





Internet Engineering Task Force (IETF)                         B. Halevy
Request for Comments: 8435
Category: Standards Track                                      T. Haynes
ISSN: 2070-1721                                              Hammerspace
                                                             August 2018


                Parallel NFS (pNFS) Flexible File Layout

Abstract

   Parallel NFS (pNFS) allows a separation between the metadata (onto a
   metadata server) and data (onto a storage device) for a file.  The
   flexible file layout type is defined in this document as an extension
   to pNFS that allows the use of storage devices that require only a
   limited degree of interaction with the metadata server and use
   already-existing protocols.  Client-side mirroring is also added to
   provide replication of files.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8435.

Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1. Introduction ....................................................3
      1.1. Definitions ................................................4
      1.2. Requirements Language ......................................6
   2. Coupling of Storage Devices .....................................6
      2.1. LAYOUTCOMMIT ...............................................7
      2.2. Fencing Clients from the Storage Device ....................7
           2.2.1. Implementation Notes for Synthetic uids/gids ........8
           2.2.2. Example of Using Synthetic uids/gids ................9
      2.3. State and Locking Models ..................................10
           2.3.1. Loosely Coupled Locking Model ......................11
           2.3.2. Tightly Coupled Locking Model ......................12
   3. XDR Description of the Flexible File Layout Type ...............13
      3.1. Code Components Licensing Notice ..........................14
   4. Device Addressing and Discovery ................................16
      4.1. ff_device_addr4 ...........................................16
      4.2. Storage Device Multipathing ...............................17
   5. Flexible File Layout Type ......................................18
      5.1. ff_layout4 ................................................19
           5.1.1. Error Codes from LAYOUTGET .........................23
           5.1.2. Client Interactions with FF_FLAGS_NO_IO_THRU_MDS ...23
      5.2. LAYOUTCOMMIT ..............................................24
      5.3. Interactions between Devices and Layouts ..................24
      5.4. Handling Version Errors ...................................24
   6. Striping via Sparse Mapping ....................................25
   7. Recovering from Client I/O Errors ..............................25
   8. Mirroring ......................................................26
      8.1. Selecting a Mirror ........................................26
      8.2. Writing to Mirrors ........................................27
           8.2.1. Single Storage Device Updates Mirrors ..............27
           8.2.2. Client Updates All Mirrors .........................27
           8.2.3. Handling Write Errors ..............................28
           8.2.4. Handling Write COMMITs .............................28
      8.3. Metadata Server Resilvering of the File ...................29
   9. Flexible File Layout Type Return ...............................29
      9.1. I/O Error Reporting .......................................30
           9.1.1. ff_ioerr4 ..........................................30
      9.2. Layout Usage Statistics ...................................31
           9.2.1. ff_io_latency4 .....................................31
           9.2.2. ff_layoutupdate4 ...................................32
           9.2.3. ff_iostats4 ........................................33
      9.3. ff_layoutreturn4 ..........................................34
   10. Flexible File Layout Type LAYOUTERROR .........................35
   11. Flexible File Layout Type LAYOUTSTATS .........................35
   12. Flexible File Layout Type Creation Hint .......................35
      12.1. ff_layouthint4 ...........................................35
   13. Recalling a Layout ............................................36



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      13.1. CB_RECALL_ANY ............................................36
   14. Client Fencing ................................................37
   15. Security Considerations .......................................37
      15.1. RPCSEC_GSS and Security Services .........................39
           15.1.1. Loosely Coupled ...................................39
           15.1.2. Tightly Coupled ...................................39
   16. IANA Considerations ...........................................39
   17. References ....................................................40
      17.1. Normative References .....................................40
      17.2. Informative References ...................................41
   Acknowledgments ...................................................42
   Authors' Addresses ................................................42

1.  Introduction

   In Parallel NFS (pNFS), the metadata server returns layout type
   structures that describe where file data is located.  There are
   different layout types for different storage systems and methods of
   arranging data on storage devices.  This document defines the
   flexible file layout type used with file-based data servers that are
   accessed using the NFS protocols: NFSv3 [RFC1813], NFSv4.0 [RFC7530],
   NFSv4.1 [RFC5661], and NFSv4.2 [RFC7862].

   To provide a global state model equivalent to that of the files
   layout type, a back-end control protocol might be implemented between
   the metadata server and NFSv4.1+ storage devices.  An implementation
   can either define its own proprietary mechanism or it could define a
   control protocol in a Standards Track document.  The requirements for
   a control protocol are specified in [RFC5661] and clarified in
   [RFC8434].

   The control protocol described in this document is based on NFS.  It
   does not provide for knowledge of stateids to be passed between the
   metadata server and the storage devices.  Instead, the storage
   devices are configured such that the metadata server has full access
   rights to the data file system and then the metadata server uses
   synthetic ids to control client access to individual files.

   In traditional mirroring of data, the server is responsible for
   replicating, validating, and repairing copies of the data file.  With
   client-side mirroring, the metadata server provides a layout that
   presents the available mirrors to the client.  The client then picks
   a mirror to read from and ensures that all writes go to all mirrors.
   The client only considers the write transaction to have succeeded if
   all mirrors are successfully updated.  In case of error, the client
   can use the LAYOUTERROR operation to inform the metadata server,
   which is then responsible for the repairing of the mirrored copies of
   the file.



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1.1.  Definitions

   control communication requirements:  the specification for
      information on layouts, stateids, file metadata, and file data
      that must be communicated between the metadata server and the
      storage devices.  There is a separate set of requirements for each
      layout type.

   control protocol:  the particular mechanism that an implementation of
      a layout type would use to meet the control communication
      requirement for that layout type.  This need not be a protocol as
      normally understood.  In some cases, the same protocol may be used
      as a control protocol and storage protocol.

   client-side mirroring:  a feature in which the client, not the
      server, is responsible for updating all of the mirrored copies of
      a layout segment.

   (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.

   fencing:  the process by which the metadata server prevents the
      storage devices from processing I/O from a specific client to a
      specific file.

   file layout type:  a layout type in which the storage devices are
      accessed via the NFS protocol (see Section 13 of [RFC5661]).

   gid:  the group id, a numeric value that identifies to which group a
      file belongs.

   layout:  the information a client uses to access file data on a
      storage device.  This information includes specification of the
      protocol (layout type) and the identity of the storage devices to
      be used.

   layout iomode:  a grant of either read-only or read/write I/O to the
      client.

   layout segment:  a sub-division of a layout.  That sub-division might
      be by the layout iomode (see Sections 3.3.20 and 12.2.9 of
      [RFC5661]), a striping pattern (see Section 13.3 of [RFC5661]), or
      requested byte range.




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   layout stateid:  a 128-bit quantity returned by a server that
      uniquely defines the layout state provided by the server for a
      specific layout that describes a layout type and file (see
      Section 12.5.2 of [RFC5661]).  Further, Section 12.5.3 of
      [RFC5661] describes differences in handling between layout
      stateids and other stateid types.

   layout type:  a specification of both the storage protocol used to
      access the data and the aggregation scheme used to lay out the
      file data on the underlying storage devices.

   loose coupling:  when the control protocol is a storage 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.  It is also responsible for
      generating, recalling, and revoking layouts for file system
      objects, for performing directory operations, and for performing
      I/O operations to regular files when the clients direct these to
      the metadata server itself.

   mirror:  a copy of a layout segment.  Note that if one copy of the
      mirror is updated, then all copies must be updated.

   recalling a layout:  a graceful recall, via a callback, of a specific
      layout by the metadata server to the client.  Graceful here means
      that the client would have the opportunity to flush any WRITEs,
      etc., before returning the layout to the metadata server.

   revoking a layout:  an invalidation of a specific layout by the
      metadata server.  Once revocation occurs, the metadata server will
      not accept as valid any reference to the revoked layout, and a
      storage device will not accept any client access based on the
      layout.

   resilvering:  the act of rebuilding a mirrored copy of a layout
      segment from a known good copy of the layout segment.  Note that
      this can also be done to create a new mirrored copy of the layout
      segment.

   rsize:  the data transfer buffer size used for READs.






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   stateid:  a 128-bit quantity returned by a server that uniquely
      defines the set of locking-related state provided by the server.
      Stateids may designate state related to open files, byte-range
      locks, delegations, or layouts.

   storage device:  the target to which clients may direct I/O requests
      when they hold an appropriate layout.  See Section 2.1 of
      [RFC8434] for further discussion of the difference between a data
      server and a storage device.

   storage protocol:  the protocol used by clients to do I/O operations
      to the storage device.  Each layout type specifies the set of
      storage protocols.

   tight coupling:  an arrangement in which the control protocol is one
      designed specifically for control communication.  It may be either
      a proprietary protocol adapted specifically to a particular
      metadata server or a protocol based on a Standards Track document.

   uid:  the user id, a numeric value that identifies which user owns a
      file.

   wsize:  the data transfer buffer size used for WRITEs.

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.  Coupling of Storage Devices

   A server implementation may choose either a loosely coupled model or
   a tightly coupled model between the metadata server and the storage
   devices.  [RFC8434] describes the general problems facing pNFS
   implementations.  This document details how the new flexible file
   layout type addresses these issues.  To implement the tightly coupled
   model, a control protocol has to be defined.  As the flexible file
   layout imposes no special requirements on the client, the control
   protocol will need to provide:

   (1)  management of both security and LAYOUTCOMMITs and

   (2)  a global stateid model and management of these stateids.





