[Qemu-devel] [PATCH] block/io: optimize bdrv_co_pwritev for small requests

[Peter Lieven <pl@kamp.de>]

in a read-modify-write cycle a small request might cause
head and tail to fall into the same alignment. Currently
QEMU reads the same block twice in this case which is
not necessary.

Signed-off-by: Peter Lieven <pl@kamp.de>
---
 block/io.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/block/io.c b/block/io.c
index 60a6bd8..fa40121 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1430,6 +1430,18 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs,
 
         bytes += offset & (align - 1);
         offset = offset & ~(align - 1);
+
+        /* if head and tail fall into the same alignment
+         * we can omit the second read as it would read
+         * the same block again */
+        if ((offset + bytes) & (align - 1) &&
+            offset / align == (offset + bytes) / align) {
+            size_t tail_offs;
+            tail_offs = (offset + bytes) & (align - 1);
+            qemu_iovec_add(&local_qiov, head_buf + tail_offs,
+                           align - tail_offs);
+            bytes += align - tail_offs;
+        }
     }
 
     if ((offset + bytes) & (align - 1)) {
-- 
1.9.1