qcow2: Skip copy-on-write when allocating a zero cluster

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>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <6d77cab968c501c44d6e1089b9bc91b04170b49e.1603731354.git.berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/block/io.c b/block/io.c
index 6fe1b27..c33cecd 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2447,6 +2447,33 @@ 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 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 coroutine_fn bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset,
+                                      int64_t bytes)
+{
+    int ret;
+    int64_t pnum = bytes;
+
+    if (!bytes) {
+        return 1;
+    }
+
+    ret = bdrv_common_block_status_above(bs, NULL, false, false, offset,
+                                         bytes, &pnum, NULL, NULL);
+
+    if (ret < 0) {
+        return ret;
+    }
+
+    return (pnum == bytes) && (ret & BDRV_BLOCK_ZERO);
+}
+
 int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t offset,
                                    int64_t bytes, int64_t *pnum)
 {