Re: [nfsv4] Progressing RFC errata for RFC 5661

[Rick Macklem <[e-mail]>]

Trond Myklebust wrote:
>Rick,
>
>That errata predates most of the Linux pNFS client implementation. We
>wrote the implementation to conform to the errata.
>
>So no. It's not a bug. It's a deliberate design based on a decision
>that was discussed in the IETF WG, on the mailing list
>
>https://mailarchive.ietf.org/arch/msg/nfsv4/_KTtO6uz-MvRoStbhPuOXWZr6yI
This actually appears to be a discussion related to the offset and length arguments for LayoutCommit, but...
>
>and in a special session of the IETF:
>
>https://mailarchive.ietf.org/arch/msg/nfsv4/Rpw9XCwCARxfaU4ym5L2TauV6ao
Ok. This was long before I got around to implementing it, so I wouldn't have
understood the implications.
--> I would have been interested in hearing the rationale behind not doing
      LayoutCommit for FILE_SYNC4 writes, since it seems to me that RFC-5661
      had gotten it right when it required them.

As I said, the FreeBSD server can handle this case, it just results in a lot of
overhead synchronizing Size, Change, Time_Modify between MDS and DS
whenever a RW layout is issued to a client for the file.

Thanks for pointing this out, rick

On Wed, 18 Sep 2019 at 12:39, Rick Macklem <[e-mail]> wrote:
>
> Mkrtchyan, Tigran wrote:
> [stuff snipped]
> >Hi Rick,
> >
> >here is the public link to errata
> >
> >https://www.rfc-editor.org/errata/eid2751
> >
> >Tigran.
> Thanks Tigran.
>
> Ok, so now that I've read it I have to admit I think it is rewriting the RFC
> to conform with what the Linux client does.
>
> I think this para. from Sec. 13.10 of RFC-5661 is clear:
>    The NFSv4.1 protocol only provides close-to-open file data cache
>    semantics; meaning that when the file is closed, all modified data is
>    written to the server.  When a subsequent OPEN of the file is done,
>    the change attribute is inspected for a difference from a cached
>    value for the change attribute.  For the case above, this means that
>    a LAYOUTCOMMIT will be done at close (along with the data WRITEs) and
>    will update the file's size and change attribute.  Access from
>    another client after that point will result in the appropriate size
>    being returned.
>
> It states "will be done". It doesn't say anything about UNSTABLE4 vs FILE_SYNC4.
> (I think most POSIX-like clients would consider the fsync(2) syscall to require
>  the same treatment as "close" above, but that is a POSIX-specific client issue.)
> I can see the argument that, since there is no "must" in the statement, that a
> client can choose not to do this, but that would also imply that the client will
> need to live with the consequences of it.
>
> I think the second sentence of the first para. of the errata is bogus:
> For file layouts, WRITEs to a Data Server that return a stable_how4 value of
> FILE_SYNC4 guarantee that data and file system metadata are on stable
> storage.  This means that a LAYOUTCOMMIT is not needed in order to make the
> data and metadata visible to the metadata server and other clients.
>
> Why?
> The FILE_SYNC4 was returned by the DS. This would imply the DS
> has committed data and metadata to stable storage on the DS.
> However, I am not aware of anything in RFC-5661 that would imply that
> the Size, Time_Modify and Change attributes or anything else must have
> been updated or in stable storage on the MDS at this time.
>
> If a server does not require LayoutCommit operations for correct behaviour
> then it can simply reply NFS4ERR_NOTSUPP (as I believe the Netapp filer
> does) and the client then no longer needs to do them.
>
> If there is somewhere in RFC-5661 that it is stated that LayoutCommits are
> not required when the DS replies FILE_SYNC4, then I missed it and there
> is a problem with RFC-5661 that needs to be addressed.
>
> Otherwise, sorry, but it seems that the bug is in the Linux client implementation
> and not RFC-5661.
>
> Is there a File Layout pNFS server implementation where the DSs return
> FILE_SYNC4 that will break if the client does a LayoutCommit for this case?
> (If so, then something may need to be done.)
>
> rick
>
>
>