NFSv4                                                          B. Halevy
Intended status: Standards Track                               T. Haynes
Expires: January 22, July 25, 2016                                      Primary Data
                                                           July 21, 2015
                                                        January 22, 2016

                Parallel NFS (pNFS) Flexible File Layout


   The Parallel Network File System (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 to allow the use of storage devices
   in a fashion such that they require only a quite limited degree of
   interaction with the metadata server, using already existing
   protocols.  Client side mirroring is also added to provide
   replication of files.

Status of This Memo

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

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 22, July 25, 2016.

Copyright Notice

   Copyright (c) 2015 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Difference Between a Data Server and a Storage Device . .   5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   2.  Coupling of Storage Devices . . . . . . . . . . . . . . . . .   6
     2.1.  LAYOUTCOMMIT  . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Fencing Clients from the Data Server  . . . . . . . . . .   6
       2.2.1.  Implementation Notes for Synthetic uids/gids  . . . .   7
       2.2.2.  Example of using Synthetic uids/gids  . . . . . . . .   7
     2.3.  State and Locking Models  . . . . . . . . . . . . . . . .   8
   3.  XDR Description of the Flexible File Layout Type  . . . . . .   9
     3.1.  Code Components Licensing Notice  . . . . . . . . . . . .   9  10
   4.  Device Addressing and Discovery . . . . . . . . . . . . . . .  11
     4.1.  ff_device_addr4 . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Storage Device Multipathing . . . . . . . . . . . . . . .  12  13
   5.  Flexible File Layout Type . . . . . . . . . . . . . . . . . .  13  14
     5.1.  ff_layout4  . . . . . . . . . . . . . . . . . . . . . . .  14
       5.1.1.  Error codes from LAYOUTGET  . . . . . . . . . . . . .  17
       5.1.2.  Client Interactions with FF_FLAGS_NO_IO_THRU_MDS  . .  18
     5.2.  Interactions Between Devices and Layouts  . . . . . . . .  17  18
     5.3.  Handling Version Errors . . . . . . . . . . . . . . . . .  18
   6.  Striping via Sparse Mapping . . . . . . . . . . . . . . . . .  18  19
   7.  Recovering from Client I/O Errors . . . . . . . . . . . . . .  19
   8.  Mirroring . . . . . . . . . . . . . . . . . . . . . . . . . .  19  20
     8.1.  Selecting a Mirror  . . . . . . . . . . . . . . . . . . .  20  21
     8.2.  Writing to Mirrors  . . . . . . . . . . . . . . . . . . .  20  21
     8.3.  Metadata Server Resilvering of the File . . . . . . . . .  21  22
   9.  Flexible Files Layout Type Return . . . . . . . . . . . . . .  21  22
     9.1.  I/O Error Reporting . . . . . . . . . . . . . . . . . . .  22  23
       9.1.1.  ff_ioerr4 . . . . . . . . . . . . . . . . . . . . . .  22  23
     9.2.  Layout Usage Statistics . . . . . . . . . . . . . . . . .  23  24
       9.2.1.  ff_io_latency4  . . . . . . . . . . . . . . . . . . .  23  24
       9.2.2.  ff_layoutupdate4  . . . . . . . . . . . . . . . . . .  24  25
       9.2.3.  ff_iostats4 . . . . . . . . . . . . . . . . . . . . .  24  25
     9.3.  ff_layoutreturn4  . . . . . . . . . . . . . . . . . . . .  25  26
   10. Flexible Files Layout Type LAYOUTERROR  . . . . . . . . . . .  26  27
   11. Flexible Files Layout Type LAYOUTSTATS  . . . . . . . . . . .  26  27
   12. Flexible File Layout Type Creation Hint . . . . . . . . . . .  26  27
     12.1.  ff_layouthint4 . . . . . . . . . . . . . . . . . . . . .  27  28
   13. Recalling Layouts . a Layout  . . . . . . . . . . . . . . . . . . . . .  27  28
     13.1.  CB_RECALL_ANY  . . . . . . . . . . . . . . . . . . . . .  27  28

