[Qemu-devel] [PATCH 38/56] nbd: Allow larger requests

[Michael Roth]

From: Eric Blake <address@hidden>

The NBD layer was breaking up request at a limit of 2040 sectors
(just under 1M) to cater to old qemu-nbd. But the server limit
was raised to 32M in commit 2d8214885 to match the kernel, more
than three years ago; and the upstream NBD Protocol is proposing
documentation that without any explicit communication to state
otherwise, a client should be able to safely assume that a 32M
transaction will work.  It is time to rely on the larger sizing,
and any downstream distro that cares about maximum
interoperability to older qemu-nbd servers can just tweak the
value of #define NBD_MAX_SECTORS.

Signed-off-by: Eric Blake <address@hidden>
Reviewed-by: Kevin Wolf <address@hidden>
Acked-by: Paolo Bonzini <address@hidden>
Cc: address@hidden
Reviewed-by: Fam Zheng <address@hidden>
Reviewed-by: Stefan Hajnoczi <address@hidden>
Signed-off-by: Kevin Wolf <address@hidden>

(cherry picked from commit 476b923c32ece0e268580776aaf1fab4ab4459a8)
Conflicts:
        include/block/nbd.h

* removed context dependency on 943cec86

Signed-off-by: Michael Roth <address@hidden>
---
 block/nbd-client.c  | 4 ----
 include/block/nbd.h | 1 +
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/block/nbd-client.c b/block/nbd-client.c
index 878e879..6f6df46 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -269,10 +269,6 @@ static int nbd_co_writev_1(BlockDriverState *bs, int64_t 
sector_num,
     return -reply.error;
 }
 
-/* qemu-nbd has a limit of slightly less than 1M per request.  Try to
- * remain aligned to 4K. */
-#define NBD_MAX_SECTORS 2040
-
 int nbd_client_co_readv(BlockDriverState *bs, int64_t sector_num,
                         int nb_sectors, QEMUIOVector *qiov)
 {
diff --git a/include/block/nbd.h b/include/block/nbd.h
index b86a976..36dde24 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -76,6 +76,7 @@ enum {
 
 /* Maximum size of a single READ/WRITE data buffer */
 #define NBD_MAX_BUFFER_SIZE (32 * 1024 * 1024)
+#define NBD_MAX_SECTORS (NBD_MAX_BUFFER_SIZE / BDRV_SECTOR_SIZE)
 
 ssize_t nbd_wr_syncv(QIOChannel *ioc,
                      struct iovec *iov,
-- 
1.9.1