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   When implementing the loosely coupled model, the only control
   protocol will be a version of NFS, with no ability to provide a
   global stateid model or to prevent clients from using layouts
   inappropriately.  To enable client use in that environment, this
   document will specify how security, state, and locking are to be
   managed.

2.1.  LAYOUTCOMMIT

   Regardless of the coupling model, the metadata server has the
   responsibility, upon receiving a LAYOUTCOMMIT (see Section 18.42 of
   [RFC5661]) to ensure that the semantics of pNFS are respected (see
   Section 3.1 of [RFC8434]).  These do include a requirement that data
   written to a data storage device be stable before the occurrence of
   the LAYOUTCOMMIT.

   It is the responsibility of the client to make sure the data file is
   stable before the metadata server begins to query the storage devices
   about the changes to the file.  If any WRITE to a storage device did
   not result with stable_how equal to FILE_SYNC, a LAYOUTCOMMIT to the
   metadata server MUST be preceded by a COMMIT to the storage devices
   written to.  Note that if the client has not done a COMMIT to the
   storage device, then the LAYOUTCOMMIT might not be synchronized to
   the last WRITE operation to the storage device.

2.2.  Fencing Clients from the Storage Device

   With loosely coupled storage devices, the metadata server uses
   synthetic uids (user ids) and gids (group ids) for the data file,
   where the uid owner of the data file is allowed read/write access and
   the gid owner is allowed read-only access.  As part of the layout
   (see ffds_user and ffds_group in Section 5.1), the client is provided
   with the user and group to be used in the Remote Procedure Call (RPC)
   [RFC5531] credentials needed to access the data file.  Fencing off of
   clients is achieved by the metadata server changing the synthetic uid
   and/or gid owners of the data file on the storage device to
   implicitly revoke the outstanding RPC credentials.  A client
   presenting the wrong credential for the desired access will get an
   NFS4ERR_ACCESS error.

   With this loosely coupled model, the metadata server is not able to
   fence off a single client; it is forced to fence off all clients.
   However, as the other clients react to the fencing, returning their
   layouts and trying to get new ones, the metadata server can hand out
   a new uid and gid to allow access.






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   It is RECOMMENDED to implement common access control methods at the
   storage device file system to allow only the metadata server root
   (super user) access to the storage device and to set the owner of all
   directories holding data files to the root user.  This approach
   provides a practical model to enforce access control and fence off
   cooperative clients, but it cannot protect against malicious clients;
   hence, it provides a level of security equivalent to AUTH_SYS.  It is
   RECOMMENDED that the communication between the metadata server and
   storage device be secure from eavesdroppers and man-in-the-middle
   protocol tampering.  The security measure could be physical security
   (e.g., the servers are co-located in a physically secure area),
   encrypted communications, or some other technique.

   With tightly coupled storage devices, the metadata server sets the
   user and group owners, mode bits, and Access Control List (ACL) of
   the data file to be the same as the metadata file.  And the client
   must authenticate with the storage device and go through the same
   authorization process it would go through via the metadata server.
   In the case of tight coupling, fencing is the responsibility of the
   control protocol and is not described in detail in this document.
   However, implementations of the tightly coupled locking model (see
   Section 2.3) will need a way to prevent access by certain clients to
   specific files by invalidating the corresponding stateids on the
   storage device.  In such a scenario, the client will be given an
   error of NFS4ERR_BAD_STATEID.

   The client need not know the model used between the metadata server
   and the storage device.  It need only react consistently to any
   errors in interacting with the storage device.  It should both return
   the layout and error to the metadata server and ask for a new layout.
   At that point, the metadata server can either hand out a new layout,
   hand out no layout (forcing the I/O through it), or deny the client
   further access to the file.

2.2.1.  Implementation Notes for Synthetic uids/gids

   The selection method for the synthetic uids and gids to be used for
   fencing in loosely coupled storage devices is strictly an
   implementation issue.  That is, an administrator might restrict a
   range of such ids available to the Lightweight Directory Access
   Protocol (LDAP) 'uid' field [RFC4519].  The administrator might also
   be able to choose an id that would never be used to grant access.
   Then, when the metadata server had a request to access a file, a
   SETATTR would be sent to the storage device to set the owner and
   group of the data file.  The user and group might be selected in a
   round-robin fashion from the range of available ids.





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   Those ids would be sent back as ffds_user and ffds_group to the
   client, who would present them as the RPC credentials to the storage
   device.  When the client is done accessing the file and the metadata
   server knows that no other client is accessing the file, it can reset
   the owner and group to restrict access to the data file.

   When the metadata server wants to fence off a client, it changes the
   synthetic uid and/or gid to the restricted ids.  Note that using a
   restricted id ensures that there is a change of owner and at least
   one id available that never gets allowed access.

   Under an AUTH_SYS security model, synthetic uids and gids of 0 SHOULD
   be avoided.  These typically either grant super access to files on a
   storage device or are mapped to an anonymous id.  In the first case,
   even if the data file is fenced, the client might still be able to
   access the file.  In the second case, multiple ids might be mapped to
   the anonymous ids.

2.2.2.  Example of Using Synthetic uids/gids

   The user loghyr creates a file "ompha.c" on the metadata server,
   which then creates a corresponding data file on the storage device.

   The metadata server entry may look like:

   -rw-r--r--    1 loghyr  staff    1697 Dec  4 11:31 ompha.c

   On the storage device, the file may be assigned some unpredictable
   synthetic uid/gid to deny access:

   -rw-r-----    1 19452   28418    1697 Dec  4 11:31 data_ompha.c

   When the file is opened on a client and accessed, the user will try
   to get a layout for the data file.  Since the layout knows nothing
   about the user (and does not care), it does not matter whether the
   user loghyr or garbo opens the file.  The client has to present an
   uid of 19452 to get write permission.  If it presents any other value
   for the uid, then it must give a gid of 28418 to get read access.

   Further, if the metadata server decides to fence the file, it should
   change the uid and/or gid such that these values neither match
   earlier values for that file nor match a predictable change based on
   an earlier fencing.

   -rw-r-----    1 19453   28419    1697 Dec  4 11:31 data_ompha.c






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   The set of synthetic gids on the storage device should be selected
   such that there is no mapping in any of the name services used by the
   storage device, i.e., each group should have no members.

   If the layout segment has an iomode of LAYOUTIOMODE4_READ, then the
   metadata server should return a synthetic uid that is not set on the
   storage device.  Only the synthetic gid would be valid.

   The client is thus solely responsible for enforcing file permissions
   in a loosely coupled model.  To allow loghyr write access, it will
   send an RPC to the storage device with a credential of 1066:1067.  To
   allow garbo read access, it will send an RPC to the storage device
   with a credential of 1067:1067.  The value of the uid does not matter
   as long as it is not the synthetic uid granted when getting the
   layout.

   While pushing the enforcement of permission checking onto the client
   may seem to weaken security, the client may already be responsible
   for enforcing permissions before modifications are sent to a server.
   With cached writes, the client is always responsible for tracking who
   is modifying a file and making sure to not coalesce requests from
   multiple users into one request.

2.3.  State and Locking Models

   An implementation can always be deployed as a loosely coupled model.
   There is, however, no way for a storage device to indicate over an
   NFS protocol that it can definitively participate in a tightly
   coupled model:

   o  Storage devices implementing the NFSv3 and NFSv4.0 protocols are
      always treated as loosely coupled.

   o  NFSv4.1+ storage devices that do not return the
      EXCHGID4_FLAG_USE_PNFS_DS flag set to EXCHANGE_ID are indicating
      that they are to be treated as loosely coupled.  From the locking
      viewpoint, they are treated in the same way as NFSv4.0 storage
      devices.

   o  NFSv4.1+ storage devices that do identify themselves with the
      EXCHGID4_FLAG_USE_PNFS_DS flag set to EXCHANGE_ID can potentially
      be tightly coupled.  They would use a back-end control protocol to
      implement the global stateid model as described in [RFC5661].

   A storage device would have to be either discovered or advertised
   over the control protocol to enable a tightly coupled model.





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2.3.1.  Loosely Coupled Locking Model

   When locking-related operations are requested, they are primarily
   dealt with by the metadata server, which generates the appropriate
   stateids.  When an NFSv4 version is used as the data access protocol,
   the metadata server may make stateid-related requests of the storage
   devices.  However, it is not required to do so, and the resulting
   stateids are known only to the metadata server and the storage
   device.

   Given this basic structure, locking-related operations are handled as
   follows:

   o  OPENs are dealt with by the metadata server.  Stateids are
      selected by the metadata server and associated with the client ID
      describing the client's connection to the metadata server.  The
      metadata server may need to interact with the storage device to
      locate the file to be opened, but no locking-related functionality
      need be used on the storage device.

      OPEN_DOWNGRADE and CLOSE only require local execution on the
      metadata server.

   o  Advisory byte-range locks can be implemented locally on the
      metadata server.  As in the case of OPENs, the stateids associated
      with byte-range locks are assigned by the metadata server and only
      used on the metadata server.

   o  Delegations are assigned by the metadata server that initiates
      recalls when conflicting OPENs are processed.  No storage device
      involvement is required.

   o  TEST_STATEID and FREE_STATEID are processed locally on the
      metadata server, without storage device involvement.