   14. Client Fencing  . . . . . . . . . . . . . . . . . . . . . . .  28  29
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  29  30
     15.1.  Kerberized File Access . . . . . . . . . . . . . . . . .  29  30
       15.1.1.  Loosely Coupled  . . . . . . . . . . . . . . . . . .  30  31
       15.1.2.  Tightly Coupled  . . . . . . . . . . . . . . . . . .  30  31
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30  31
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30  31
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  30  31
     17.2.  Informative References . . . . . . . . . . . . . . . . .  31  32
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  31  32
   Appendix B.  RFC Editor Notes . . . . . . . . . . . . . . . . . .  32  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32  33

1.  Introduction

   In the parallel Network File System (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 Network File System (NFS)
   protocols: NFSv3 [RFC1813], NFSv4.0 [RFCNFSv4], NFSv4.1 [RFC5661],
   and NFSv4.2 [NFSv42].

   To provide a global state model equivalent to that of the Files
   Layout Type, a back-end control protocol MAY be implemented between
   the metadata server and NFSv4.1+ storage devices.  It is out of scope
   for this document to specify the wire protocol of such a protocol,
   yet the requirements for the protocol are specified in [RFC5661] and
   clarified in [pNFSLayouts].

1.1.  Definitions

   control protocol:  is a set of requirements for the communication of
      information on layouts, stateids, file metadata, and file data
      between the metadata server and the storage devices (see

   client-side mirroring:  is when the client and not the server is
      responsible for updating all of the mirrored copies of a layout

   data file:  is that part of the file system object which describes
      the payload and not the object.  E.g., it is the file contents.

   data server (DS):  is one of the pNFS servers which provides the
      contents of a file system object which is a regular file.
      Depending on the layout, there might be one or more data servers
      over which the data is striped.  Note that while the metadata
      server is strictly accessed over the NFSv4.1+ protocol, depending
      on the Layout Type, the data server could be accessed via any
      protocol that meets the pNFS requirements.

   fencing:  is when the metadata server prevents the storage devices
      from processing I/O from a specific client to a specific file.

   File Layout Type:  is a Layout Type in which the storage devices are
      accessed via the NFS protocol.

   layout:  informs a client of which storage devices it needs to
      communicate with (and over which protocol) to perform I/O on a
      file.  The layout might also provide some hints about how the
      storage is physically organized.

   layout iomode:  describes whether the layout granted to the client is
      for read or read/write I/O.

   layout segment:  describes a sub-division of a layout.  That sub-
      division might be by the 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.

   layout stateid:  is 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 describes
      the difference between a layout stateid and a normal stateid.

   layout type:  describes 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:  is when the metadata server and the storage devices
      do not have a control protocol present.

   metadata file:  is that part of the file system object which
      describes the object and not the payload.  E.g., it could be the
      time since last modification, access, etc.

   metadata server (MDS):  is the pNFS server which provides metadata
      information for a file system object.  It also is responsible for
      generating layouts for file system objects.  Note that the MDS is
      responsible for directory-based operations.

   mirror:  is a copy of a layout segment.  While mirroring can be used
      for backing up a layout segment, the copies can be distributed
      such that each remote site has a locally available copy.  Note
      that if one copy of the mirror is updated, then all copies must be

   recalling a layout:  is when the metadata server uses a back channel
      to inform the client that the layout is to be returned in a
      graceful manner.  Note that the client could be able to flush any
      writes, etc., before replying to the metadata server.

   revoking a layout:  is when the metadata server invalidates the
      layout such that neither the metadata server nor any storage
      device will accept any access from the client with that layout.

   resilvering:  is 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

   rsize:  is the data transfer buffer size used for reads.

   stateid:  is a 128-bit quantity returned by a server that uniquely
      defines the open and locking states provided by the server for a
      specific open-owner or lock-owner/open-owner pair for a specific
      file and type of lock.

   storage device:  is another term used almost interchangeably with
      data server.  See Section 1.2 for the nuances between the two.

   tight coupling:  is when the metadata server and the storage devices
      do have a control protocol present.

   wsize:  is the data transfer buffer size used for writes.

