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nbd: Allow larger requests

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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 <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Eric Blake 2016-06-23 16:37:08 -06:00 committed by Kevin Wolf
parent 82524274ea
commit 476b923c32
2 changed files with 2 additions and 4 deletions
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4
block/nbd-client.c
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@ -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)
{

2
include/block/nbd.h
View file

@ -77,6 +77,8 @@ 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)
/* 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