| Message ID | 1464100220-21317-1-git-send-email-pl@kamp.de |
|---|---|
| State | New |
| Headers | show |
Am 24.05.2016 um 16:30 hat Peter Lieven geschrieben: > 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 <pl@kamp.de> Reviewed-by: Kevin Wolf <kwolf@redhat.com>
On Tue, 05/24 16:30, Peter Lieven wrote: > 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 <pl@kamp.de> Thanks, applied to my block branch: https://github.com/famz/qemu/tree/block
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)) {
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 <pl@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(+)