1.2.  Difference Between a Data Server and a Storage Device

   We defined a data server as a pNFS server, which implies that it can
   utilize the NFSv4.1+ protocol to communicate with the client.  As
   such, only the File Layout Type would currently meet this
   requirement.  The more generic concept is a storage device, which can
   use any protocol to communicate with the client.  The requirements
   for a storage device to act together with the metadata server to
   provide data to a client are that there is a Layout Type
   specification for the given protocol and that the metadata server has
   granted a layout to the client.  Note that nothing precludes there
   being multiple supported Layout Types (i.e., protocols) between a
   metadata server, storage devices, and client.

   As storage device is the more encompassing terminology, this document
   utilizes it over data server.

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Coupling of Storage Devices

   The coupling of the metadata server with the storage devices can be
   either tight or loose.  In a tight coupling, there is a control
   protocol present to manage security, LAYOUTCOMMITs, etc.  With a
   loose coupling, the only control protocol might be a version of NFS.
   As such, semantics for managing security, state, and locking models
   MUST be defined.


   With a tightly coupled system, when the metadata server receives a
   LAYOUTCOMMIT (see Section 18.42 of [RFC5661]), the semantics of the
   File Layout Type MUST be met (see Section 12.5.4 of [RFC5661]).  With
   a loosely coupled system, a LAYOUTCOMMIT to the metadata server MUST
   be proceeded with a COMMIT to the storage device.  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.  With a loosely coupled system, 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
   with a COMMIT to the storage device.  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 Data Server

   With loosely coupled storage devices, the metadata server uses
   synthetic uids and gids 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.

   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.

   Note: it is recommended to implement common access control methods at
   the storage device filesystem 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 can not protect against malicious
   clients; hence it provides a level of security equivalent to

   With tightly coupled storage devices, the metadata server sets the
   user and group owners, mode bits, and 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.

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.  An implementation might allow  I.e., an administrator
   to might restrict a range
   of such ids in available to the name servers. Lightweight Directory Access Protocol
   (LDAP) 'uid' field [RFC4519].  She might also be able to choose an id
   that would never be used to grant acccess.  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.

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

   When the metadata server wanted to fence off a client, it would
   change 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.

2.2.2.  Example of using Synthetic uids/gids

   The user loghyr creates a file "ompha.c" on the metadata server and
   it 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, it may be assigned some random 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, since the layout knows nothing
   about the user (and does not care), whether loghyr or garbo opens the
   file does not matter.  The owner and group are modified and those
   values are returned.

   -rw-r-----    1 1066    1067     1697 Dec  4 11:31 data_ompha.c

   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 it when getting the

   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

   Metadata file OPEN, LOCK, and DELEGATION operations are always
   executed only against the metadata server.

   The metadata server responds to state changing operations by
   executing them against the respective data files on the storage
   devices.  It then sends the storage device open stateid as part of
   the layout (see the ffm_stateid in Section 5.1) and it is then used
   by the client for executing READ/WRITE operations against the storage


   NFSv4.1+ storage devices that do not return the
   EXCHGID4_FLAG_USE_PNFS_DS flag set to EXCHANGE_ID are used indicating that
   they are loosely coupled.  As such, they are treated the same way as
   NFSv4 storage devices.

   NFSv4.1+ clustered storage devices that do identify themselves with the
   EXCHGID4_FLAG_USE_PNFS_DS flag set to EXCHANGE_ID use are stongly
   coupled.  They will be using a back-end control protocol as described
   in [RFC5661] to implement a global stateid model as defined there.

