Re: [PATCH 2/2] nbd/proto: add NBD_CMD_WRITE_ZEROES64

[Eric Blake]

On 2/5/20 10:43 AM, Vladimir Sementsov-Ogievskiy wrote:
> Add new structured request type to represent 64bit version of
> NBD_CMD_WRITE_ZEROES.

Initial reaction: why do we need a new NBD_CMD?  Why can't we reuse the 
existing NBD_CMD_WRITE_ZEROES, and merely document that if structured 
messages were negotiated, then calling NBD_CMD_WRITE_ZEROES with a 
structured request gives a larger range of access?

> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>   doc/proto.md | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
>
> diff --git a/doc/proto.md b/doc/proto.md
> index cb0ac56..378a800 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -1096,6 +1096,8 @@ The field has the following format:
>     is set.
>   - bit 12, `NBD_FLAG_STRUCTURED_REQUEST`; allow clients to use
>     structured requests.
> +- bit 13, `NBD_FLAG_SEND_WRITE_ZEROES64`: documents that the server
> +  understands `NBD_CMD_WRITE_ZEROES64` structured request chunk type.

Why do we need this bit?  Could we instead rely on the existing 
NBD_FLAG_SEND_WRITE_ZEROES combined with successful negotiation of 
structured requests as being sufficient to imply that the server 
therefore supports 64-bit zero requests?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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