Add bdrv_aio_multiwrite

One performance problem of qcow2 during the initial image growth are
sequential writes that are not cluster aligned. In this case, when a first
requests requires to allocate a new cluster but writes only to the first
couple of sectors in that cluster, the rest of the cluster is zeroed - just
to be overwritten by the following second request that fills up the cluster.

Let's try to merge sequential write requests to the same cluster, so we can
avoid to write the zero padding to the disk in the first place.

As a nice side effect, also other formats take advantage of dealing with less
and larger requests.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/block_int.h b/block_int.h
index 0902fd4733..3e4b4a387b 100644
--- a/block_int.h
+++ b/block_int.h
@@ -70,6 +70,12 @@ struct BlockDriver {
         int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
         BlockDriverCompletionFunc *cb, void *opaque);
 
+    int (*bdrv_aio_multiwrite)(BlockDriverState *bs, BlockRequest *reqs,
+        int num_reqs);
+    int (*bdrv_merge_requests)(BlockDriverState *bs, BlockRequest* a,
+        BlockRequest *b);
+
+
     const char *protocol_name;
     int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset);
     int64_t (*bdrv_getlength)(BlockDriverState *bs);