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Parallel NFS (pNFS) SCSI Layout

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This is an older version of an Internet-Draft that was ultimately published as RFC 8154.
Author Christoph Hellwig
Last updated 2015-11-03
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NFSv4                                                         C. Hellwig
Internet-Draft                                         November 03, 2015
Intended status: Standards Track
Expires: May 6, 2016

                    Parallel NFS (pNFS) SCSI 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 SCSI Layout Type is defined in this document
   as an extension to pNFS to allow the use SCSI based block storage

Status of this Memo

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

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

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

   This Internet-Draft will expire on May 6, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( 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.  Conventions Used in This Document  . . . . . . . . . . . .  4
     1.2.  General Definitions  . . . . . . . . . . . . . . . . . . .  4
     1.3.  Code Components Licensing Notice . . . . . . . . . . . . .  4
     1.4.  XDR Description  . . . . . . . . . . . . . . . . . . . . .  4
   2.  SCSI Layout Description  . . . . . . . . . . . . . . . . . . .  6
     2.1.  Background and Architecture  . . . . . . . . . . . . . . .  6
     2.2.  layouttype4  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  GETDEVICEINFO  . . . . . . . . . . . . . . . . . . . . . .  8
       2.3.1.  Volume Identification  . . . . . . . . . . . . . . . .  8
       2.3.2.  Volume Topology  . . . . . . . . . . . . . . . . . . .  9
     2.4.  Data Structures: Extents and Extent Lists  . . . . . . . . 12
       2.4.1.  Layout Requests and Extent Lists . . . . . . . . . . . 14
       2.4.2.  Layout Commits . . . . . . . . . . . . . . . . . . . . 16
       2.4.3.  Layout Returns . . . . . . . . . . . . . . . . . . . . 16
       2.4.4.  Client Copy-on-Write Processing  . . . . . . . . . . . 17
       2.4.5.  Extents are Permissions  . . . . . . . . . . . . . . . 18
       2.4.6.  End-of-file Processing . . . . . . . . . . . . . . . . 19
       2.4.7.  Layout Hints . . . . . . . . . . . . . . . . . . . . . 20
       2.4.8.  Client Fencing . . . . . . . . . . . . . . . . . . . . 20
     2.5.  Crash Recovery Issues  . . . . . . . . . . . . . . . . . . 22
     2.6.  Recalling Resources: CB_RECALL_ANY . . . . . . . . . . . . 22
     2.7.  Transient and Permanent Errors . . . . . . . . . . . . . . 23
     2.8.  Volatile write caches  . . . . . . . . . . . . . . . . . . 23
   3.  Enforcing NFSv4 Semantics  . . . . . . . . . . . . . . . . . . 24
     3.1.  Use of Open Stateids . . . . . . . . . . . . . . . . . . . 24
     3.2.  Enforcing Security Restrictions  . . . . . . . . . . . . . 25
     3.3.  Enforcing Locking Restrictions . . . . . . . . . . . . . . 25
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   6.  Normative References . . . . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 28
   Appendix B.  RFC Editor Notes  . . . . . . . . . . . . . . . . . . 28
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29

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1.  Introduction

   Figure 1 shows the overall architecture of a Parallel NFS (pNFS)

       |+-----------+                                 +-----------+
       ||+-----------+                                |           |
       |||           |       NFSv4.1 + pNFS           |           |
       +||  Clients  |<------------------------------>|   Server  |
        +|           |                                |           |
         +-----------+                                |           |
              |||                                     +-----------+
              |||                                           |
              |||                                           |
              ||| Storage        +-----------+              |
              ||| Protocol       |+-----------+             |
              ||+----------------||+-----------+  Control   |
              |+-----------------|||           |    Protocol|
              +------------------+||  Storage  |------------+
                                  +|  Systems  |

                                 Figure 1

   The overall approach is that pNFS-enhanced clients obtain sufficient
   information from the server to enable them to access the underlying
   storage (on the storage systems) directly.  See the Section 12 of
   [RFC5661] for more details.  This document is concerned with access
   from pNFS clients to storage devices over block storage protocols
   based on the the SCSI Architecture Model ([SAM-4]), e.g., Fibre
   Channel Protocol (FCP) for Fibre Channel, Internet SCSI (iSCSI) or
   Serial Attached SCSI (SAS). pNFS SCSI layout requires block based
   SCSI command sets, for example SCSI Block Commands ([SBC3]).  While
   SCSI command set for non-block based access exist these are not
   supported by the SCSI layout type, and all future references to SCSI
   storage devices will imply a block based SCSI command set.