   All I/O operations to the storage device are done using the anonymous
   stateid.  Thus, the storage device has no information about the
   openowner and lockowner responsible for issuing a particular I/O
   operation.  As a result:

   o  Mandatory byte-range locking cannot be supported because the
      storage device has no way of distinguishing I/O done on behalf of
      the lock owner from those done by others.

   o  Enforcement of share reservations is the responsibility of the
      client.  Even though I/O is done using the anonymous stateid, the
      client must ensure that it has a valid stateid associated with the
      openowner.



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   In the event that a stateid is revoked, the metadata server is
   responsible for preventing client access, since it has no way of
   being sure that the client is aware that the stateid in question has
   been revoked.

   As the client never receives a stateid generated by a storage device,
   there is no client lease on the storage device and no prospect of
   lease expiration, even when access is via NFSv4 protocols.  Clients
   will have leases on the metadata server.  In dealing with lease
   expiration, the metadata server may need to use fencing to prevent
   revoked stateids from being relied upon by a client unaware of the
   fact that they have been revoked.

2.3.2.  Tightly Coupled Locking Model

   When locking-related operations are requested, they are primarily
   dealt with by the metadata server, which generates the appropriate
   stateids.  These stateids must be made known to the storage device
   using control protocol facilities, the details of which are not
   discussed in this document.

   Given this basic structure, locking-related operations are handled as
   follows:

   o  OPENs are dealt with primarily on the metadata server.  Stateids
      are selected by the metadata server and associated with the client
      ID describing the client's connection to the metadata server.  The
      metadata server needs to interact with the storage device to
      locate the file to be opened and to make the storage device aware
      of the association between the metadata-server-chosen stateid and
      the client and openowner that it represents.

      OPEN_DOWNGRADE and CLOSE are executed initially on the metadata
      server, but the state change made must be propagated to the
      storage device.

   o  Advisory byte-range locks can be implemented locally on the
      metadata server.  As in the case of OPENs, the stateids associated
      with byte-range locks are assigned by the metadata server and are
      available for use on the metadata server.  Because I/O operations
      are allowed to present lock stateids, the metadata server needs
      the ability to make the storage device aware of the association
      between the metadata-server-chosen stateid and the corresponding
      open stateid it is associated with.

   o  Mandatory byte-range locks can be supported when both the metadata
      server and the storage devices have the appropriate support.  As
      in the case of advisory byte-range locks, these are assigned by



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      the metadata server and are available for use on the metadata
      server.  To enable mandatory lock enforcement on the storage
      device, the metadata server needs the ability to make the storage
      device aware of the association between the metadata-server-chosen
      stateid and the client, openowner, and lock (i.e., lockowner,
      byte-range, and lock-type) that it represents.  Because I/O
      operations are allowed to present lock stateids, this information
      needs to be propagated to all storage devices to which I/O might
      be directed rather than only to storage device that contain the
      locked region.

   o  Delegations are assigned by the metadata server that initiates
      recalls when conflicting OPENs are processed.  Because I/O
      operations are allowed to present delegation stateids, the
      metadata server requires the ability (1) to make the storage
      device aware of the association between the metadata-server-chosen
      stateid and the filehandle and delegation type it represents and
      (2) to break such an association.

   o  TEST_STATEID is processed locally on the metadata server, without
      storage device involvement.

   o  FREE_STATEID is processed on the metadata server, but the metadata
      server requires the ability to propagate the request to the
      corresponding storage devices.

   Because the client will possess and use stateids valid on the storage
   device, there will be a client lease on the storage device, and the
   possibility of lease expiration does exist.  The best approach for
   the storage device is to retain these locks as a courtesy.  However,
   if it does not do so, control protocol facilities need to provide the
   means to synchronize lock state between the metadata server and
   storage device.

   Clients will also have leases on the metadata server that are subject
   to expiration.  In dealing with lease expiration, the metadata server
   would be expected to use control protocol facilities enabling it to
   invalidate revoked stateids on the storage device.  In the event the
   client is not responsive, the metadata server may need to use fencing
   to prevent revoked stateids from being acted upon by the storage
   device.

3.  XDR Description of the Flexible File Layout Type

   This document contains the External Data Representation (XDR)
   [RFC4506] description of the flexible file layout type.  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



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   reader can feed this document into the following shell script to
   produce the machine-readable XDR description of the flexible file
   layout type:

   <CODE BEGINS>

   #!/bin/sh
   grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'

   <CODE ENDS>

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

   sh extract.sh < spec.txt > flex_files_prot.x

   The effect of the script is to remove leading white space from each
   line, plus a sentinel sequence of "///".

   The embedded XDR file header follows.  Subsequent 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 NFSv4.1 nfs4_prot.x file [RFC5662].  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.

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 "Trust
   Legal Provisions (TLP)" [LEGAL].  These Code Components are licensed
   according to the terms of that document.

   <CODE BEGINS>

   /// /*
   ///  * Copyright (c) 2018 IETF Trust and the persons identified
   ///  * as authors of the code.  All rights reserved.
   ///  *
   ///  * Redistribution and use in source and binary forms, with
   ///  * or without modification, are permitted provided that the
   ///  * following conditions are met:
   ///  *
   ///  * - Redistributions of source code must retain the above
   ///  *   copyright notice, this list of conditions and the
   ///  *   following disclaimer.



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   ///  *
   ///  * - Redistributions in binary form must reproduce the above
   ///  *   copyright notice, this list of conditions and the
   ///  *   following disclaimer in the documentation and/or other
   ///  *   materials provided with the distribution.
   ///  *
   ///  * - Neither the name of Internet Society, IETF or IETF
   ///  *   Trust, nor the names of specific contributors, may be
   ///  *   used to endorse or promote products derived from this
   ///  *   software without specific prior written permission.
   ///  *
   ///  *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
   ///  *   AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
   ///  *   WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   ///  *   IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   ///  *   FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
   ///  *   EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
   ///  *   LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   ///  *   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   ///  *   NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   ///  *   SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   ///  *   INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   ///  *   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
   ///  *   OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
   ///  *   IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
   ///  *   ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   ///  *
   ///  * This code was derived from RFC 8435.
   ///  * Please reproduce this note if possible.
   ///  */
   ///
   /// /*
   ///  * flex_files_prot.x
   ///  */
   ///
   /// /*
   ///  * The following include statements are for example only.
   ///  * The actual XDR definition files are generated separately
   ///  * and independently and are likely to have a different name.
   ///  * %#include <nfsv42.x>
   ///  * %#include <rpc_prot.x>
   ///  */
   ///

   <CODE ENDS>






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4.  Device Addressing and Discovery

   Data operations to a storage device require the client to know the
   network address of the storage device.  The NFSv4.1+ GETDEVICEINFO
   operation (Section 18.40 of [RFC5661]) is used by the client to
   retrieve that information.

4.1.  ff_device_addr4

   The ff_device_addr4 data structure is returned by the server as the
   layout-type-specific opaque field da_addr_body in the device_addr4
   structure by a successful GETDEVICEINFO operation.

   <CODE BEGINS>

   /// struct ff_device_versions4 {
   ///         uint32_t        ffdv_version;
   ///         uint32_t        ffdv_minorversion;
   ///         uint32_t        ffdv_rsize;
   ///         uint32_t        ffdv_wsize;
   ///         bool            ffdv_tightly_coupled;
   /// };
   ///

   /// struct ff_device_addr4 {
   ///         multipath_list4     ffda_netaddrs;
   ///         ff_device_versions4 ffda_versions<>;
   /// };
   ///

   <CODE ENDS>

   The ffda_netaddrs field is used to locate the storage device.  It
   MUST be set by the server to a list holding one or more of the device
   network addresses.

   The ffda_versions array allows the metadata server to present choices
   as to NFS version, minor version, and coupling strength to the
   client.  The ffdv_version and ffdv_minorversion represent the NFS
   protocol to be used to access the storage device.  This layout
   specification defines the semantics for ffdv_versions 3 and 4.  If
   ffdv_version equals 3, then the server MUST set ffdv_minorversion to
   0 and ffdv_tightly_coupled to false.  The client MUST then access the
   storage device using the NFSv3 protocol [RFC1813].  If ffdv_version
   equals 4, then the server MUST set ffdv_minorversion to one of the
   NFSv4 minor version numbers, and the client MUST access the storage
   device using NFSv4 with the specified minor version.




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   Note that while the client might determine that it cannot use any of
   the configured combinations of ffdv_version, ffdv_minorversion, and
   ffdv_tightly_coupled, when it gets the device list from the metadata
   server, there is no way to indicate to the metadata server as to
   which device it is version incompatible.  However, if the client
   waits until it retrieves the layout from the metadata server, it can
   at that time clearly identify the storage device in question (see
   Section 5.4).

