Re: [Qemu-devel] [PATCH] proto: add 'shift' extension.

[Alex Bligh]

> On 26 Sep 2016, at 13:46, Vladimir Sementsov-Ogievskiy <address@hidden> wrote:
> 
> #### Option reply types
> 
> These values are used in the "reply type" field, sent by the server
> @@ -872,7 +883,13 @@ valid may depend on negotiation during the handshake 
> phase.
>   
> [extension](https://github.com/yoe/nbd/blob/extension-write-zeroes/doc/proto.md).
> - bit 2, `NBD_CMD_FLAG_DF`; defined by the experimental `STRUCTURED_REPLY`
>   
> [extension](https://github.com/yoe/nbd/blob/extension-structured-reply/doc/proto.md).
> -
> +- bit 3, `NBD_CMD_FLAG_SHIFT`; This flag is valid for `NBD_CMD_TRIM` and
> +  `NBD_CMD_WRITE_ZEROES`. If this flag is set the server shifts request
> +  *length* and *offset* left by N bits, where N is defined by `NBD_OPT_SHIFT`
> +  option or is assumed to be 16 bits by default if `NBD_OPT_SHIFT` option is
> +  not specified. If after shift `(offset + length)` exceeds disk size, length
> +  should be reduced to `(<disk size> - offset)`. However, `(offset + length)`
> +  must not exceed disk size by more than `(1 << N) - 1`.

Is there a good reason the shift size can't be either the minimum block size
or otherwise negotiated using NBD_OPT_INFO?

-- 
Alex Bligh