Re: [Qemu-block] [PATCH v2 05/17] nbd: Advertise realistic limits to block layer

[Stefan Hajnoczi]

On Tue, Jun 14, 2016 at 03:30:27PM -0600, Eric Blake wrote:
> We were basing the advertisement of maximum discard and transfer
> length off of UINT32_MAX, but since the rest of the block layer
> has signed int limits on a transaction, nothing could ever reach
> that maximum, and we risk overflowing an int once things are
> converted to byte-based rather than sector-based limits.  What's
> more, we DO have a much smaller limit: both the current kernel
> and qemu-nbd have a hard limit of 32M on a read or write
> transaction, and while they may also permit up to a full 32 bits
> on a discard transaction, the upstream NBD protocol is proposing
> wording that without any explicit advertisement otherwise,
> clients should limit ALL requests to the same limits as read and
> write, even though the other requests do not actually require as
> many bytes across the wire.  So the better limit to tell the
> block layer is 32M for both values.
> 
> Signed-off-by: Eric Blake <address@hidden>
> 
> ---
> v2: new patch
> ---
>  block/nbd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Stefan Hajnoczi <address@hidden>

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