   The ffdv_rsize and ffdv_wsize are used to communicate the maximum
   rsize and wsize supported by the storage device.  As the storage
   device can have a different rsize or wsize than the metadata server,
   the ffdv_rsize and ffdv_wsize allow the metadata server to
   communicate that information on behalf of the storage device.

   ffdv_tightly_coupled informs the client as to whether or not the
   metadata server is tightly coupled with the storage devices.  Note
   that even if the data protocol is at least NFSv4.1, it may still be
   the case that there is loose coupling in effect.  If
   ffdv_tightly_coupled is not set, then the client MUST commit writes
   to the storage devices for the file before sending a LAYOUTCOMMIT to
   the metadata server.  That is, the writes MUST be committed by the
   client to stable storage via issuing WRITEs with stable_how ==
   FILE_SYNC or by issuing a COMMIT after WRITEs with stable_how !=
   FILE_SYNC (see Section 3.3.7 of [RFC1813]).

4.2.  Storage Device Multipathing

   The flexible file layout type supports multipathing to multiple
   storage device addresses.  Storage-device-level multipathing is used
   for bandwidth scaling via trunking and for higher availability of use
   in the event of a storage device failure.  Multipathing allows the
   client to switch to another storage device address that may be that
   of another storage device that is exporting the same data stripe
   unit, without having to contact the metadata server for a new layout.

   To support storage device multipathing, ffda_netaddrs contains an
   array of one or more storage device network addresses.  This array
   (data type multipath_list4) represents a list of storage devices
   (each identified by a network address), with the possibility that
   some storage device will appear in the list multiple times.

   The client is free to use any of the network addresses as a
   destination to send storage device requests.  If some network
   addresses are less desirable paths to the data than others, then the
   metadata server SHOULD NOT include those network addresses in
   ffda_netaddrs.  If less desirable network addresses exist to provide
   failover, the RECOMMENDED method to offer the addresses is to provide



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   them in a replacement device-ID-to-device-address mapping or a
   replacement device ID.  When a client finds no response from the
   storage device using all addresses available in ffda_netaddrs, it
   SHOULD send a GETDEVICEINFO to attempt to replace the existing
   device-ID-to-device-address mappings.  If the metadata server detects
   that all network paths represented by ffda_netaddrs are unavailable,
   the metadata server SHOULD send a CB_NOTIFY_DEVICEID (if the client
   has indicated it wants device ID notifications for changed device
   IDs) to change the device-ID-to-device-address mappings to the
   available addresses.  If the device ID itself will be replaced, the
   metadata server SHOULD recall all layouts with the device ID and thus
   force the client to get new layouts and device ID mappings via
   LAYOUTGET and GETDEVICEINFO.

   Generally, if two network addresses appear in ffda_netaddrs, they
   will designate the same storage device.  When the storage device is
   accessed over NFSv4.1 or a higher minor version, the two storage
   device addresses will support the implementation of client ID or
   session trunking (the latter is RECOMMENDED) as defined in [RFC5661].
   The two storage device addresses will share the same server owner or
   major ID of the server owner.  It is not always necessary for the two
   storage device addresses to designate the same storage device with
   trunking being used.  For example, the data could be read-only, and
   the data consist of exact replicas.

5.  Flexible File Layout Type

   The original layouttype4 introduced in [RFC5662] is modified to be:

   <CODE BEGINS>

       enum layouttype4 {
           LAYOUT4_NFSV4_1_FILES   = 1,
           LAYOUT4_OSD2_OBJECTS    = 2,
           LAYOUT4_BLOCK_VOLUME    = 3,
           LAYOUT4_FLEX_FILES      = 4
       };

       struct layout_content4 {
           layouttype4             loc_type;
           opaque                  loc_body<>;
       };









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       struct layout4 {
           offset4                 lo_offset;
           length4                 lo_length;
           layoutiomode4           lo_iomode;
           layout_content4         lo_content;
       };

   <CODE ENDS>

   This document defines structures associated with the layouttype4
   value LAYOUT4_FLEX_FILES.  [RFC5661] specifies the loc_body structure
   as an XDR type "opaque".  The opaque layout is uninterpreted by the
   generic pNFS client layers but is interpreted by the flexible file
   layout type implementation.  This section defines the structure of
   this otherwise opaque value, ff_layout4.

5.1.  ff_layout4

   <CODE BEGINS>

   /// const FF_FLAGS_NO_LAYOUTCOMMIT   = 0x00000001;
   /// const FF_FLAGS_NO_IO_THRU_MDS    = 0x00000002;
   /// const FF_FLAGS_NO_READ_IO        = 0x00000004;
   /// const FF_FLAGS_WRITE_ONE_MIRROR  = 0x00000008;

   /// typedef uint32_t            ff_flags4;
   ///

   /// struct ff_data_server4 {
   ///     deviceid4               ffds_deviceid;
   ///     uint32_t                ffds_efficiency;
   ///     stateid4                ffds_stateid;
   ///     nfs_fh4                 ffds_fh_vers<>;
   ///     fattr4_owner            ffds_user;
   ///     fattr4_owner_group      ffds_group;
   /// };
   ///

   /// struct ff_mirror4 {
   ///     ff_data_server4         ffm_data_servers<>;
   /// };
   ///

   /// struct ff_layout4 {
   ///     length4                 ffl_stripe_unit;
   ///     ff_mirror4              ffl_mirrors<>;
   ///     ff_flags4               ffl_flags;
   ///     uint32_t                ffl_stats_collect_hint;



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   /// };
   ///

   <CODE ENDS>

   The ff_layout4 structure specifies a layout in that portion of the
   data file described in the current layout segment.  It is either a
   single instance or a set of mirrored copies of that portion of the
   data file.  When mirroring is in effect, it protects against loss of
   data in layout segments.

   While not explicitly shown in the above XDR, each layout4 element
   returned in the logr_layout array of LAYOUTGET4res (see
   Section 18.43.2 of [RFC5661]) describes a layout segment.  Hence,
   each ff_layout4 also describes a layout segment.  It is possible that
   the file is concatenated from more than one layout segment.  Each
   layout segment MAY represent different striping parameters.

   The ffl_stripe_unit field is the stripe unit size in use for the
   current layout segment.  The number of stripes is given inside each
   mirror by the number of elements in ffm_data_servers.  If the number
   of stripes is one, then the value for ffl_stripe_unit MUST default to
   zero.  The only supported mapping scheme is sparse and is detailed in
   Section 6.  Note that there is an assumption here that both the
   stripe unit size and the number of stripes are the same across all
   mirrors.

   The ffl_mirrors field is the array of mirrored storage devices that
   provide the storage for the current stripe; see Figure 1.

   The ffl_stats_collect_hint field provides a hint to the client on how
   often the server wants it to report LAYOUTSTATS for a file.  The time
   is in seconds.


















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                      +-----------+
                      |           |
                      |           |
                      |   File    |
                      |           |
                      |           |
                      +-----+-----+
                            |
               +------------+------------+
               |                         |
          +----+-----+             +-----+----+
          | Mirror 1 |             | Mirror 2 |
          +----+-----+             +-----+----+
               |                         |
          +-----------+            +-----------+
          |+-----------+           |+-----------+
          ||+-----------+          ||+-----------+
          +||  Storage  |          +||  Storage  |
           +|  Devices  |           +|  Devices  |
            +-----------+            +-----------+

                           Figure 1

   The ffs_mirrors field represents an array of state information for
   each mirrored copy of the current layout segment.  Each element is
   described by a ff_mirror4 type.

   ffds_deviceid provides the deviceid of the storage device holding the
   data file.

   ffds_fh_vers is an array of filehandles of the data file matching the
   available NFS versions on the given storage device.  There MUST be
   exactly as many elements in ffds_fh_vers as there are in
   ffda_versions.  Each element of the array corresponds to a particular
   combination of ffdv_version, ffdv_minorversion, and
   ffdv_tightly_coupled provided for the device.  The array allows for
   server implementations that have different filehandles for different
   combinations of version, minor version, and coupling strength.  See
   Section 5.4 for how to handle versioning issues between the client
   and storage devices.

   For tight coupling, ffds_stateid provides the stateid to be used by
   the client to access the file.  For loose coupling and an NFSv4
   storage device, the client will have to use an anonymous stateid to
   perform I/O on the storage device.  With no control protocol, the
   metadata server stateid cannot be used to provide a global stateid
   model.  Thus, the server MUST set the ffds_stateid to be the
   anonymous stateid.



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   This specification of the ffds_stateid restricts both models for
   NFSv4.x storage protocols:

   loosely coupled model:  the stateid has to be an anonymous stateid

   tightly coupled model:  the stateid has to be a global stateid

   A number of issues stem from a mismatch between the fact that
   ffds_stateid is defined as a single item while ffds_fh_vers is
   defined as an array.  It is possible for each open file on the
   storage device to require its own open stateid.  Because there are
   established loosely coupled implementations of the version of the
   protocol described in this document, such potential issues have not
   been addressed here.  It is possible for future layout types to be
   defined that address these issues, should it become important to
   provide multiple stateids for the same underlying file.

   For loosely coupled storage devices, ffds_user and ffds_group provide
   the synthetic user and group to be used in the RPC credentials that
   the client presents to the storage device to access the data files.
   For tightly coupled storage devices, the user and group on the
   storage device will be the same as on the metadata server; that is,
   if ffdv_tightly_coupled (see Section 4.1) is set, then the client
   MUST ignore both ffds_user and ffds_group.

