If COW areas of the newly allocated clusters are zeroes on the backing image,
efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the whole
cluster instead of writing explicit zero buffers later in perform_cow().
iotest 060:
write to the discarded cluster does not trigger COW anymore.
Use a backing image instead.
Signed-off-by: Anton Nefedov
Reviewed-by: Vladimir Sementsov-Ogievskiy
---
qapi/block-core.json | 4 +-
block/qcow2.h | 6 +++
block/qcow2-cluster.c | 2 +-
block/qcow2.c | 91 +-
block/trace-events | 1 +
tests/qemu-iotests/060 | 7 ++-
tests/qemu-iotests/060.out | 5 ++-
7 files changed, 110 insertions(+), 6 deletions(-)
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 762000f31f..204528b3f6 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -3009,6 +3009,8 @@
#
# @cor_write: a write due to copy-on-read (since 2.11)
#
+# @cluster_alloc_space: an allocation of file space for a cluster (since 4.0)
+#
# Since: 2.9
##
{ 'enum': 'BlkdebugEvent', 'prefix': 'BLKDBG',
@@ -3027,7 +3029,7 @@
'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
'l1_shrink_write_table', 'l1_shrink_free_l2_clusters',
-'cor_write'] }
+'cor_write', 'cluster_alloc_space'] }
##
# @BlkdebugInjectErrorOptions:
diff --git a/block/qcow2.h b/block/qcow2.h
index 438a1dee9e..dad4b1c7ca 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -389,6 +389,12 @@ typedef struct QCowL2Meta
*/
Qcow2COWRegion cow_end;
+/*
+ * Indicates that COW regions are already handled and do not require
+ * any more processing.
+ */
+bool skip_cow;
+
/**
* The I/O vector with the data from the actual guest write request.
* If non-NULL, this is meant to be merged together with the data
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 30eca26c47..e5f936a82c 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -806,7 +806,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
assert(start->offset + start->nb_bytes <= end->offset);
assert(!m->data_qiov || m->data_qiov->size == data_bytes);
-if (start->nb_bytes == 0 && end->nb_bytes == 0) {
+if ((start->nb_bytes == 0 && end->nb_bytes == 0) || m->skip_cow) {
return 0;
}
diff --git a/block/qcow2.c b/block/qcow2.c
index 4897abae5e..05a7cbebbd 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2021,6 +2021,11 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
continue;
}
+/* If COW regions are handled already, skip this too */
+if (m->skip_cow) {
+continue;
+}
+
/* The data (middle) region must be immediately after the
* start region */
if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
@@ -2046,6 +2051,79 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
return false;
}
+static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t bytes)
+{
+int64_t nr;
+return !bytes ||
+(!bdrv_is_allocated_above(bs, NULL, offset, bytes, ) && nr ==
bytes);
+}
+
+static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
+{
+/*
+ * This check is designed for optimization shortcut so it must be
+ * efficient.
+ * Instead of is_zero(), use is_unallocated() as it is faster (but not
+ * as accurate and can result in false negatives).
+ */
+return is_unallocated(bs, m->offset + m->cow_start.offset,
+ m->cow_start.nb_bytes) &&
+ is_unallocated(bs, m->offset + m->cow_end.offset,
+ m->cow_end.nb_bytes);
+}
+
+static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
+{
+BDRVQcow2State *s = bs->opaque;
+QCowL2Meta *m;
+
+if (!(bs->file->bs->supported_zero_flags & BDRV_REQ_ALLOCATE)) {
+return 0;
+}
+
+if (bs->encrypted) {
+return 0;
+}
+
+for (m = l2meta; m != NULL; m = m->next) {
+int ret;
+
+if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
+continue;
+}
+
+if (!is_zero_cow(bs, m)) {
+continue;
+}
+
+/*
+ * instead of writing zero COW buffers,
+ * efficiently zero out the whole clusters
+ */
+
+ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset,
+m->nb_clusters * s->cluster_size);
+if (ret < 0) {
+return ret;
+}
+
+BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
+ret = bdrv_co_pwrite_zeroes(bs->file, m->alloc_offset,
+m->nb_clusters * s->cluster_size,
+BDRV_REQ_ALLOCATE);
+if
