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Add LAYOUT_WCC to NFSv4.2's Flex File Layout Type

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Thomas Haynes , Trond Myklebust
Last updated 2023-02-21
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Network File System Version 4                                  T. Haynes
Internet-Draft                                              T. Myklebust
Updates: 8435 (if approved)                                  Hammerspace
Intended status: Standards Track                        21 February 2023
Expires: 25 August 2023

           Add LAYOUT_WCC to NFSv4.2's Flex File Layout Type


   The Parallel Network File System (pNFS) Flexible File Layout allows
   for a file's metadata (MDS) and data (DS) to be on different servers.
   It does not provide a mechanism for the data server to update the
   metadata server of changes to the data part of the file.  The client
   has knowledge of such updates, but lacks the ability to update the
   metadata server.  This document presents a refinement to RFC8435 to
   allow the client to update the metadata server to changes on the data


   This note is to be removed before publishing as an RFC.

   Discussion of this draft takes place on the NFSv4 working group
   mailing list (, which is archived at  Working Group
   information can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 August 2023.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Operation 77: LAYOUT_WCC - Layout Weak Cache Consistency  . .   3
     2.4.  Allowed Errors  . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Extension of Existing Implementations . . . . . . . . . .   5
     2.6.  Flex Files Layout Type  . . . . . . . . . . . . . . . . .   6
   3.  Extraction of XDR . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Code Components Licensing Notice  . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   In the Network File System version4 (NFSv4) with a Parallel NFS
   (pNFS) Flexible File Layout ([RFC8435]) server, there is no mechanism
   for the data servers to update the metadata servers for when the data
   portion of the file is modified.  The metadata server needs this
   knowledge to correspondingly update the metadata portion of the file.
   If the client is using NFSv3 as the protocol with the data server, it
   can leverage weak cache consistency (WCC) to update the metadata
   server of the attribute changes.  In this document, we introduce a
   new operation called LAYOUT_WCC which allows the client to
   periodically report the attributes of the data files to the metadata

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   Using the process detailed in [RFC8178], the revisions in this
   document become an extension of NFSv4.2 [RFC7862].  They are built on
   top of the external data representation (XDR) [RFC4506] generated
   from [RFC7863].

1.1.  Definitions

   (file) data:  that part of the file system object that contains the
      data to be read or written.  It is the contents of the object
      rather than the attributes of the object.

   data server (DS):  a pNFS server that provides the file's data when
      the file system object is accessed over a file-based protocol.

   (file) metadata:  the part of the file system object that contains
      various descriptive data relevant to the file object, as opposed
      to the file data itself.  This could include the time of last
      modification, access time, EOF position, etc.

   metadata server (MDS):  the pNFS server that provides metadata
      information for a file system object.

   weak cache consistency (WCC):  In NFSv3, WCC allows the client to
      check for file attribute changes before and after an operation.
      (See Section 2.6 of [RFC1813])

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Operation 77: LAYOUT_WCC - Layout Weak Cache Consistency


   /// struct LAYOUT_WCC4args {
   ///         stateid4        lowa_stateid;
   ///         layouttype4     lowa_type;
   ///         opaque          lowa_body<>;
   /// };

2.2.  RESULT

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   /// struct LAYOUT_WCC4res {
   ///         nfsstat4                lowr_status;
   /// };


   When using pNFS (See Section 12 of [RFC8881]), the client is most
   likely to be performing file operations to the storage device and not
   the metadata server.  With a NFSv3 data server in the flexible files
   layout type (in [RFC8435]) there is no control protocol ([RFC8434])
   between the metadata server and the storage device.  In order to
   update the metadata state of the file, the metadata server will need
   to track the metadata state of the data file - once the layout is
   issued, it is not able to see the NFSv3 file operations from the
   client to the storage device.  Thus the metadata server will be
   required to query the storage device for the data file attributes.