   The allowed values for both ffds_user and ffds_group are specified as
   owner and owner_group, respectively, in Section 5.9 of [RFC5661].
   For NFSv3 compatibility, user and group strings that consist of
   decimal numeric values with no leading zeros can be given a special
   interpretation by clients and servers that choose to provide such
   support.  The receiver may treat such a user or group string as
   representing the same user as would be represented by an NFSv3 uid or
   gid having the corresponding numeric value.  Note that if using
   Kerberos for security, the expectation is that these values will be a
   name@domain string.

   ffds_efficiency describes the metadata server's evaluation as to the
   effectiveness of each mirror.  Note that this is per layout and not
   per device as the metric may change due to perceived load,
   availability to the metadata server, etc.  Higher values denote
   higher perceived utility.  The way the client can select the best
   mirror to access is discussed in Section 8.1.

   ffl_flags is a bitmap that allows the metadata server to inform the
   client of particular conditions that may result from more or less
   tight coupling of the storage devices.





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   FF_FLAGS_NO_LAYOUTCOMMIT:  can be set to indicate that the client is
      not required to send LAYOUTCOMMIT to the metadata server.

   FF_FLAGS_NO_IO_THRU_MDS:  can be set to indicate that the client
      should not send I/O operations to the metadata server.  That is,
      even if the client could determine that there was a network
      disconnect to a storage device, the client should not try to proxy
      the I/O through the metadata server.

   FF_FLAGS_NO_READ_IO:  can be set to indicate that the client should
      not send READ requests with the layouts of iomode
      LAYOUTIOMODE4_RW.  Instead, it should request a layout of iomode
      LAYOUTIOMODE4_READ from the metadata server.

   FF_FLAGS_WRITE_ONE_MIRROR:  can be set to indicate that the client
      only needs to update one of the mirrors (see Section 8.2).

5.1.1.  Error Codes from LAYOUTGET

   [RFC5661] provides little guidance as to how the client is to proceed
   with a LAYOUTGET that returns an error of either
   NFS4ERR_LAYOUTTRYLATER, NFS4ERR_LAYOUTUNAVAILABLE, and NFS4ERR_DELAY.
   Within the context of this document:

   NFS4ERR_LAYOUTUNAVAILABLE:  there is no layout available and the I/O
      is to go to the metadata server.  Note that it is possible to have
      had a layout before a recall and not after.

   NFS4ERR_LAYOUTTRYLATER:  there is some issue preventing the layout
      from being granted.  If the client already has an appropriate
      layout, it should continue with I/O to the storage devices.

   NFS4ERR_DELAY:  there is some issue preventing the layout from being
      granted.  If the client already has an appropriate layout, it
      should not continue with I/O to the storage devices.

5.1.2.  Client Interactions with FF_FLAGS_NO_IO_THRU_MDS

   Even if the metadata server provides the FF_FLAGS_NO_IO_THRU_MDS
   flag, the client can still perform I/O to the metadata server.  The
   flag functions as a hint.  The flag indicates to the client that the
   metadata server prefers to separate the metadata I/O from the data I/
   O, most likely for performance reasons.








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5.2.  LAYOUTCOMMIT

   The flexible file layout does not use lou_body inside the
   loca_layoutupdate argument to LAYOUTCOMMIT.  If lou_type is
   LAYOUT4_FLEX_FILES, the lou_body field MUST have a zero length (see
   Section 18.42.1 of [RFC5661]).

5.3.  Interactions between Devices and Layouts

   In [RFC5661], the file layout type is defined such that the
   relationship between multipathing and filehandles can result in
   either 0, 1, or N filehandles (see Section 13.3).  Some rationales
   for this are clustered servers that share the same filehandle or
   allow for multiple read-only copies of the file on the same storage
   device.  In the flexible file layout type, while there is an array of
   filehandles, they are independent of the multipathing being used.  If
   the metadata server wants to provide multiple read-only copies of the
   same file on the same storage device, then it should provide multiple
   mirrored instances, each with a different ff_device_addr4.  The
   client can then determine that, since the each of the ffds_fh_vers
   are different, there are multiple copies of the file for the current
   layout segment available.

5.4.  Handling Version Errors

   When the metadata server provides the ffda_versions array in the
   ff_device_addr4 (see Section 4.1), the client is able to determine
   whether or not it can access a storage device with any of the
   supplied combinations of ffdv_version, ffdv_minorversion, and
   ffdv_tightly_coupled.  However, due to the limitations of reporting
   errors in GETDEVICEINFO (see Section 18.40 in [RFC5661]), the client
   is not able to specify which specific device it cannot communicate
   with over one of the provided ffdv_version and ffdv_minorversion
   combinations.  Using ff_ioerr4 (see Section 9.1.1) inside either the
   LAYOUTRETURN (see Section 18.44 of [RFC5661]) or the LAYOUTERROR (see
   Section 15.6 of [RFC7862] and Section 10 of this document), the
   client can isolate the problematic storage device.

   The error code to return for LAYOUTRETURN and/or LAYOUTERROR is
   NFS4ERR_MINOR_VERS_MISMATCH.  It does not matter whether the mismatch
   is a major version (e.g., client can use NFSv3 but not NFSv4) or
   minor version (e.g., client can use NFSv4.1 but not NFSv4.2), the
   error indicates that for all the supplied combinations for
   ffdv_version and ffdv_minorversion, the client cannot communicate
   with the storage device.  The client can retry the GETDEVICEINFO to
   see if the metadata server can provide a different combination, or it
   can fall back to doing the I/O through the metadata server.




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6.  Striping via Sparse Mapping

   While other layout types support both dense and sparse mapping of
   logical offsets to physical offsets within a file (see, for example,
   Section 13.4 of [RFC5661]), the flexible file layout type only
   supports a sparse mapping.

   With sparse mappings, the logical offset within a file (L) is also
   the physical offset on the storage device.  As detailed in
   Section 13.4.4 of [RFC5661], this results in holes across each
   storage device that does not contain the current stripe index.

   L: logical offset within the file

   W: stripe width
       W = number of elements in ffm_data_servers

   S: number of bytes in a stripe
       S = W * ffl_stripe_unit

   N: stripe number
       N = L / S

7.  Recovering from Client I/O Errors

   The pNFS client may encounter errors when directly accessing the
   storage devices.  However, it is the responsibility of the metadata
   server to recover from the I/O errors.  When the LAYOUT4_FLEX_FILES
   layout type is used, the client MUST report the I/O errors to the
   server at LAYOUTRETURN time using the ff_ioerr4 structure (see
   Section 9.1.1).

   The metadata server analyzes the error and determines the required
   recovery operations such as recovering media failures or
   reconstructing missing data files.

   The metadata server MUST recall any outstanding layouts to allow it
   exclusive write access to the stripes being recovered and to prevent
   other clients from hitting the same error condition.  In these cases,
   the server MUST complete recovery before handing out any new layouts
   to the affected byte ranges.

   Although the client implementation has the option to propagate a
   corresponding error to the application that initiated the I/O
   operation and drop any unwritten data, the client should attempt to
   retry the original I/O operation by either requesting a new layout or
   sending the I/O via regular NFSv4.1+ READ or WRITE operations to the
   metadata server.  The client SHOULD attempt to retrieve a new layout



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   and retry the I/O operation using the storage device first and only
   retry the I/O operation via the metadata server if the error
   persists.

8.  Mirroring

   The flexible file layout type has a simple model in place for the
   mirroring of the file data constrained by a layout segment.  There is
   no assumption that each copy of the mirror is stored identically on
   the storage devices.  For example, one device might employ
   compression or deduplication on the data.  However, the over-the-wire
   transfer of the file contents MUST appear identical.  Note, this is a
   constraint of the selected XDR representation in which each mirrored
   copy of the layout segment has the same striping pattern (see
   Figure 1).

   The metadata server is responsible for determining the number of
   mirrored copies and the location of each mirror.  While the client
   may provide a hint to how many copies it wants (see Section 12), the
   metadata server can ignore that hint; in any event, the client has no
   means to dictate either the storage device (which also means the
   coupling and/or protocol levels to access the layout segments) or the
   location of said storage device.

   The updating of mirrored layout segments is done via client-side
   mirroring.  With this approach, the client is responsible for making
   sure modifications are made on all copies of the layout segments it
   is informed of via the layout.  If a layout segment is being
   resilvered to a storage device, that mirrored copy will not be in the
   layout.  Thus, the metadata server MUST update that copy until the
   client is presented it in a layout.  If the FF_FLAGS_WRITE_ONE_MIRROR
   is set in ffl_flags, the client need only update one of the mirrors
   (see Section 8.2).  If the client is writing to the layout segments
   via the metadata server, then the metadata server MUST update all
   copies of the mirror.  As seen in Section 8.3, during the
   resilvering, the layout is recalled, and the client has to make
   modifications via the metadata server.

8.1.  Selecting a Mirror

   When the metadata server grants a layout to a client, it MAY let the
   client know how fast it expects each mirror to be once the request
   arrives at the storage devices via the ffds_efficiency member.  While
   the algorithms to calculate that value are left to the metadata
   server implementations, factors that could contribute to that
   calculation include speed of the storage device, physical memory
   available to the device, operating system version, current load, etc.




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   However, what should not be involved in that calculation is a
   perceived network distance between the client and the storage device.
   The client is better situated for making that determination based on
   past interaction with the storage device over the different available
   network interfaces between the two; that is, the metadata server
   might not know about a transient outage between the client and
   storage device because it has no presence on the given subnet.