   The Server to Storage System protocol, called the "Control Protocol",
   is not of concern for interoperability, although it will typically be
   the same SCSI based storage protocol.

   This document is based on and updates [RFC5663] to provide a better
   pNFS layout protocol for SCSI based storage devices, and functionally
   obsoletes [RFC6688] by providing mandatory disk access protection as
   part of the protocol.  Unlike [RFC5663] this document can make use of
   SCSI protocol features and thus can provide reliable fencing by using

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   SCSI Persistent Reservations, and it can provide reliable and
   efficient device discovery by using SCSI device identifiers instead
   of having to rely on probing all devices potentially attached to a
   client for a signature.  The document also optimizes the I/O path by
   reducing the size of the LAYOUTCOMMIT payload.

1.1.  Conventions Used in This Document

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

1.2.  General Definitions

   The following definitions are provided for the purpose of providing
   an appropriate context for the reader.

   Byte  This document defines a byte as an octet, i.e., a datum exactly
      8 bits in length.

   Client  The "client" is the entity that accesses the NFS server's
      resources.  The client may be an application that contains the
      logic to access the NFS server directly.  The client may also be
      the traditional operating system client that provides remote file
      system services for a set of applications.

   Server  The "server" is the entity responsible for coordinating
      client access to a set of file systems and is identified by a
      server owner.

1.3.  Code Components Licensing Notice

   The external data representation (XDR) description and scripts 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 Section
   4 of "Legal Provisions Relating to IETF Documents".

1.4.  XDR Description

   This document contains the XDR [RFC4506] description of the NFSv4.1
   SCSI layout protocol.  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 NFSv4.1 SCSI layout:


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   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 > scsi_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.

    /// /*
    ///  * This code was derived from RFCTBD10
    ///  * Please reproduce this note if possible.
    ///  */
    /// /*
    ///  * Copyright (c) 2010,2015 IETF Trust and the persons identified
    ///  * as the document authors.  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.
    ///  *
    ///  * - 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.
    ///  *

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    ///  */
    /// /*
    ///  *      nfs4_scsi_layout_prot.x
    ///  */
    /// %#include "nfsv41.h"

2.  SCSI Layout Description

2.1.  Background and Architecture

   The fundamental storage abstraction supported by SCSI storage devices
   is a Logical Unit (LU) consisting of a sequential series of fixed-
   size blocks.  This can be thought of as a logical disk; it may be
   realized by the storage system as a physical disk, a portion of a
   physical disk, or something more complex (e.g., concatenation,
   striping, RAID, and combinations thereof) involving multiple physical
   disks or portions thereof.  Logical units used as devices for NFS
   scsi layouts, and the SCSI initiators used for the pNFS Metadata
   Served and clients MUST support SCSI persistent reservations.

   A pNFS layout for this SCSI class of storage is responsible for
   mapping from an NFS file (or portion of a file) to the blocks of
   storage volumes that contain the file.  The blocks are expressed as
   extents with 64-bit offsets and lengths using the existing NFSv4
   offset4 and length4 types.  Clients MUST be able to perform I/O to
   the block extents without affecting additional areas of storage
   (especially important for writes); therefore, extents MUST be aligned
   to 512-byte boundaries, and writable extents MUST be aligned to the
   block size used by the NFSv4 server in managing the actual file
   system (4 kilobytes and 8 kilobytes are common block sizes).  This
   block size is available as the NFSv4.1 layout_blksize attribute.
   [RFC5661].  Readable extents SHOULD be aligned to the block size used

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   by the NFSv4 server, but in order to support legacy file systems with
   fragments, alignment to 512-byte boundaries is acceptable.