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
   reader can feed this document into the following shell script to
   produce the machine readable XDR description of the Flexible File
   Layout Type:


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


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

   sh < spec.txt > 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 "Legal
   Provisions Relating to IETF Documents" [LEGAL].  These Code
   Components are licensed according to the terms of that document.


   /// /*
   ///  * Copyright (c) 2012 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:
   ///  *
   ///  * o Redistributions of source code must retain the above
   ///  *   copyright notice, this list of conditions and the
   ///  *   following disclaimer.
   ///  *
   ///  * o 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.
   ///  *
   ///  * o 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 code was derived from RFCTBD10.

   ///  * 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>
   ///  */


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
   storage protocol specific opaque field da_addr_body in the
   device_addr4 structure by a successful GETDEVICEINFO operation.


   /// 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<>;
   /// };

   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
   multiple choices
   as to NFS versions and/or version, minor versions 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 the 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.

   Note that while the client might determine that it can not cannot use any of
   the configured ffdv_version or 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.  If however however, 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 5.3).

   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 the metadata
   server is tightly coupled with the storage devices or not.  Note that
   even if the data protocol is at least NFSv4.1, it may still be the
   case that there is no control protocol present. loose coupling is 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.  I.e., 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 case event of a storage device failure.  Multipathing allows the
   client to switch to another storage device address which 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 device 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 optimal desirable paths to the data than others, then the
   MDS SHOULD NOT include those network addresses in ffda_netaddrs.  If
   less optimal desirable network addresses exist to provide failover, the
   RECOMMENDED method to offer the addresses is to provide 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 MDS detects that all network paths
   represented by ffda_netaddrs are unavailable, the MDS 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 MDS 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 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 layout4 type is defined in [RFC5662] as follows:


       enum layouttype4 {
           LAYOUT4_NFSV4_1_FILES   = 1,
           LAYOUT4_OSD2_OBJECTS    = 2,
           LAYOUT4_BLOCK_VOLUME    = 3,
           LAYOUT4_FLEX_FILES      = 4
   [[RFC Editor: please modify the LAYOUT4_FLEX_FILES
     to be the layouttype assigned by IANA]]

       struct layout_content4 {
           layouttype4             loc_type;
           opaque                  loc_body<>;

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


   This document defines structure 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 obviously must be 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


   /// const FF_FLAGS_NO_LAYOUTCOMMIT   = 1; 0x00000001;
   /// const FF_FLAGS_NO_IO_THRU_MDS    = 0x00000002;

   /// 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;
   /// };


   The ff_layout4 structure specifies a layout over a set of mirrored
   copies of that portion of the data file described in the current
   layout segment.  This mirroring protects against loss of data in
   layout segments.  Note that while not explicitly shown in the above
   XDR, each layout4 element returned in the logr_layout array of
   LAYOUTGET4res (see Section 18.43.1 of [RFC5661]) descibes a layout
   segment.  Hence each ff_layout4 also descibes 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, applying respectively only to the layout segment byte

   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 is the same across all

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

                      |           |
                      |           |
                      |   File    |
                      |           |
                      |           |
               |                         |

   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.

                      |           |
                      |           |
                      |   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 to
   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 each
   ffdv_version a particular
   combination of ffdv_version, ffdv_minorversion, and ffdv_minorversion
   ffdv_tightly_coupled provided for the device.  The array allows for
   server implementations which have different filehandles for different version and
   combinations of version, minor version combinations. version, and coupling strength.  See
   Section 5.3 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 a NFSv4
   storage device, the client may use an anonymous stateid to perform I/
   O on the storage device as there is no use for the metadata server
   stateid (no control protocol).  In such a scenario, the server MUST
   set the ffds_stateid to be zero. the anonymous stateid.

