From 117bc3fa22d359db6c4f0c10f34c8c9e00ed64af Mon Sep 17 00:00:00 2001 From: Peter Lieven Date: Mon, 30 May 2016 13:31:13 +0200 Subject: [PATCH] block/io: optimize bdrv_co_pwritev for small requests 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 Message-id: 1464607873-28206-1-git-send-email-pl@kamp.de Signed-off-by: Stefan Hajnoczi --- block/io.c | 8 ++++++++ tests/qemu-iotests/077 | 12 +----------- tests/qemu-iotests/077.out | 26 -------------------------- 3 files changed, 9 insertions(+), 37 deletions(-) diff --git a/block/io.c b/block/io.c index 7ac9897559..e12f303df5 100644 --- a/block/io.c +++ b/block/io.c @@ -1427,6 +1427,14 @@ 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)) { diff --git a/tests/qemu-iotests/077 b/tests/qemu-iotests/077 index 4dc680b7fc..d2d2a2d687 100755 --- a/tests/qemu-iotests/077 +++ b/tests/qemu-iotests/077 @@ -60,7 +60,7 @@ EOF # Sequential RMW requests on the same physical sector off=0x1000 -for ev in "head" "after_head" "tail" "after_tail"; do +for ev in "head" "after_head"; do cat <