   As such, it is the client that decides which mirror to access for
   reading the file.  The requirements for writing to mirrored layout
   segments are presented below.

8.2.  Writing to Mirrors

8.2.1.  Single Storage Device Updates Mirrors

   If the FF_FLAGS_WRITE_ONE_MIRROR flag in ffl_flags is set, the client
   only needs to update one of the copies of the layout segment.  For
   this case, the storage device MUST ensure that all copies of the
   mirror are updated when any one of the mirrors is updated.  If the
   storage device gets an error when updating one of the mirrors, then
   it MUST inform the client that the original WRITE had an error.  The
   client then MUST inform the metadata server (see Section 8.2.3).  The
   client's responsibility with respect to COMMIT is explained in
   Section 8.2.4.  The client may choose any one of the mirrors and may
   use ffds_efficiency as described in Section 8.1 when making this
   choice.

8.2.2.  Client Updates All Mirrors

   If the FF_FLAGS_WRITE_ONE_MIRROR flag in ffl_flags is not set, the
   client is responsible for updating all mirrored copies of the layout
   segments that it is given in the layout.  A single failed update is
   sufficient to fail the entire operation.  If all but one copy is
   updated successfully and the last one provides an error, then the
   client needs to inform the metadata server about the error.  The
   client can use either LAYOUTRETURN or LAYOUTERROR to inform the
   metadata server that the update failed to that storage device.  If
   the client is updating the mirrors serially, then it SHOULD stop at
   the first error encountered and report that to the metadata server.
   If the client is updating the mirrors in parallel, then it SHOULD
   wait until all storage devices respond so that it can report all
   errors encountered during the update.








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8.2.3.  Handling Write Errors

   When the client reports a write error to the metadata server, the
   metadata server is responsible for determining if it wants to remove
   the errant mirror from the layout, if the mirror has recovered from
   some transient error, etc.  When the client tries to get a new
   layout, the metadata server informs it of the decision by the
   contents of the layout.  The client MUST NOT assume that the contents
   of the previous layout will match those of the new one.  If it has
   updates that were not committed to all mirrors, then it MUST resend
   those updates to all mirrors.

   There is no provision in the protocol for the metadata server to
   directly determine that the client has or has not recovered from an
   error.  For example, if a storage device was network partitioned from
   the client and the client reported the error to the metadata server,
   then the network partition would be repaired, and all of the copies
   would be successfully updated.  There is no mechanism for the client
   to report that fact, and the metadata server is forced to repair the
   file across the mirror.

   If the client supports NFSv4.2, it can use LAYOUTERROR and
   LAYOUTRETURN to provide hints to the metadata server about the
   recovery efforts.  A LAYOUTERROR on a file is for a non-fatal error.
   A subsequent LAYOUTRETURN without a ff_ioerr4 indicates that the
   client successfully replayed the I/O to all mirrors.  Any
   LAYOUTRETURN with a ff_ioerr4 is an error that the metadata server
   needs to repair.  The client MUST be prepared for the LAYOUTERROR to
   trigger a CB_LAYOUTRECALL if the metadata server determines it needs
   to start repairing the file.

8.2.4.  Handling Write COMMITs

   When stable writes are done to the metadata server or to a single
   replica (if allowed by the use of FF_FLAGS_WRITE_ONE_MIRROR), it is
   the responsibility of the receiving node to propagate the written
   data stably, before replying to the client.

   In the corresponding cases in which unstable writes are done, the
   receiving node does not have any such obligation, although it may
   choose to asynchronously propagate the updates.  However, once a
   COMMIT is replied to, all replicas must reflect the writes that have
   been done, and this data must have been committed to stable storage
   on all replicas.







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   In order to avoid situations in which stale data is read from
   replicas to which writes have not been propagated:

   o  A client that has outstanding unstable writes made to single node
      (metadata server or storage device) MUST do all reads from that
      same node.

   o  When writes are flushed to the server (for example, to implement
      close-to-open semantics), a COMMIT must be done by the client to
      ensure that up-to-date written data will be available irrespective
      of the particular replica read.

8.3.  Metadata Server Resilvering of the File

   The metadata server may elect to create a new mirror of the layout
   segments at any time.  This might be to resilver a copy on a storage
   device that was down for servicing, to provide a copy of the layout
   segments on storage with different storage performance
   characteristics, etc.  As the client will not be aware of the new
   mirror and the metadata server will not be aware of updates that the
   client is making to the layout segments, the metadata server MUST
   recall the writable layout segment(s) that it is resilvering.  If the
   client issues a LAYOUTGET for a writable layout segment that is in
   the process of being resilvered, then the metadata server can deny
   that request with an NFS4ERR_LAYOUTUNAVAILABLE.  The client would
   then have to perform the I/O through the metadata server.

9.  Flexible File Layout Type Return

   layoutreturn_file4 is used in the LAYOUTRETURN operation to convey
   layout-type-specific information to the server.  It is defined in
   Section 18.44.1 of [RFC5661] as follows:

   <CODE BEGINS>

      /* Constants used for LAYOUTRETURN and CB_LAYOUTRECALL */
      const LAYOUT4_RET_REC_FILE      = 1;
      const LAYOUT4_RET_REC_FSID      = 2;
      const LAYOUT4_RET_REC_ALL       = 3;

      enum layoutreturn_type4 {
              LAYOUTRETURN4_FILE = LAYOUT4_RET_REC_FILE,
              LAYOUTRETURN4_FSID = LAYOUT4_RET_REC_FSID,
              LAYOUTRETURN4_ALL  = LAYOUT4_RET_REC_ALL
      };

   struct layoutreturn_file4 {
           offset4         lrf_offset;



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           length4         lrf_length;
           stateid4        lrf_stateid;
           /* layouttype4 specific data */
           opaque          lrf_body<>;
   };

   union layoutreturn4 switch(layoutreturn_type4 lr_returntype) {
           case LAYOUTRETURN4_FILE:
                   layoutreturn_file4      lr_layout;
           default:
                   void;
   };

   struct LAYOUTRETURN4args {
           /* CURRENT_FH: file */
           bool                    lora_reclaim;
           layouttype4             lora_layout_type;
           layoutiomode4           lora_iomode;
           layoutreturn4           lora_layoutreturn;
   };

   <CODE ENDS>

   If the lora_layout_type layout type is LAYOUT4_FLEX_FILES and the
   lr_returntype is LAYOUTRETURN4_FILE, then the lrf_body opaque value
   is defined by ff_layoutreturn4 (see Section 9.3).  This allows the
   client to report I/O error information or layout usage statistics
   back to the metadata server as defined below.  Note that while the
   data structures are built on concepts introduced in NFSv4.2, the
   effective discriminated union (lora_layout_type combined with
   ff_layoutreturn4) allows for an NFSv4.1 metadata server to utilize
   the data.

9.1.  I/O Error Reporting

9.1.1.  ff_ioerr4

   <CODE BEGINS>

   /// struct ff_ioerr4 {
   ///         offset4        ffie_offset;
   ///         length4        ffie_length;
   ///         stateid4       ffie_stateid;
   ///         device_error4  ffie_errors<>;
   /// };
   ///

   <CODE ENDS>



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   Recall that [RFC7862] defines device_error4 as:

   <CODE BEGINS>

   struct device_error4 {
           deviceid4       de_deviceid;
           nfsstat4        de_status;
           nfs_opnum4      de_opnum;
   };

   <CODE ENDS>

   The ff_ioerr4 structure is used to return error indications for data
   files that generated errors during data transfers.  These are hints
   to the metadata server that there are problems with that file.  For
   each error, ffie_errors.de_deviceid, ffie_offset, and ffie_length
   represent the storage device and byte range within the file in which
   the error occurred; ffie_errors represents the operation and type of
   error.  The use of device_error4 is described in Section 15.6 of
   [RFC7862].

   Even though the storage device might be accessed via NFSv3 and
   reports back NFSv3 errors to the client, the client is responsible
   for mapping these to appropriate NFSv4 status codes as de_status.
   Likewise, the NFSv3 operations need to be mapped to equivalent NFSv4
   operations.

9.2.  Layout Usage Statistics

9.2.1.  ff_io_latency4

   <CODE BEGINS>

   /// struct ff_io_latency4 {
   ///         uint64_t       ffil_ops_requested;
   ///         uint64_t       ffil_bytes_requested;
   ///         uint64_t       ffil_ops_completed;
   ///         uint64_t       ffil_bytes_completed;
   ///         uint64_t       ffil_bytes_not_delivered;
   ///         nfstime4       ffil_total_busy_time;
   ///         nfstime4       ffil_aggregate_completion_time;
   /// };
   ///

   <CODE ENDS>






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   Both operation counts and bytes transferred are kept in the
   ff_io_latency4.  As seen in ff_layoutupdate4 (see Section 9.2.2),
   READ and WRITE operations are aggregated separately.  READ operations
   are used for the ff_io_latency4 ffl_read.  Both WRITE and COMMIT
   operations are used for the ff_io_latency4 ffl_write.  "Requested"
   counters track what the client is attempting to do, and "completed"
   counters track what was done.  There is no requirement that the
   client only report completed results that have matching requested
   results from the reported period.

   ffil_bytes_not_delivered is used to track the aggregate number of
   bytes requested but not fulfilled due to error conditions.
   ffil_total_busy_time is the aggregate time spent with outstanding RPC
   calls. ffil_aggregate_completion_time is the sum of all round-trip
   times for completed RPC calls.