   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.  I.e., 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 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 the more or less
   tight coupling of the storage devices.  FF_FLAGS_NO_LAYOUTCOMMIT,  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 IO operations to the
   metadata server.  I.e., even if a storage device is partitioned from
   the client, the client SHOULD not try to proxy the IO through the
   metadata server.

5.1.1.  Error codes from LAYOUTGET

   [RFC5661] provides little guidance as to how the client is to proceed
   with a LAYOUTEGT which returns an error of either

   NFS4ERR_LAYOUTUNAVAILABLE:  there is no layout available and the IO
      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 IO 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 IO to the storage devices.

5.1.2.  Client Interactions with FF_FLAGS_NO_IO_THRU_MDS

   If the client does not ask for a layout for a file, then the IO will
   go through the metadata server.  Thus, even if the metadata server
   sets the FF_FLAGS_NO_IO_THRU_MDS flag, it can recall the layout and
   either not set the flag on the new layout or not provide a layout.
   When a client encounters an error with a storage device, it typically
   returns the layout to the metadata server and requests a new layout.
   The client's IO would then proceed according to the status codes as
   outlined in Section 5.1.1.

5.2.  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 rationals for
   this are clustered servers which share the same filehandle or
   allowing 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 ff_device_addr4, each as a mirror.  The client can
   then determine that since the ffds_fh_vers are different, then there
   are multiple copies of the file for the current layout segment

5.3.  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 if
   it can not access a storage device with any of the supplied
   combinations of ffdv_version, ffdv_minorversion, and ffdv_minorversion combinations.
   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 can not 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 [NFSv42] 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 can not 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.

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 which does not contain the current stripe index.

   L: logical offset into 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 SHOULD 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 it MAY be acceptable for the client 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 requesting a new layout using
   LAYOUTGET and retry the I/O operation(s) using the new layout, or the
   client MAY just retry the I/O operation(s) using regular NFS READ or
   WRITE operations via the metadata server.  The client SHOULD attempt
   to retrieve a new layout and retry the I/O operation using the
   storage device first and only if the error persists, retry the I/O
   operation via the metadata server.

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, i.e., 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 construct
   of the selected XDR representation that 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 and in any event, the client has
   no means to dictate neither the storage device (which also means the
   coupling and/or protocol levels to access the layout segments) nor
   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 get to all copies of the layout segments it is
   informed of via the layout.  If a layout segments 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.  Also, if the client is writing to the
   layout segments via the metadata server, e.g., using an earlier
   version of the protocol, 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 can 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.

   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.  I.e., 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 which decides which mirror to access for
   reading the file.  The requirements for writing to a mirrored layout
   segments are presented below.

8.2.  Writing to Mirrors

   The client is responsible for updating all mirrored copies of the
   layout segments that it is given in the layout.  If  A single failed
   update is suffcient to fail the entire operation.  I.e., if all but
   one copy is updated successfully and the last one provides an error,
   then the client needs to return the layout to the metadata server
   with an error indicating 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 such that it
   can report all errors encountered during the update.

   The metadata server is then 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 make any assumptions
   that the contents of the previous layout will match those of the new
   one.  If it has updates that were not committed, it MUST resend those
   updates to all mirrors.

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 which 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 which is in
   the process of being resilvered, then the metadata server MUST deny
   that request with a NFS4ERR_LAYOUTTRYLATER.  The client can then
   perform the I/O through the metadata server.

