Re: [PATCH v2 2/6] spec: Tweak description of maximum block size

[Wouter Verhelst]

Hi Eric,

Busy days, busy times. Sorry about the insane delays here.

On Mon, Nov 14, 2022 at 04:46:51PM -0600, Eric Blake wrote:
> Commit 9f30fedb improved the spec to allow non-payload requests that
> exceed any advertised maximum block size.  Take this one step further
> by permitting the server to use NBD_EOVERFLOW as a hint to the client
> when a request is oversize (while permitting NBD_EINVAL for
> back-compat), and by rewording the text to explicitly call out that
> what is being advertised is the maximum payload length, not maximum
> block size.  This becomes more important when we add 64-bit
> extensions, where it becomes possible to extend `NBD_CMD_BLOCK_STATUS`
> to have both an effect length (how much of the image does the client
> want status on - may be larger than 32 bits) and an optional payload
> length (a way to filter the response to a subset of negotiated
> metadata contexts).  In the shorter term, it means that a server may
> (but not must) accept a read request larger than the maximum block
> size if it can use structured replies to keep each chunk of the
> response under the maximum payload limits.
> ---
>  doc/proto.md | 127 +++++++++++++++++++++++++++++----------------------
>  1 file changed, 73 insertions(+), 54 deletions(-)
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 8f08583..53c334a 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -745,8 +745,8 @@ text unless the client insists on TLS.
> 
>  During transmission phase, several operations are constrained by the
>  export size sent by the final `NBD_OPT_EXPORT_NAME` or `NBD_OPT_GO`,
> -as well as by three block size constraints defined here (minimum,
> -preferred, and maximum).
> +as well as by three block size constraints defined here (minimum
> +block, preferred block, and maximum payload).
> 
>  If a client can honour server block size constraints (as set out below
>  and under `NBD_INFO_BLOCK_SIZE`), it SHOULD announce this during the
> @@ -772,15 +772,15 @@ learn the server's constraints without committing to 
> them.
> 
>  If block size constraints have not been advertised or agreed on
>  externally, then a server SHOULD support a default minimum block size
> -of 1, a preferred block size of 2^12 (4,096), and a maximum block size
> -that is effectively unlimited (0xffffffff, or the export size if that
> -is smaller), while a client desiring maximum interoperability SHOULD
> -constrain its requests to a minimum block size of 2^9 (512), and limit
> -`NBD_CMD_READ` and `NBD_CMD_WRITE` commands to a maximum block size of
> -2^25 (33,554,432).  A server that wants to enforce block sizes other
> -than the defaults specified here MAY refuse to go into transmission
> -phase with a client that uses `NBD_OPT_EXPORT_NAME` (via a hard
> -disconnect) or which uses `NBD_OPT_GO` without requesting
> +of 1, a preferred block size of 2^12 (4,096), and a maximum block
> +payload size that is at least 2^25 (33,554,432) (even if the export
> +size is smaller); while a client desiring maximum interoperability
> +SHOULD constrain its requests to a minimum block size of 2^9 (512),
> +and limit `NBD_CMD_READ` and `NBD_CMD_WRITE` commands to a maximum
> +block size of 2^25 (33,554,432).  A server that wants to enforce block
> +sizes other than the defaults specified here MAY refuse to go into
> +transmission phase with a client that uses `NBD_OPT_EXPORT_NAME` (via
> +a hard disconnect) or which uses `NBD_OPT_GO` without requesting

This does more than what the commit message says: it also limits payload
size from 0xffffffff to 2^25. We already have a "A server that desires
maximum interoperability" clause that mentions the 2^25, so I'm not
entirely sure why we need to restrict that for the default cause.

Remember, apart from specifying how something should be implemented, the
spec also documents current and historic behavior. I am probably
convinced that it makes more sense to steer people towards limiting to
2^25, but it should be done in such a way that servers which accept a
0xffffffff block size are not suddenly noncompliant. I don't think this
does that.

[no further comments on this one]
-- 
     w@uter.{be,co.za}
wouter@{grep.be,fosdem.org,debian.org}

I will have a Tin-Actinium-Potassium mixture, thanks.