   The pNFS operation for requesting a layout (LAYOUTGET) includes the
   "layoutiomode4 loga_iomode" argument, which indicates whether the
   requested layout is for read-only use or read-write use.  A read-only
   layout may contain holes that are read as zero, whereas a read-write
   layout will contain allocated, but un-initialized storage in those
   holes (read as zero, can be written by client).  This document also
   supports client participation in copy-on-write (e.g., for file
   systems with snapshots) by providing both read-only and un-
   initialized storage for the same range in a layout.  Reads are
   initially performed on the read-only storage, with writes going to
   the un-initialized storage.  After the first write that initializes
   the un-initialized storage, all reads are performed to that now-
   initialized writable storage, and the corresponding read-only storage
   is no longer used.

   The SCSI layout solution expands the security responsibilities of the
   pNFS clients, and there are a number of environments where the
   mandatory to implement security properties for NFS cannot be
   satisfied.  The additional security responsibilities of the client
   follow, and a full discussion is present im Section 4, "Security

   o  Typically, SCSI storage devices provide access control mechanisms
      (e.g., Logical Unit Number (LUN) mapping and/or masking), which
      operate at the granularity of individual hosts, not individual
      blocks.  For this reason, block-based protection must be provided
      by the client software.

   o  Similarly, SCSI storage devices typically are not able to validate
      NFS locks that apply to file regions.  For instance, if a file is
      covered by a mandatory read-only lock, the server can ensure that
      only readable layouts for the file are granted to pNFS clients.
      However, it is up to each pNFS client to ensure that the readable
      layout is used only to service read requests, and not to allow
      writes to the existing parts of the file.

   Since SCSI storage devices are generally not capable of enforcing
   such file-based security, in environments where pNFS clients cannot
   be trusted to enforce such policies, pNFS SCSI layouts SHOULD NOT be

2.2.  layouttype4

   The layout4 type defined in [RFC5662] is extended with a new value as

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       enum layouttype4 {
           LAYOUT4_NFSV4_1_FILES   = 1,
           LAYOUT4_OSD2_OBJECTS    = 2,
           LAYOUT4_BLOCK_VOLUME    = 3,
           LAYOUT4_SCSI            = 0x80000005
   [[RFC Editor: please modify the LAYOUT4_SCSI
     to be the layouttype assigned by IANA]]

   This document defines structure associated with the layouttype4 value
   LAYOUT4_SCSI.  [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 interpreted by the Layout
   Type implementation.


2.3.1.  Volume Identification

   SCSI targets implementing [SPC3] export unique LU names for each LU
   through the Device Identification VPD page (page code 0x83), which
   can be obtained using the INQUIRY command with the EVPD bit set to
   one.  This document uses a subset of this information to identify LUs
   backing pNFS SCSI layouts.  It is similar to the "Identification
   Descriptor Target Descriptor" specified in [SPC3], but limits the
   allowed values to those that uniquely identify a LU.  Device
   Identification VPD page descriptors used to identify LUs for use with
   pNFS SCSI layouts must adhere to the following restrictions:

   1.  The "ASSOCIATION" MUST be set to 0 (The DESIGNATOR field is
       associated with the addressed logical unit).

   2.  The "DESIGNATOR TYPE" MUST be set to one of four values that are
       required for the mandatory logical unit name in [SPC3], as
       explicitly listed in the "pnfs_scsi_designator_type" enumeration:

       PS_DESIGNATOR_T10  T10 vendor ID based

       PS_DESIGNATOR_EUI64  EUI-64-based


       PS_DESIGNATOR_NAME  SCSI name string

       Any other associate or designator type MUST NOT be used.

   The "CODE SET" VPD page field is stored in the "sbv_code_set" field
   of the "pnfs_scsi_base_volume_info4" structure, the "DESIGNATOR TYPE"

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   is stored in "sbv_designator_type", and the DESIGNATOR is stored in
   "sbv_designator".  Due to the use of a XDR array the "DESIGNATOR
   LENGTH" field does not need to be set separately.  Only certain
   combinations of "sbv_code_set" and "sbv_designator_type" are valid,
   please refer to [SPC3] for details, and note that ASCII may be used
   as the code set for UTF-8 text that contains only printable ASCII
   characters.  Note that a Device Identification VPD page MAY contain
   multiple descriptors with the same association, code set and
   designator type.  NFS clients thus MUST check all the descriptors for
   a possible match to "sbv_code_set", "sbv_designator_type" and

   Storage devices such as storage arrays can have multiple physical
   network ports that need not be connected to a common network,
   resulting in a pNFS client having simultaneous multipath access to
   the same storage volumes via different ports on different networks.
   Selection of one or multiple ports to access the storage device is
   left up to the client.