9.  Flexible Files Layout Type Return

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


   struct layoutreturn_file4 {
           offset4         lrf_offset;
           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;
   struct LAYOUTRETURN4args {
           /* CURRENT_FH: file */
           bool                    lora_reclaim;
           layoutreturn_stateid    lora_recallstateid;
           layouttype4             lora_layout_type;
           layoutiomode4           lora_iomode;
           layoutreturn4           lora_layoutreturn;


   If the lora_layout_type layout type is LAYOUT4_FLEX_FILES, then the
   lrf_body opaque value is defined by ff_layoutreturn4 (See
   Section 9.3).  It allows the client to report I/O error information
   or layout usage statistics back to the metadata server as defined

9.1.  I/O Error Reporting

9.1.1.  ff_ioerr4


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


   Recall that [NFSv42] defines device_error4 as:


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


   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

   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

9.2.  Layout Usage Statistics

9.2.1.  ff_io_latency4


   /// 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;
   /// };


   Both operation counts and bytes transferred are kept in the
   ff_io_latency4.  READ operations are used for read latencies.  Both
   WRITE and COMMIT operations are used for write latencies.
   "Requested" counters track what the client is attempting to do and
   "completed" counters track what was done.  Note that 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 by 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 latencies for
   completed RPC calls.

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

9.2.2.  ff_layoutupdate4


   /// 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;
   /// };


   ffl_addr differentiates which network address the client 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 respectively for both read and write operations.
   ffl_duration is used to indicate the time period over which the
   statistics were collected.  ffl_local if true 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


   /// 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;
   /// };


   Recall that [NFSv42] defines io_info4 as:


   struct io_info4 {
           uint64_t        ii_count;
           uint64_t        ii_bytes;


   With pNFS, the data transfers are performed directly between the pNFS
   client and the storage devices.  Therefore, the metadata server has
   no visibility to the I/O stream and cannot use any statistical
   information about client I/O to optimize 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.  Since it is
   infeasible 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 is out of
   the scope of this document.  It is suggested for client
   implementation to provide reasonable default values and an optional
   run-time management interface to control these parameters.  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, respectively,
   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 identify both
   the storage path and the network route to it.  Finally, the
   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


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

   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.

10.  Flexible Files 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 can MAY use
   LAYOUTERROR (see Section 15.6 of [NFSv42]) to communicate that
   information.  For the Flexible Files Layout Type, this means that
   LAYOUTERROR4args is treated the same as ff_ioerr4.

11.  Flexible Files 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 can MAY use
   LAYOUTSTATS (see Section 15.7 of [NFSv42]) to communicate that
   information.  For the Flexible Files 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:


   struct layouthint4 {
       layouttype4        loh_type;
       opaque             loh_body<>;


   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


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

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


   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 Layouts

   The a Layout

   While Section 12.5.5 of [RFC5661] discusses layout type independent
   reasons for recalling a layout, the Flexible File Layout Type
   metadata server should recall outstanding layouts in the following

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

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

   o  When there are sharing conflicts.

   o  When a file is being resilvered, either due to being repaired
      after a write error or to load balance.


   The metadata server can use the CB_RECALL_ANY callback operation to
   notify the client to return some or all of its layouts.  The
   [RFC5661] defines the following types:

   const RCA4_TYPE_MASK_FF_LAYOUT_MIN     = -2;
   const RCA4_TYPE_MASK_FF_LAYOUT_MAX     = -1;
   [[RFC Editor: please insert assigned constants]]

   struct  CB_RECALL_ANY4args      {
       uint32_t        craa_layouts_to_keep;
       bitmap4         craa_type_mask;


   [[AI13: No, 5661 does not define these above values.  The ask here is
   to create these and _add_ them to 5661.  --TH]]

   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 Flexible
   File Layout Type mask flags are defined as follows:


   /// enum ff_cb_recall_any_mask {
   ///     FF_RCA4_TYPE_MASK_READ = -2,
   ///     FF_RCA4_TYPE_MASK_RW   = -1
   [[RFC Editor: please insert assigned constants]]
   /// };


   They 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, at the least the server MAY revoke client layouts and/
   or device address mappings and
   reassign these resources to other clients (see "Recalling a Layout" Section 12.5.5 in

   [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 pNFS extension partitions the NFSv4.1+ file system protocol into
   two parts, the control path and the data path (storage protocol).
   The control path contains all the new operations described by this
   extension; all existing NFSv4 security mechanisms and features apply
   to the control path.  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, including security
   countermeasures to defend against threats that NFSv4.1+ provides
   defenses for in environments where these threats are considered

