qcow2: handle_dependencies(): relax conflict detection

There is no conflict and no dependency if we have parallel writes to
different subclusters of one cluster when the cluster itself is already
allocated. So, relax extra dependency.

Measure performance:
First, prepare build/qemu-img-old and build/qemu-img-new images.

cd scripts/simplebench
./img_bench_templater.py

Paste the following to stdin of running script:

qemu_img=../../build/qemu-img-{old|new}
$qemu_img create -f qcow2 -o extended_l2=on /ssd/x.qcow2 1G
$qemu_img bench -c 100000 -d 8 [-s 2K|-s 2K -o 512|-s $((1024*2+512))] \
        -w -t none -n /ssd/x.qcow2

The result:

All results are in seconds

------------------  ---------  ---------
                    old        new
-s 2K               6.7 ± 15%  6.2 ± 12%
                                 -7%
-s 2K -o 512        13 ± 3%    11 ± 5%
                                 -16%
-s $((1024*2+512))  9.5 ± 4%   8.4
                                 -12%
------------------  ---------  ---------

So small writes are more independent now and that helps to keep deeper
io queue which improves performance.

271 iotest output becomes racy for three allocation in one cluster.
Second and third writes may finish in different order. Second and
third requests don't depend on each other any more. Still they both
depend on first request anyway. Filter out second and third write
offsets to cover both possible outputs.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210824101517.59802-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
[hreitz: s/ an / and /]
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

block/qcow2-cluster.c

@@ -1403,6 +1403,17 @@ static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
             continue;
         }
 
+        if (old_alloc->keep_old_clusters &&
+            (end <= l2meta_cow_start(old_alloc) ||
+             start >= l2meta_cow_end(old_alloc)))
+        {
+            /*
+             * Clusters intersect but COW areas don't. And cluster itself is
+             * already allocated. So, there is no actual conflict.
+             */
+            continue;
+        }
+
         /* Conflict */
 
         if (start < old_start) {

tests/qemu-iotests/271

@@ -893,7 +893,10 @@ EOF
 }
 
 _make_test_img -o extended_l2=on 1M
-_concurrent_io     | $QEMU_IO | _filter_qemu_io
+# Second and third writes in _concurrent_io() are independent and may finish in
+# different order. So, filter offset out to match both possible variants.
+_concurrent_io     | $QEMU_IO | _filter_qemu_io | \
+    $SED -e 's/\(20480\|40960\)/OFFSET/'
 _concurrent_verify | $QEMU_IO | _filter_qemu_io
 
 # success, all done

tests/qemu-iotests/271.out

@@ -719,8 +719,8 @@ blkdebug: Suspended request 'A'
 blkdebug: Resuming request 'A'
 wrote 2048/2048 bytes at offset 30720
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-wrote 2048/2048 bytes at offset 20480
+wrote 2048/2048 bytes at offset OFFSET
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-wrote 2048/2048 bytes at offset 40960
+wrote 2048/2048 bytes at offset OFFSET
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done