in a read-modify-write cycle a small request might cause
head and tail to fall into the same aligned block. Currently
QEMU reads the same block twice in this case which is
not necessary.
Signed-off-by: Peter Lieven <p...@kamp.de>
---
v1->v2: following Paolos suggestions to simplify the if condition and
adjusting the comment
block/io.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/block/io.c b/block/io.c
index 60a6bd8..7459dfb 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1430,6 +1430,14 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs,
bytes += offset & (align - 1);
offset = offset & ~(align - 1);
+
+ /* We have read the tail already if the request is smaller
+ * than one aligned block.
+ */
+ if (bytes < align) {
+ qemu_iovec_add(&local_qiov, head_buf + bytes, align - bytes);
+ bytes = align;
+ }
}
if ((offset + bytes) & (align - 1)) {
--
1.9.1