   Additionally the server returns a Persistent Reservation key in the
   "sbv_pr_key" field.  See Section 2.4.8 for more details on the use of
   Persistent Reservations.

2.3.2.  Volume Topology

   The pNFS SCSI layout volume topology is expressed as an arbitrary
   combination of base volume types enumerated in the following data
   structures.  The individual components of the topology are contained
   in an array and components may refer to other components by using
   array indices.

      /// enum pnfs_scsi_volume_type4 {
      ///     PNFS_SCSI_VOLUME_SLICE  = 1,  /* volume is a slice of
      ///                                      another volume */
      ///     PNFS_SCSI_VOLUME_CONCAT = 2,  /* volume is a
      ///                                      concatenation of
      ///                                      multiple volumes */
      ///     PNFS_SCSI_VOLUME_STRIPE = 3   /* volume is striped across
      ///                                      multiple volumes */
      ///     PNFS_SCSI_VOLUME_BASE   = 4,  /* volume maps to a single
      ///                                      LU */
      /// };

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    /// /*
    ///  * Code sets from SPC-3.
    ///  */
    /// enum pnfs_scsi_code_set {
    ///     PS_CODE_SET_BINARY     = 1,
    ///     PS_CODE_SET_ASCII      = 2,
    ///     PS_CODE_SET_UTF8       = 3
    /// };
    /// /*
    ///  * Designator types from taken from SPC-3.
    ///  *
    ///  * Other values are allocated in SPC-3, but not mandatory to
    ///  * implement or aren't Logical Unit names.
    ///  */
    /// enum pnfs_scsi_designator_type {
    ///     PS_DESIGNATOR_T10      = 1,
    ///     PS_DESIGNATOR_EUI64    = 2,
    ///     PS_DESIGNATOR_NAA      = 3,
    ///     PS_DESIGNATOR_NAME     = 8
    /// };
    /// /*
    ///  * Logical Unit name + reservation key.
    ///  */
    /// struct pnfs_scsi_base_volume_info4 {
    ///     pnfs_scsi_code_set             sbv_code_set;
    ///     pnfs_scsi_designator_type      sbv_designator_type;
    ///     opaque                         sbv_designator<>;
    ///     uint64_t                       sbv_pr_key;
    /// };

   /// struct pnfs_scsi_slice_volume_info4 {
   ///     offset4  ssv_start;            /* offset of the start of the
   ///                                       slice in bytes */
   ///     length4  ssv_length;           /* length of slice in bytes */
   ///     uint32_t ssv_volume;           /* array index of sliced
   ///                                       volume */
   /// };

     /// struct pnfs_scsi_concat_volume_info4 {
     ///     uint32_t  scv_volumes<>;       /* array indices of volumes
     ///                                       which are concatenated */

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     /// };

   /// struct pnfs_scsi_stripe_volume_info4 {
   ///     length4  ssv_stripe_unit;      /* size of stripe in bytes */
   ///     uint32_t ssv_volumes<>;        /* array indices of volumes
   ///                                       which are striped across --
   ///                                       MUST be same size */
   /// };

      /// union pnfs_scsi_volume4 switch (pnfs_scsi_volume_type4 type) {
      ///     case PNFS_SCSI_VOLUME_BASE:
      ///         pnfs_scsi_base_volume_info4 sv_simple_info;
      ///     case PNFS_SCSI_VOLUME_SLICE:
      ///         pnfs_scsi_slice_volume_info4 sv_slice_info;
      ///     case PNFS_SCSI_VOLUME_CONCAT:
      ///         pnfs_scsi_concat_volume_info4 sv_concat_info;
      ///     case PNFS_SCSI_VOLUME_STRIPE:
      ///         pnfs_scsi_stripe_volume_info4 sv_stripe_info;
      /// };