   In Section 3.3.1 of [RFC5661], the nfstime4 is defined as the number
   of seconds and nanoseconds since midnight or zero hour January 1,
   1970 Coordinated Universal Time (UTC).  The use of nfstime4 in
   ff_io_latency4 is to store time since the start of the first I/O from
   the client after receiving the layout.  In other words, these are to
   be decoded as duration and not as a date and time.

   Note that LAYOUTSTATS are cumulative, i.e., not reset each time the
   operation is sent.  If two LAYOUTSTATS operations for the same file
   and layout stateid originate from the same NFS client and are
   processed at the same time by the metadata server, then the one
   containing the larger values contains the most recent time series
   data.

9.2.2.  ff_layoutupdate4

   <CODE BEGINS>

   /// struct ff_layoutupdate4 {
   ///         netaddr4       ffl_addr;
   ///         nfs_fh4        ffl_fhandle;
   ///         ff_io_latency4 ffl_read;
   ///         ff_io_latency4 ffl_write;
   ///         nfstime4       ffl_duration;
   ///         bool           ffl_local;
   /// };
   ///

   <CODE ENDS>






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   ffl_addr differentiates which network address the client is connected
   to on the storage device.  In the case of multipathing, ffl_fhandle
   indicates which read-only copy was selected. ffl_read and ffl_write
   convey the latencies for both READ and WRITE operations,
   respectively.  ffl_duration is used to indicate the time period over
   which the statistics were collected.  If true, ffl_local indicates
   that the I/O was serviced by the client's cache.  This flag allows
   the client to inform the metadata server about "hot" access to a file
   it would not normally be allowed to report on.

9.2.3.  ff_iostats4

   <CODE BEGINS>

   /// struct ff_iostats4 {
   ///         offset4           ffis_offset;
   ///         length4           ffis_length;
   ///         stateid4          ffis_stateid;
   ///         io_info4          ffis_read;
   ///         io_info4          ffis_write;
   ///         deviceid4         ffis_deviceid;
   ///         ff_layoutupdate4  ffis_layoutupdate;
   /// };
   ///

   <CODE ENDS>

   [RFC7862] defines io_info4 as:

   <CODE BEGINS>

   struct io_info4 {
           uint64_t        ii_count;
           uint64_t        ii_bytes;
   };

   <CODE ENDS>

   With pNFS, data transfers are performed directly between the pNFS
   client and the storage devices.  Therefore, the metadata server has
   no direct knowledge of the I/O operations being done and thus cannot
   create on its own statistical information about client I/O to
   optimize the data storage location.  ff_iostats4 MAY be used by the
   client to report I/O statistics back to the metadata server upon
   returning the layout.






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   Since it is not feasible for the client to report every I/O that used
   the layout, the client MAY identify "hot" byte ranges for which to
   report I/O statistics.  The definition and/or configuration mechanism
   of what is considered "hot" and the size of the reported byte range
   are out of the scope of this document.  For client implementation,
   providing reasonable default values and an optional run-time
   management interface to control these parameters is suggested.  For
   example, a client can define the default byte-range resolution to be
   1 MB in size and the thresholds for reporting to be 1 MB/second or 10
   I/O operations per second.

   For each byte range, ffis_offset and ffis_length represent the
   starting offset of the range and the range length in bytes.
   ffis_read.ii_count, ffis_read.ii_bytes, ffis_write.ii_count, and
   ffis_write.ii_bytes represent the number of contiguous READ and WRITE
   I/Os and the respective aggregate number of bytes transferred within
   the reported byte range.

   The combination of ffis_deviceid and ffl_addr uniquely identifies
   both the storage path and the network route to it.  Finally,
   ffl_fhandle allows the metadata server to differentiate between
   multiple read-only copies of the file on the same storage device.

9.3.  ff_layoutreturn4

   <CODE BEGINS>

   /// struct ff_layoutreturn4 {
   ///         ff_ioerr4     fflr_ioerr_report<>;
   ///         ff_iostats4   fflr_iostats_report<>;
   /// };
   ///

   <CODE ENDS>

   When data file I/O operations fail, fflr_ioerr_report<> is used to
   report these errors to the metadata server as an array of elements of
   type ff_ioerr4.  Each element in the array represents an error that
   occurred on the data file identified by ffie_errors.de_deviceid.  If
   no errors are to be reported, the size of the fflr_ioerr_report<>
   array is set to zero.  The client MAY also use fflr_iostats_report<>
   to report a list of I/O statistics as an array of elements of type
   ff_iostats4.  Each element in the array represents statistics for a
   particular byte range.  Byte ranges are not guaranteed to be disjoint
   and MAY repeat or intersect.






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10.  Flexible File Layout Type LAYOUTERROR

   If the client is using NFSv4.2 to communicate with the metadata
   server, then instead of waiting for a LAYOUTRETURN to send error
   information to the metadata server (see Section 9.1), it MAY use
   LAYOUTERROR (see Section 15.6 of [RFC7862]) to communicate that
   information.  For the flexible file layout type, this means that
   LAYOUTERROR4args is treated the same as ff_ioerr4.

11.  Flexible File Layout Type LAYOUTSTATS

   If the client is using NFSv4.2 to communicate with the metadata
   server, then instead of waiting for a LAYOUTRETURN to send I/O
   statistics to the metadata server (see Section 9.2), it MAY use
   LAYOUTSTATS (see Section 15.7 of [RFC7862]) to communicate that
   information.  For the flexible file layout type, this means that
   LAYOUTSTATS4args.lsa_layoutupdate is overloaded with the same
   contents as in ffis_layoutupdate.

12.  Flexible File Layout Type Creation Hint

   The layouthint4 type is defined in the [RFC5661] as follows:

   <CODE BEGINS>

   struct layouthint4 {
       layouttype4        loh_type;
       opaque             loh_body<>;
   };

   <CODE ENDS>

   The layouthint4 structure is used by the client to pass a hint about
   the type of layout it would like created for a particular file.  If
   the loh_type layout type is LAYOUT4_FLEX_FILES, then the loh_body
   opaque value is defined by the ff_layouthint4 type.

12.1.  ff_layouthint4

   <CODE BEGINS>

   /// union ff_mirrors_hint switch (bool ffmc_valid) {
   ///     case TRUE:
   ///         uint32_t    ffmc_mirrors;
   ///     case FALSE:
   ///         void;
   /// };
   ///



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   /// struct ff_layouthint4 {
   ///     ff_mirrors_hint    fflh_mirrors_hint;
   /// };
   ///

   <CODE ENDS>

   This type conveys hints for the desired data map.  All parameters are
   optional so the client can give values for only the parameter it
   cares about.

13.  Recalling a Layout

   While Section 12.5.5 of [RFC5661] discusses reasons independent of
   layout type for recalling a layout, the flexible file layout type
   metadata server should recall outstanding layouts in the following
   cases:

   o  When the file's security policy changes, i.e., ACLs or permission
      mode bits are set.

   o  When the file's layout changes, rendering outstanding layouts
      invalid.

   o  When existing layouts are inconsistent with the need to enforce
      locking constraints.

   o  When existing layouts are inconsistent with the requirements
      regarding resilvering as described in Section 8.3.

13.1.  CB_RECALL_ANY

   The metadata server can use the CB_RECALL_ANY callback operation to
   notify the client to return some or all of its layouts.  Section 22.3
   of [RFC5661] defines the allowed types of the "NFSv4 Recallable
   Object Types Registry".

   <CODE BEGINS>

   /// const RCA4_TYPE_MASK_FF_LAYOUT_MIN     = 16;
   /// const RCA4_TYPE_MASK_FF_LAYOUT_MAX     = 17;
   ///

   struct  CB_RECALL_ANY4args      {
       uint32_t        craa_layouts_to_keep;
       bitmap4         craa_type_mask;
   };




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   <CODE ENDS>

   Typically, CB_RECALL_ANY will be used to recall client state when the
   server needs to reclaim resources.  The craa_type_mask bitmap
   specifies the type of resources that are recalled, and the
   craa_layouts_to_keep value specifies how many of the recalled
   flexible file layouts the client is allowed to keep.  The mask flags
   for the flexible file layout type are defined as follows:

   <CODE BEGINS>

   /// enum ff_cb_recall_any_mask {
   ///     PNFS_FF_RCA4_TYPE_MASK_READ = 16,
   ///     PNFS_FF_RCA4_TYPE_MASK_RW   = 17
   /// };
   ///

   <CODE ENDS>

   The flags represent the iomode of the recalled layouts.  In response,
   the client SHOULD return layouts of the recalled iomode that it needs
   the least, keeping at most craa_layouts_to_keep flexible file
   layouts.

   The PNFS_FF_RCA4_TYPE_MASK_READ flag notifies the client to return
   layouts of iomode LAYOUTIOMODE4_READ.  Similarly, the
   PNFS_FF_RCA4_TYPE_MASK_RW flag notifies the client to return layouts
   of iomode LAYOUTIOMODE4_RW.  When both mask flags are set, the client
   is notified to return layouts of either iomode.