   For example, the metadata server would issue a NFSv3 GETATTR to the
   storage device.  These queries are most likely triggered in response
   to a NFSv4 GETATTR to the metadata server.  Not only are these NFSv3
   GETATTRs to the storage device individually expensive, the storage
   device can become inundated by a storm of such requests.  NFSv3
   solved a similar issue by having the READ and WRITE operations employ
   a post-operation attribute to report the weak cache consistency (WCC)
   data (See Section 2.6 of [RFC1813]).

   Each NFSv3 operation corresponds to one round trip between the client
   and server.  So a WRITE followed by a GETATTR would require two round
   trips.  In that scenario, the attribute information retrieved is
   considered to be strict server-client consistency.  For NFSv4, the
   WRITE and GETATTR can be issued together inside a compound, which
   only requires one round trip between the client and server.  And this
   is also considered to be a strict server-client consistency.  In
   essence, the NFSv4 READ and WRITE operations drop the post-operation
   attributes, allowing the client to decide if it needs that

   With the flexible files layout type, the client can leverage the
   NFSv3 WCC to service the proxying of times (See Section 4 of
   [delstid]).  But the granularity of this data is limited.  With
   client side mirroring (See Section 8 of [RFC8435]), the client has to
   aggregate the N mirrored files in order to send one piece of
   information instead of N pieces of information.  Also, the client is
   limited to sending that information only when it returns the

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   The current filehandle and the lowa_stateid identifies the particular
   layout for the LAYOUT_WCC operation.  The lowa_type indicates how to
   unpack the layout type specific payload inside the lowa_body field.
   The lowa_type is defined to be a value from the IANA registry for
   "pNFS Layout Types Registry".

   The lowa_body will contain the data file attributes.  The client will
   be responsible for mapping the NFSv3 post-operation attributes to
   those in a fattr4.  Just as the post-operation attributes may be
   ignored by the client, the server may ignore the attributes inside
   the LAYOUT_WCC.  But the server can also use those attributes to
   avoid querying the storage device for the data file attributes.  Note
   that as these attributes are optional and there is nothing the client
   can do if the server ignores one, there is no need to return a
   bitmap4 of which attributes were accepted in the result of the

2.4.  Allowed Errors

   The LAYOUT_WCC operation can raise the errors in Table 1.  When an
   error is encountered, the metadata server can decide to ignore the
   entire operation or depending on the layout type specific payload, it
   could decide to apply a portion of the payload.

   Valid Error Returns for LAYOUT_WCC

   | Errors                                                            |
   | NFS4ERR_INVAL, NFS4ERR_ISDIR, NFS4ERR_MOVED,                      |
   | NFS4ERR_OLD_STATEID, NFS4ERR_OP_NOT_IN_SESSION,                   |
   | NFS4ERR_WRONG_TYPE                                                |

                                  Table 1

2.5.  Extension of Existing Implementations

   The new LAYOUT_WCC operation is OPTIONAL for both NFSv4.2 ([RFC7863])
   and the flexible files layout type ([RFC8435]).

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2.6.  Flex Files Layout Type

   /// struct ff_data_server_wcc4 {
   ///             deviceid4            ffdsw_deviceid;
   ///             stateid4             ffdsw_stateid;
   ///             nfs_fh4              ffdsw_fh_vers<>;
   ///             fattr4               ffdsw_attributes;
   /// };
   /// struct ff_mirror_wcc4 {
   ///             ff_data_server_wcc4  ffmw_data_servers<>;
   /// };
   /// struct ff_layout_wcc4 {
   ///             ff_mirror_wcc4       fflw_mirrors<>;
   /// };

   The flex file layout type specific results MUST correspond to the
   ff_layout4 data structure as defined in Section 5.1 of [RFC8435].
   There MUST be a one-to-one correspondence between:

   *  ff_data_server4 -> ff_data_server_wcc4

   *  ff_mirror4 -> ff_mirror_wcc4

   *  ff_layout4 -> ff_layout_wcc4

   Each ff_layout4 has an array of ff_mirror4, which have an array of
   ff_data_server4.  Based on the current filehandle and the
   lowa_stateid, the server can match the reported attributes.

