Re: [nfsv4] Spencer Dawkins' Yes on draft-ietf-nfsv4-versioning-09: (with COMMENT)

[Trond Myklebust <[e-mail]>]

On Fri, 2017-05-12 at 07:36 -0700, Spencer Dawkins wrote:
> 
> I did have one question that came up during AD Evaluation that I
> wanted
> to mention.
> 
> The first two drafts that used this mechanism (umask and xattrs) used
> two
> different idioms for discovering support. The xaddrs draft defines an
> xaddr_support attribute, while umask does not.
> 
> In conversations with the working group, the reasoning was that
> xattrs
> defines a number of operations, so discovering that the complete
> mechanism is supported before you start trying to use the attributes
> makes sense, while umask defines only one attribute, and for any
> attribute, you can find out if it is supported within a given file
> system
> by interrogating the appropriate bit position in the REQUIRED
> attribute
> supported_attrs, so there is no advantage to adding a umask_support
> attribute. 
> 
> That all made perfect sense to me, but the explanation was helpful
> enough
> to me that I wonder if it's worth a sentence or two, pointing out
> that
> some protocol designers may choose one idiom, while other protocol
> designers choose the other, and saying that's not a problem.
> 

It's an extra mechanism that could be a convenience, but is not
something that a client can or should rely on. The only reliable
mechanism for verifying that the feature is available is to try it, and
handle the resulting NFS4ERR_NOTSUPP and/or NFS4ERR_OP_ILLEGAL error.

This is how the Linux client handles all the other NFSv4.x optional
features as well.

Cheers
  Trond