   The metadata server enforces the file access-control policy at
   LAYOUTGET time.  The client should use suitable authorization
   credentials for getting the layout for the requested iomode (READ or
   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.  If access is
   allowed, the client uses the corresponding (READ or RW) 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 it
   MUST fence off the clients holding outstanding layouts for the
   respective file by implicitly invalidating the outstanding
   credentials on all data files comprising before committing to the new
   permissions and ACL.  Doing this will ensure that clients re-
   authorize their layouts according to the modified permissions and ACL
   by requesting new layouts.  Recalling the layouts in this case is
   courtesy of the server intended to prevent clients from getting an
   error on I/Os done after the client was fenced off.

15.1.  Kerberized File Access
15.1.1.  Loosely Coupled

   Under this coupling model, the principal used to authenticate the
   metadata file is different than that used to authenticate the data
   file.  I.e., the synthetic principals generated to control access to
   the data file could prove to be difficult to manage.

   While RPCSEC_GSS version 3 (RPCSEC_GSSv3) [rpcsec_gssv3] could be
   used to authorize the client to the storage device on behalf of the
   metadata server, such a requirement exceeds the loose coupling model.
   I.e., each of the metadata server, storage device, and client would
   have to implement RPCSEC_GSSv3.

   In all, while either an elaborate schema could be used to
   automatically authenticate principals or RPCSEC_GSSv3 aware clients,
   metadata server, and storage devices could be deployed, if more
   secure authentication is desired, tight coupling should be considered
   as described in the next section.

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.  Thus there are
   no security issues related to using Kerberos with a tightly coupled

16.  IANA Considerations

   As described in [RFC5661], new layout type numbers have been assigned
   by IANA.  This document defines the protocol associated with the
   existing layout type number, LAYOUT4_FLEX_FILES.

17.  References

17.1.  Normative References

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

   [NFSv42]   Haynes, T., "NFS Version 4 Minor Version 2", draft-ietf-
              nfsv4-minorversion2-28 (Work In Progress), November 2014.

   [RFC1813]  IETF, "NFS Version 3 Protocol Specification", RFC 1813,
              June 1995.

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

   [RFC4506]  Eisler, M., "XDR: External Data Representation Standard",
              STD 67, RFC 4506, May 2006.

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, May 2009.

   [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              Protocol", RFC 5661, January 2010.

   [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, January 2010.

              Haynes, T. and D. Noveck, "NFS Version 4 Protocol", draft-
              ietf-nfsv4-rfc3530bis-35 (work in progress), Dec 2014.

              Haynes, T., "Considerations for a New pNFS Layout Type",
              draft-ietf-nfsv4-layout-types-02 (Work In Progress),
              October 2014.

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,

              Adamson, W. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", November 2014.

Appendix A.  Acknowledgments

   Those who provided miscellaneous comments to early drafts of this
   document include: Matt W. Benjamin, Adam Emerson, J. Bruce Fields,
   and Lev Solomonov.

   Those who provided miscellaneous comments to the final drafts of this
   document include: Anand Ganesh, Robert Wipfel, Gobikrishnan
   Sundharraj, and Trond Myklebust.

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

   Dave Noveck provided a comprehensive review of the document during
   the working group last call.

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

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

Appendix B.  RFC Editor Notes

   [RFC Editor: please remove this section prior to publishing this
   document as an RFC]

   [RFC Editor: prior to publishing this document as an RFC, please
   replace all occurrences of RFCTBD10 with RFCxxxx where xxxx is the
   RFC number of this document]

Authors' Addresses

   Benny Halevy


   Thomas Haynes
   Primary Data, Inc.
   4300 El Camino Real Ste 100
   Los Altos, CA  94022

   Phone: +1 408 215 1519