Reporting of Errors via LAYOUTRETURN in NFSv4.2
draft-ietf-nfsv4-layrec-01

Network File System Version 4                                  T. Haynes
Internet-Draft                                              T. Myklebust
Intended status: Standards Track                             Hammerspace
Expires: 22 September 2024                                 21 March 2024

            Reporting of Errors via LAYOUTRETURN in NFSv4.2
                       draft-ietf-nfsv4-layrec-01

Abstract

   The Parallel Network File System (pNFS) allows for a file's metadata
   (MDS) and data (DS) to be on different servers.  When the metadata
   server is restarted, the client can still modify the data file
   component.  During the recovery phase of startup, the metadata server
   and the data servers work together to recover state (which files are
   open, last modification time, size, etc).  If the client has not
   encountered errors with the data files, then the state can be
   recovered, avoiding resilvering of the data files.  With any errors,
   there is no means by which the client can report errors to the
   metadata server.  As such, the metadata server has to assume that
   file needs resilvering.  This document presents an extension to
   RFC8435 to allow the client to update the metadata and avoid the
   resilvering.

Note

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

   Discussion of this draft takes place on the NFSv4 working group
   mailing list (nfsv4@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/nfsv4/.  Working Group
   information can be found at https://datatracker.ietf.org/wg/nfsv4/
   about/.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Layout State Recovery . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   In the Network File System version4 (NFSv4) with a Parallel NFS
   (pNFS) Flexible File Layout ([RFC8435]) server, during recovery after
   a restart, there is no mechanism for the client to inform the
   metadata server about an error which occurred during a WRITE (see
   Section 18.32 of [RFC8881]) operation to the data servers in the
   period of the outage.

   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].

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1.1.  Definitions

   See Section 1.1 of [RFC8435] for a more complete set of 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.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.  Layout State Recovery

   When a metadata server restarts, clients are provided a grace period
   where they are allowed to recover any state that they had
   established.  With open files, the client can send an OPEN (see
   Section 18.16 of [RFC8881]) operation with a claim type of
   CLAIM_PREVIOUS (see Section 9.11 of [RFC8881]).  The client uses the
   RECLAIM_COMPLETE (see Section 18.51 of [RFC8881]) operation to notify
   the metadata server that it is done reclaiming state.

   The NFSv4 Flexible File Layout Type allows for the client to mirror
   files (see Section 8 of [RFC8435]).  With client side mirroring, it
   is important for the client to inform the metadata server of any I/O
   errors encountered with one of the mirrors.  This is the only way for
   the metadata server to determine one or more of the mirrors is
   corrupt and then repair the mirrors via resilvering.  The client can
   use LAYOUTRETURN (see Section 18.44 of [RFC8881]) and the ff_ioerr4
   (see Section 9.1.1 of [RFC8435]) structure to inform the metadata
   server of I/O errors.

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   A problem is that if the metadata server restarts and the client has
   errors it needs to report, it can not do so.  The LAYOUTRETURN needs
   a layout stateid to proceed and there is no way for the client to
   recover layout state.  As such, clients have no choice but to not
   recover files with I/O errors.  In turn, the metadata server MUST
   assume that the mirrors are inconsistent and pick one for
   resilvering.  It is a MUST because even if the metadata server can
   determine that the client did modify data during the outage, it MUST
   NOT assume those modifications were consistent.

   If the server were to allow the client to use the anonymous stateid
   of all zeros (see Section 8.2.3 of [RFC8881]) for lrf_stateid in
   LAYOUTRETURN (see Section 18.44.1 of [RFC8881]), then the client
   could inform the metadata server of errors encountered.  That in turn
   would allow the metadata server to accurately resilver the file by
   picking the correct mirror(s).

   There are two error scenarios that can occur:

   During the grace period:  If the client were to send any lrf_stateid
      in the LAYOUTRETURN other than the anonymous stateid of all zeros,
      then the metadata server would respond with an error of
      NFS4ERR_GRACE (see Section of 15.1.9.2 [RFC8881]).

   After the grace period:  If the client were to send any lrf_stateid
      in the LAYOUTRETURN with the anonymous stateid of all zeros, then
      the metadata server would respond with an error of
      NFS4ERR_NO_GRACE (see Section 15.1.9.3 of [RFC8881]).

   Also, when the metadata server builds the reply to the LAYOUTRETURN,
   it MUST NOT bump the seqid of the lorr_stateid.

   The metadata server MUST NOT resilver a file if there are clients
   with outstanding layouts with iomode of LAYOUTIOMODE4_RW.  Whether
   the metadata server prevents all I/O to the file until the
   resilvering is done or forces all I/O to go through the metadata
   server or allows a proxy server to update the new data file as it is
   being reslivered is all an implementation choice.  The constraint is
   that the metadata server is responsible for the reconstruction of the
   data file and for the consistency of the mirrors.  The metadata
   server MUST NOT have been resilvering the file such that it has a
   different layout (set of mirror instances) than the client before the
   restart of the metadata server.  Further, the metadata server MUST
   NOT start a new resilvering of the file during the grace period
   unless there are no outstanding layouts with iomode of
   LAYOUTIOMODE4_RW.

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   If the metadata server detects that the layout being returned in the
   LAYOUTRETURN does not match the current mirror instances found for
   the file, then it should ignore the LAYOUTRETURN and resilver the
   file in question.

   Finally, the metadata server MUST determine that any files which are
   neither explicitly recovered with a CLAIM_PREVIOUS nor have a
   reported error via a LAYOUTRETURN, have to be resilvered.  The client
   has most likely restarted and lost any state.

3.  Security Considerations

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

4.  IANA Considerations

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

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <https://www.rfc-editor.org/info/rfc4506>.

   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
              November 2016, <https://www.rfc-editor.org/info/rfc7862>.

   [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, <https://www.rfc-editor.org/info/rfc7863>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8178]  Noveck, D., "Rules for NFSv4 Extensions and Minor
              Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
              <https://www.rfc-editor.org/info/rfc8178>.

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   [RFC8435]  Halevy, B. and T. Haynes, "Parallel NFS (pNFS) Flexible
              File Layout", RFC 8435, DOI 10.17487/RFC8435, August 2018,
              <https://www.rfc-editor.org/info/rfc8435>.

   [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,
              <https://www.rfc-editor.org/info/rfc8881>.

Appendix A.  Acknowledgments

   Tigran Mkrtchyan, Jeff Layton, and Rick Macklem provided reviews of
   the document.

Authors' Addresses

   Thomas Haynes
   Hammerspace
   Email: loghyr@hammerspace.com

   Trond Myklebust
   Hammerspace
   Email: trondmy@hammerspace.com

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