[Qemu-devel] [PATCH 1/1] nbd: increase maximum size of the PWRITE_ZERO request

[Edgar Kaziakhmedov]

Upstream NBD protocol implementation supports an efficient zero out
mechanism over the wire, along with the ability to check whether a
client allows using a hole.

Accordingly, since PWRITE_ZERO doesn't involve any payload on the wire,
increase a maximum size of the PWRITE_ZERO request up to 1Gb (aligned).
Moreover, such change will decrease the number of PWRITE_ZERO NBD commands
in comparison with the current 32M limit. The benefits of
the larger constraint can be examined in a block mirroring over NBD.

Signed-off-by: Edgar Kaziakhmedov <address@hidden>
---
 block/nbd.c         | 2 +-
 include/block/nbd.h | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 94220f6d14..3641d9244e 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -477,7 +477,7 @@ static void nbd_refresh_limits(BlockDriverState *bs, Error 
**errp)
     uint32_t max = MIN_NON_ZERO(NBD_MAX_BUFFER_SIZE, s->info.max_block);

     bs->bl.max_pdiscard = max;
-    bs->bl.max_pwrite_zeroes = max;
+    bs->bl.max_pwrite_zeroes = NBD_MAX_PWRITE_ZERO_SIZE;
     bs->bl.max_transfer = max;

     if (s->info.opt_block &&
diff --git a/include/block/nbd.h b/include/block/nbd.h
index ee74ec391a..e2f18e2332 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -182,6 +182,9 @@ enum {
 /* Maximum size of a single READ/WRITE data buffer */
 #define NBD_MAX_BUFFER_SIZE (32 * 1024 * 1024)

+/* Maximum size of a single PWRITE_ZERO request 1Gb */
+#define NBD_MAX_PWRITE_ZERO_SIZE (1024 * 1024 * 1024)
+
 /* Maximum size of an export name. The NBD spec requires 256 and
  * suggests that servers support up to 4096, but we stick to only the
  * required size so that we can stack-allocate the names, and because
--
2.11.0