14.  Client Fencing

   In cases where clients are uncommunicative and their lease has
   expired or when clients fail to return recalled layouts within a
   lease period, the server MAY revoke client layouts and reassign these
   resources to other clients (see Section 12.5.5 of [RFC5661]).  To
   avoid data corruption, the metadata server MUST fence off the revoked
   clients from the respective data files as described in Section 2.2.

15.  Security Considerations

   The combination of components in a pNFS system is required to
   preserve the security properties of NFSv4.1+ with respect to an
   entity accessing data via a client.  The pNFS feature partitions the
   NFSv4.1+ file system protocol into two parts: the control protocol
   and the data protocol.  As the control protocol in this document is
   NFS, the security properties are equivalent to the version of NFS
   being used.  The flexible file layout further divides the data



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   protocol into metadata and data paths.  The security properties of
   the metadata path are equivalent to those of NFSv4.1x (see Sections
   1.7.1 and 2.2.1 of [RFC5661]).  And the security properties of the
   data path are equivalent to those of the version of NFS used to
   access the storage device, with the provision that the metadata
   server is responsible for authenticating client access to the data
   file.  The metadata server provides appropriate credentials to the
   client to access data files on the storage device.  It is also
   responsible for revoking access for a client to the storage device.

   The metadata server enforces the file access control policy at
   LAYOUTGET time.  The client should use RPC authorization credentials
   for getting the layout for the requested iomode ((LAYOUTIOMODE4_READ
   or LAYOUTIOMODE4_RW), and the server verifies the permissions and ACL
   for these credentials, possibly returning NFS4ERR_ACCESS if the
   client is not allowed the requested iomode.  If the LAYOUTGET
   operation succeeds, the client receives, as part of the layout, a set
   of credentials allowing it I/O access to the specified data files
   corresponding to the requested iomode.  When the client acts on I/O
   operations on behalf of its local users, it MUST authenticate and
   authorize the user by issuing respective OPEN and ACCESS calls to the
   metadata server, similar to having NFSv4 data delegations.

   The combination of filehandle, synthetic uid, and gid in the layout
   is the way that the metadata server enforces access control to the
   data server.  The client only has access to filehandles of file
   objects and not directory objects.  Thus, given a filehandle in a
   layout, it is not possible to guess the parent directory filehandle.
   Further, as the data file permissions only allow the given synthetic
   uid read/write permission and the given synthetic gid read
   permission, knowing the synthetic ids of one file does not
   necessarily allow access to any other data file on the storage
   device.

   The metadata server can also deny access at any time by fencing the
   data file, which means changing the synthetic ids.  In turn, that
   forces the client to return its current layout and get a new layout
   if it wants to continue I/O to the data file.

   If access is allowed, the client uses the corresponding (read-only or
   read/write) credentials to perform the I/O operations at the data
   file's storage devices.  When the metadata server receives a request
   to change a file's permissions or ACL, it SHOULD recall all layouts
   for that file and then MUST fence off any clients still holding
   outstanding layouts for the respective files by implicitly
   invalidating the previously distributed credential on all data file
   comprising the file in question.  It is REQUIRED that this be done
   before committing to the new permissions and/or ACL.  By requesting



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   new layouts, the clients will reauthorize access against the modified
   access control metadata.  Recalling the layouts in this case is
   intended to prevent clients from getting an error on I/Os done after
   the client was fenced off.

15.1.  RPCSEC_GSS and Security Services

   Because of the special use of principals within the loosely coupled
   model, the issues are different depending on the coupling model.

15.1.1.  Loosely Coupled

   RPCSEC_GSS version 3 (RPCSEC_GSSv3) [RFC7861] contains facilities
   that would allow it to be used to authorize the client to the storage
   device on behalf of the metadata server.  Doing so would require that
   each of the metadata server, storage device, and client would need to
   implement RPCSEC_GSSv3 using an RPC-application-defined structured
   privilege assertion in a manner described in Section 4.9.1 of
   [RFC7862].  The specifics necessary to do so are not described in
   this document.  This is principally because any such specification
   would require extensive implementation work on a wide range of
   storage devices, which would be unlikely to result in a widely usable
   specification for a considerable time.

   As a result, the layout type described in this document will not
   provide support for use of RPCSEC_GSS together with the loosely
   coupled model.  However, future layout types could be specified,
   which would allow such support, either through the use of
   RPCSEC_GSSv3 or in other ways.

15.1.2.  Tightly Coupled

   With tight coupling, the principal used to access the metadata file
   is exactly the same as used to access the data file.  The storage
   device can use the control protocol to validate any RPC credentials.
   As a result, there are no security issues related to using RPCSEC_GSS
   with a tightly coupled system.  For example, if Kerberos V5 Generic
   Security Service Application Program Interface (GSS-API) [RFC4121] is
   used as the security mechanism, then the storage device could use a
   control protocol to validate the RPC credentials to the metadata
   server.

16.  IANA Considerations

   [RFC5661] introduced the "pNFS Layout Types Registry"; new layout
   type numbers in this registry need to be assigned by IANA.  This
   document defines the protocol associated with an existing layout type
   number: LAYOUT4_FLEX_FILES.  See Table 1.



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   +--------------------+------------+----------+-----+----------------+
   | Layout Type Name   | Value      | RFC      | How | Minor Versions |
   +--------------------+------------+----------+-----+----------------+
   | LAYOUT4_FLEX_FILES | 0x00000004 | RFC 8435 | L   | 1              |
   +--------------------+------------+----------+-----+----------------+

                     Table 1: Layout Type Assignments

   [RFC5661] also introduced the "NFSv4 Recallable Object Types
   Registry".  This document defines new recallable objects for
   RCA4_TYPE_MASK_FF_LAYOUT_MIN and RCA4_TYPE_MASK_FF_LAYOUT_MAX (see
   Table 2).

   +------------------------------+-------+--------+-----+-------------+
   | Recallable Object Type Name  | Value | RFC    | How | Minor       |
   |                              |       |        |     | Versions    |
   +------------------------------+-------+--------+-----+-------------+
   | RCA4_TYPE_MASK_FF_LAYOUT_MIN | 16    | RFC    | L   | 1           |
   |                              |       | 8435   |     |             |
   | RCA4_TYPE_MASK_FF_LAYOUT_MAX | 17    | RFC    | L   | 1           |
   |                              |       | 8435   |     |             |
   +------------------------------+-------+--------+-----+-------------+

                Table 2: Recallable Object Type Assignments

17.  References

17.1.  Normative References

   [LEGAL]    IETF Trust, "Trust Legal Provisions (TLP)",
              <https://trustee.ietf.org/trust-legal-provisions.html>.

   [RFC1813]  Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
              Version 3 Protocol Specification", RFC 1813,
              DOI 10.17487/RFC1813, June 1995,
              <https://www.rfc-editor.org/info/rfc1813>.

   [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>.

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              DOI 10.17487/RFC4121, July 2005,
              <https://www.rfc-editor.org/info/rfc4121>.




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   [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>.

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
              May 2009, <https://www.rfc-editor.org/info/rfc5531>.

   [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
              <https://www.rfc-editor.org/info/rfc5661>.

   [RFC5662]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              External Data Representation Standard (XDR) Description",
              RFC 5662, DOI 10.17487/RFC5662, January 2010,
              <https://www.rfc-editor.org/info/rfc5662>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <https://www.rfc-editor.org/info/rfc7530>.

   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <https://www.rfc-editor.org/info/rfc7861>.

   [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>.

   [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>.

   [RFC8434]  Haynes, T., "Requirements for Parallel NFS (pNFS) Layout
              Types", RFC 8434, DOI 10.17487/RFC8434, August 2018,
              <https://www.rfc-editor.org/info/rfc8434>.

17.2.  Informative References

   [RFC4519]  Sciberras, A., Ed., "Lightweight Directory Access Protocol
              (LDAP): Schema for User Applications", RFC 4519,
              DOI 10.17487/RFC4519, June 2006,
              <https://www.rfc-editor.org/info/rfc4519>.






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Acknowledgments

   The following individuals provided miscellaneous comments to early
   draft versions of this document: Matt W. Benjamin, Adam Emerson,
   J. Bruce Fields, and Lev Solomonov.

   The following individuals provided miscellaneous comments to the
   final draft versions of this document: Anand Ganesh, Robert Wipfel,
   Gobikrishnan Sundharraj, Trond Myklebust, Rick Macklem, and Jim
   Sermersheim.

   Idan Kedar caught a nasty bug in the interaction of client-side
   mirroring and the minor versioning of devices.

   Dave Noveck provided comprehensive reviews of the document during the
   working group last calls.  He also rewrote Section 2.3.

   Olga Kornievskaia made a convincing case against the use of a
   credential versus a principal in the fencing approach.  Andy Adamson
   and Benjamin Kaduk helped to sharpen the focus.

   Benjamin Kaduk and Olga Kornievskaia also helped provide concrete
   scenarios for loosely coupled security mechanisms.  In the end, Olga
   proved that as defined, the loosely coupled model would not work with
   RPCSEC_GSS.

   Tigran Mkrtchyan provided the use case for not allowing the client to
   proxy the I/O through the data server.

   Rick Macklem provided the use case for only writing to a single
   mirror.

Authors' Addresses

   Benny Halevy

   Email: bhalevy@gmail.com


   Thomas Haynes
   Hammerspace
   4300 El Camino Real Ste 105
   Los Altos, CA  94022
   United States of America

   Email: loghyr@gmail.com





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