Re: [Qemu-devel] [PATCH V2] block/io: optimize bdrv_co_pwritev for small requests

[Fam Zheng <famz@redhat.com>]

On Thu, 05/26 14:50, Fam Zheng wrote:
> 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
> 

Looks like this breaks iotests 077 (hang), the blkdebug break points expected
by the script are not hit now. While squashing in below patch fixes the case, I
think it is more appropriate to keep the patch as is and fix the case itself.

Dropped from my queue, please send another version with test case update so I
can apply together.

diff --git a/block/io.c b/block/io.c
index d480097..a6523cf 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1435,8 +1435,10 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs,
          * than one aligned block.
          */
         if (bytes < align) {
+            bdrv_debug_event(bs, BLKDBG_PWRITEV_RMW_TAIL);
             qemu_iovec_add(&local_qiov, head_buf + bytes, align - bytes);
             bytes = align;
+            bdrv_debug_event(bs, BLKDBG_PWRITEV_RMW_AFTER_TAIL);
         }
     }