   But the positional correspondence between the elements is not
   sufficient to determine the attributes to update.  Consider the case
   where a layout had three mirrors and two of them had updated
   attributes, but the third did not.  A client could decide to present
   all three mirrors, with one mirror having an attribute mask with no
   attributes present.  Or it could decide to present only the two
   mirrors which had been changed.

   In either case, the combination of ffdsw_deviceid, ffdsw_stateid, and
   ffdsw_fh_vers will uniquely identify the attributes to be updated.
   All three arguments are required.  A layout might have multiple data
   files on the same storage device, in which case the ffdsw_deviceid
   and ffdsw_stateid would match, but the ffdsw_fh_vers would not.

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   The ffdsw_attributes are processed similar to the obj_attributes in
   the SETATTR arguments (See Section 18.34 of [RFC8881]).

3.  Extraction of XDR

   This document contains the external data representation (XDR)
   [RFC4506] description of the new open flags for delegating the file
   to the client.  The XDR description is embedded in this document in a
   way that makes it simple for the reader to extract into a ready-to-
   compile form.  The reader can feed this document into the following
   shell script to produce the machine readable XDR description of the
   new flags:

   grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'

   That is, if the above script is stored in a file called "",
   and this document is in a file called "spec.txt", then the reader can

   sh < spec.txt > layout_wcc.x

   The effect of the script is to remove leading white space from each
   line, plus a sentinel sequence of "///".  XDR descriptions with the
   sentinel sequence are embedded throughout the document.

   Note that the XDR code contained in this document depends on types
   from the NFSv4.2 nfs4_prot.x file (generated from [RFC7863]).  This
   includes both nfs types that end with a 4, such as offset4, length4,
   etc., as well as more generic types such as uint32_t and uint64_t.

   While the XDR can be appended to that from [RFC7863], the various
   code snippets belong in their respective areas of the that XDR.

3.1.  Code Components Licensing Notice

   Both the XDR description and the scripts used for extracting the XDR
   description are Code Components as described in Section 4 of "Legal
   Provisions Relating to IETF Documents" [LEGAL].  These Code
   Components are licensed according to the terms of that document.

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4.  Security Considerations

   There are no new security considerations beyond those in [RFC7862].

5.  IANA Considerations

   IANA should use the current document (RFC-TBD) as the reference for
   the new entries.

6.  References

6.1.  Normative References

   [delstid]  Haynes, T. and T. Myklebust, "Extending the Opening of
              Files in NFSv4.2", draft-ietf-nfsv4-delstid-02.xml (Work
              In Progress), February 2023.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <>.

   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
              November 2016, <>.

   [RFC7863]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 External Data Representation Standard (XDR)
              Description", RFC 7863, DOI 10.17487/RFC7863, November
              2016, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8178]  Noveck, D., "Rules for NFSv4 Extensions and Minor
              Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,

   [RFC8434]  Haynes, T., "Requirements for Parallel NFS (pNFS) Layout
              Types", RFC 8434, DOI 10.17487/RFC8434, August 2018,

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   [RFC8435]  Halevy, B. and T. Haynes, "Parallel NFS (pNFS) Flexible
              File Layout", RFC 8435, DOI 10.17487/RFC8435, August 2018,

   [RFC8881]  Noveck, D., Ed. and C. Lever, "Network File System (NFS)
              Version 4 Minor Version 1 Protocol", RFC 8881,
              DOI 10.17487/RFC8881, August 2020,

6.2.  Informative References

   [LEGAL]    IETF Trust, "Legal Provisions Relating to IETF Documents",
              November 2008, <

   [RFC1813]  Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
              Version 3 Protocol Specification", RFC 1813,
              DOI 10.17487/RFC1813, June 1995,

Appendix A.  Acknowledgments

   Trond Myklebust and David Flynn all worked on the prototype at

   Dave Noveck and Tigran Mkrtchyan provided reviews of the document.

Authors' Addresses

   Thomas Haynes

   Trond Myklebust

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