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

[Paolo Bonzini]

On 14/06/2016 23:30, Eric Blake wrote:
> 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)
> +#define NBD_MAX_SECTORS (NBD_MAX_BUFFER_SIZE / BDRV_SECTOR_SIZE)
> 
>  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 
> 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)
>  {
> 

Acked-by: Paolo Bonzini <address@hidden>