This commit makes the discard operation work on the subcluster level
rather than cluster level. It introduces discard_l2_subclusters()
function and makes use of it in qcow2 discard implementation, much like
it's done with zero_in_l2_slice() / zero_l2_subclusters(). It also
changes the qcow2 driver pdiscard_alignment to subcluster_size. That
way subcluster-aligned discards lead to actual fallocate(PUNCH_HOLE)
operation and free host disk space.
This feature will let us gain additional disk space on guest
TRIM/discard requests, especially when using large enough clusters
(1M, 2M) with subclusters enabled.
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
block/qcow2-cluster.c | 100 ++++++++++++++++++++++++++++++++++++++++--
block/qcow2.c | 8 ++--
2 files changed, 101 insertions(+), 7 deletions(-)
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 7c6fa5524c..cf40f2dc12 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -2042,6 +2042,74 @@ discard_in_l2_slice(BlockDriverState *bs, uint64_t
offset, uint64_t nb_clusters,
return nb_clusters;
}
+static int coroutine_fn GRAPH_RDLOCK
+discard_l2_subclusters(BlockDriverState *bs, uint64_t offset,
+ uint64_t nb_subclusters,
+ enum qcow2_discard_type type,
+ bool full_discard,
+ SubClusterRangeInfo *pscri)
+{
+ BDRVQcow2State *s = bs->opaque;
+ uint64_t new_l2_bitmap, l2_bitmap_mask;
+ int ret, sc = offset_to_sc_index(s, offset);
+ SubClusterRangeInfo scri = { 0 };
+
+ if (!pscri) {
+ ret = get_sc_range_info(bs, offset, nb_subclusters, &scri);
+ if (ret < 0) {
+ goto out;
+ }
+ } else {
+ scri = *pscri;
+ }
+
+ l2_bitmap_mask = QCOW_OFLAG_SUB_ALLOC_RANGE(sc, sc + nb_subclusters);
+ new_l2_bitmap = scri.l2_bitmap;
+ new_l2_bitmap &= ~l2_bitmap_mask;
+
+ /*
+ * If there're no allocated subclusters left, we might as well discard
+ * the entire cluster. That way we'd also update the refcount table.
+ */
+ if (!(new_l2_bitmap & QCOW_L2_BITMAP_ALL_ALLOC)) {
+ return discard_in_l2_slice(bs,
+ QEMU_ALIGN_DOWN(offset, s->cluster_size),
+ 1, type, full_discard);
+ }