[Qemu-devel] [PATCH 2/6] block: split write_zeroes always

["Denis V. Lunev" <den@openvz.org>]

We should split requests even if they are less than write_zeroes_alignment.
For example we can have the following request:
  offset 62k
  size   4k
  write_zeroes_alignment 64k
Original code will send 1 request covering 2 qcow2 clusters. One cluster
could be zeroed as a whole, another could not be. In this case we will have
both 2 clusters allocated.

After the patch 2 requests to qcow2 layer will be sent and thus only one
cluster will be allocated.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
---
 block/io.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/block/io.c b/block/io.c
index cd6d71a..2d6f48f 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1172,13 +1172,13 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
         /* Align request.  Block drivers can expect the "bulk" of the request
          * to be aligned.
          */
-        if (bs->bl.write_zeroes_alignment
-            && num > bs->bl.write_zeroes_alignment) {
+        if (bs->bl.write_zeroes_alignment) {
             if (sector_num % bs->bl.write_zeroes_alignment != 0) {
                 /* Make a small request up to the first aligned sector.  */
                 num = bs->bl.write_zeroes_alignment;
                 num -= sector_num % bs->bl.write_zeroes_alignment;
-            } else if ((sector_num + num) % bs->bl.write_zeroes_alignment != 0) {
+            } else if (num > bs->bl.write_zeroes_alignment &&
+                    (sector_num + num) % bs->bl.write_zeroes_alignment != 0) {
                 /* Shorten the request to the last aligned sector.  num cannot
                  * underflow because num > bs->bl.write_zeroes_alignment.
                  */
-- 
2.5.0