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

["Denis V. Lunev" <den@virtuozzo.com>]

On 09/26/2016 03:46 PM, Vladimir Sementsov-Ogievskiy wrote:
> This extension allows big requests for TRIM and WRITE_ZEROES through
> special 'shift' parameter, which means that offset and length should be
> shifted left by several bits.
>
> This is needed for efficient clearing large regions of the disk (up to
> the whole disk).
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>
> Here mentioned WRITE_ZEROES command which is only an experemental
> extension for now.
>
> To dicuss:
> NBD_OPT_SHIFT Data. It can be reduced to 8 bits actually... Are some
>    reserved bits needed here?
>
>  doc/proto.md | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/doc/proto.md b/doc/proto.md
> index 2de3a6a..6fd1b16 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -682,6 +682,8 @@ The field has the following format:
>    experimental `WRITE_ZEROES` [extension](https://github.com/yoe/nbd/blob/extension-write-zeroes/doc/proto.md).
>  - bit 7, `NBD_FLAG_SEND_DF`: defined by the experimental `STRUCTURED_REPLY`
>    [extension](https://github.com/yoe/nbd/blob/extension-structured-reply/doc/proto.md).
> +- bit 8, `NBD_FLAG_SEND_SHIFT` : exposes support for `NBD_CMD_FLAG_SHIFT` and
> +  `NBD_OPT_SHIFT`
>  
>  Clients SHOULD ignore unknown flags.
>  
> @@ -765,6 +767,15 @@ of the newstyle negotiation.
>  
>      Defined by the experimental `INFO` [extension](https://github.com/yoe/nbd/blob/extension-info/doc/proto.md).
>  
> +- `NBD_OPT_SHIFT` (10)
> +
> +    Defines shift used to calculate request offset and length if
> +    `NBD_CMD_FLAG_SHIFT` is set.
> +
> +    Data:
> +
> +    - 32 bits, shift (unsigned); Must not be larger than 32.
> +
>  #### 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`.
>  
>  #### Request types
>  
We could go in a different direction and export flag
'has_zero_init' which will report that the storage is
initialized with all zeroes at the moment. In this
case mirroring code will not fall into this
branch.

Den