Instead of making the backing file contents visible again after a discard
request, set the zero flag if possible (i.e. on version >= 3).
Signed-off-by: Kevin Wolf <kw...@redhat.com>
Reviewed-by: Max Reitz <mre...@redhat.com>
---
block/qcow2-cluster.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 25d45d1..9461969 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1333,13 +1333,31 @@ static int discard_single_l2(BlockDriverState *bs,
uint64_t offset,
uint64_t old_offset;
old_offset = be64_to_cpu(l2_table[l2_index + i]);
- if ((old_offset & L2E_OFFSET_MASK) == 0) {
+
+ /*
+ * 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.
+ */
+ if (!!(old_offset & QCOW_OFLAG_ZERO)) {
+ continue;
+ }
+
+ if ((old_offset & L2E_OFFSET_MASK) == 0 && !bs->backing_hd) {
continue;
}
/* First remove L2 entries */
qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
- l2_table[l2_index + i] = cpu_to_be64(0);
+ if (s->qcow_version >= 3) {
+ l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
+ } else {
+ l2_table[l2_index + i] = cpu_to_be64(0);
+ }
/* Then decrease the refcount */
qcow2_free_any_clusters(bs, old_offset, 1, type);
--
1.8.1.4
