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[Qemu-devel] [PATCH v2 04/17] nbd: Allow larger requests

From: Eric Blake
Subject: [Qemu-devel] [PATCH v2 04/17] nbd: Allow larger requests
Date: Tue, 14 Jun 2016 15:30:26 -0600

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>

v2: new patch
 include/block/nbd.h | 1 +
 block/nbd-client.c  | 4 ----
 2 files changed, 1 insertion(+), 4 deletions(-)

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)

 ssize_t nbd_wr_syncv(QIOChannel *ioc,
                      struct iovec *iov,
diff --git a/block/nbd-client.c b/block/nbd-client.c
index 4d13444..420bce8 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 
     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)

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