@@ -496,6 +496,8 @@ int bdrv_is_allocated(BlockDriverState *bs, int64_t offset, int64_t bytes,
int bdrv_is_allocated_above(BlockDriverState *top, BlockDriverState *base,
bool include_base, int64_t offset, int64_t bytes,
int64_t *pnum);
+int bdrv_is_unallocated_or_zero_above(BlockDriverState *bs, int64_t offset,
+ int64_t bytes);
bool bdrv_is_read_only(BlockDriverState *bs);
int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only,
@@ -2557,6 +2557,30 @@ int bdrv_block_status(BlockDriverState *bs, int64_t offset, int64_t bytes,
offset, bytes, pnum, map, file);
}
+/*
+ * Check @bs (and its backing chain) to see if the range defined
+ * by @offset and @bytes is unallocated or known to read as zeroes.
+ * Return 1 if that is the case, 0 otherwise and -errno on error.
+ * This test is meant to be fast rather than accurate so returning 0
+ * does not guarantee non-zero data.
+ */
+int bdrv_is_unallocated_or_zero_above(BlockDriverState *bs, int64_t offset,
+ int64_t bytes)
+{
+ int ret;
+ int64_t pnum = bytes;
+
+ ret = bdrv_common_block_status_above(bs, NULL, false, offset,
+ bytes, &pnum, NULL, NULL);
+
+ if (ret < 0) {
+ return ret;
+ }
+
+ return (pnum == bytes) &&
+ ((ret & BDRV_BLOCK_ZERO) || (!(ret & BDRV_BLOCK_ALLOCATED)));
+}
+
int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t offset,
int64_t bytes, int64_t *pnum)
{
@@ -2391,26 +2391,28 @@ 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, false, offset, bytes, &nr) &&
- nr == bytes);
-}
-
-static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
+/*
+ * Return 1 if the COW regions read as zeroes, 0 if not, < 0 on error.
+ * Note that returning 0 does not guarantee non-zero data.
+ */
+static int 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).
+ * Instead of is_zero(), use bdrv_is_unallocated_or_zero_above() 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);
+ int ret;
+
+ ret = bdrv_is_unallocated_or_zero_above(bs, m->offset + m->cow_start.offset,
+ m->cow_start.nb_bytes);
+ if (ret <= 0) {
+ return ret;
+ }
+
+ return bdrv_is_unallocated_or_zero_above(bs, m->offset + m->cow_end.offset,
+ m->cow_end.nb_bytes);
}
static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
@@ -2436,7 +2438,10 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
continue;
}
- if (!is_zero_cow(bs, m)) {
+ ret = is_zero_cow(bs, m);
+ if (ret < 0) {
+ return ret;
+ } else if (ret == 0) {
continue;
}
Since commit c8bb23cbdbe32f5c326365e0a82e1b0e68cdcd8a when a write request results in a new allocation QEMU first tries to see if the rest of the cluster outside the written area contains only zeroes. In that case, instead of doing a normal copy-on-write operation and writing explicit zero buffers to disk, the code zeroes the whole cluster efficiently using pwrite_zeroes() with BDRV_REQ_NO_FALLBACK. This improves performance very significantly but it only happens when we are writing to an area that was completely unallocated before. Zero clusters (QCOW2_CLUSTER_ZERO_*) are treated like normal clusters and are therefore slower to allocate. This happens because the code uses bdrv_is_allocated_above() rather bdrv_block_status_above(). The former is not as accurate for this purpose but it is faster. However in the case of qcow2 the underlying call does already report zero clusters just fine so there is no reason why we cannot use that information. After testing 4KB writes on an image that only contains zero clusters this patch results in almost five times more IOPS. Signed-off-by: Alberto Garcia <berto@igalia.com> --- v2: - Add new, simpler API: bdrv_is_unallocated_or_zero_above() include/block/block.h | 2 ++ block/io.c | 24 ++++++++++++++++++++++++ block/qcow2.c | 37 +++++++++++++++++++++---------------- 3 files changed, 47 insertions(+), 16 deletions(-)