BDRVQcowState *s = bs->opaque;
uint64_t *l2_table;
@@ -1365,25 +1365,31 @@ static int discard_single_l2(BlockDriverState *bs,
uint64_t offset,
old_offset = be64_to_cpu(l2_table[l2_index + i]);
/*
- * Make sure that a discarded area reads back as zeroes for v3 images
- * (we cannot do it for v2 without actually writing a zero-filled
- * buffer). We can skip the operation if the cluster is already marked
- * as zero, or if it's unallocated and we don't have a backing file.
+ * If full_discard is false, make sure that a discarded area reads back
+ * as zeroes for v3 images (we cannot do it for v2 without actually
+ * writing a zero-filled buffer). We can skip the operation if the
+ * cluster is already marked as zero, or if it's unallocated and we
+ * don't have a backing file.
*
* TODO We might want to use bdrv_get_block_status(bs) here, but we're
* holding s->lock, so that doesn't work today.
+ *
+ * In case of full_discard being true, the sector should not be read
+ * back as zeroes, but rather fall through to the backing file.
*/
- if (old_offset & QCOW_OFLAG_ZERO) {
+ if (!full_discard && (old_offset & QCOW_OFLAG_ZERO)) {
continue;
}
- if ((old_offset & L2E_OFFSET_MASK) == 0 && !bs->backing_hd) {
+ if ((old_offset & L2E_OFFSET_MASK) == 0 &&
+ (full_discard || !bs->backing_hd))
+ {
continue;
}