      /// /* SCSI layout specific type for da_addr_body */
      /// struct pnfs_scsi_deviceaddr4 {
      ///     pnfs_scsi_volume4 sda_volumes<>; /* array of volumes */
      /// };

   The "pnfs_scsi_deviceaddr4" data structure is a structure that allows
   arbitrarily complex nested volume structures to be encoded.  The
   types of aggregations that are allowed are stripes, concatenations,
   and slices.  Note that the volume topology expressed in the
   pnfs_scsi_deviceaddr4 data structure will always resolve to a set of
   pnfs_scsi_volume_type4 PNFS_SCSI_VOLUME_BASE.  The array of volumes
   is ordered such that the root of the volume hierarchy is the last
   element of the array.  Concat, slice, and stripe volumes MUST refer
   to volumes defined by lower indexed elements of the array.

   The "pnfs_scsi_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

   As noted above, all device_addr4 structures eventually resolve to a

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   set of volumes of type PNFS_SCSI_VOLUME_BASE.  Complicated volume
   hierarchies may be composed of dozens of volumes each with several
   signature components; thus, the device address may require several
   kilobytes.  The client SHOULD be prepared to allocate a large buffer
   to contain the result.  In the case of the server returning
   NFS4ERR_TOOSMALL, the client SHOULD allocate a buffer of at least
   gdir_mincount_bytes to contain the expected result and retry the

2.4.  Data Structures: Extents and Extent Lists

   A pNFS SCSI layout is a list of extents within a flat array of data
   blocks in a logical volume.  The details of the volume topology can
   be determined by using the GETDEVICEINFO operation.  The SCSI layout
   describes the individual block extents on the volume that make up the
   file.  The offsets and length contained in an extent are specified in
   units of bytes.

   /// enum pnfs_scsi_extent_state4 {
   ///     PNFS_SCSI_READ_WRITE_DATA = 0, /* the data located by this
   ///                                       extent is valid
   ///                                       for reading and writing. */
   ///     PNFS_SCSI_READ_DATA      = 1,  /* the data located by this
   ///                                       extent is valid for reading
   ///                                       only; it may not be
   ///                                       written. */
   ///     PNFS_SCSI_INVALID_DATA   = 2,  /* the location is valid; the
   ///                                       data is invalid.  It is a
   ///                                       newly (pre-) allocated
   ///                                       extent.  There is physical
   ///                                       space on the volume. */
   ///     PNFS_SCSI_NONE_DATA      = 3   /* the location is invalid.
   ///                                       It is a hole in the file.
   ///                                       There is no physical space
   ///                                       on the volume. */
   /// };

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      /// struct pnfs_scsi_extent4 {
      ///     deviceid4    se_vol_id;         /* id of logical volume on
      ///                                        which extent of file is
      ///                                        stored. */
      ///     offset4      se_file_offset;    /* starting byte offset
      ///                                        in the file */
      ///     length4      se_length;         /* size in bytes of the
      ///                                        extent */
      ///     offset4      se_storage_offset; /* starting byte offset
      ///                                        in the volume */
      ///     pnfs_scsi_extent_state4 se_state;
      ///                                     /* state of this extent */
      /// };

    /// /* SCSI layout specific type for loc_body */
    /// struct pnfs_scsi_layout4 {
    ///     pnfs_scsi_extent4 sl_extents<>;
    ///                                    /* extents which make up this
    ///                                       layout. */
    /// };

   The SCSI layout consists of a list of extents that map the logical
   regions of the file to physical locations on a volume.  The
   "se_storage_offset" field within each extent identifies a location on
   the logical volume specified by the "se_vol_id" field in the extent.
   The se_vol_id itself is shorthand for the whole topology of the
   logical volume on which the file is stored.  The client is
   responsible for translating this logical offset into an offset on the
   appropriate underlying SCSI LU.  In most cases, all extents in a
   layout will reside on the same volume and thus have the same
   se_vol_id.  In the case of copy-on-write file systems, the
   PNFS_SCSI_READ_DATA extents may have a different se_vol_id from the
   writable extents.

   Each extent maps a logical region of the file onto a portion of the
   specified LU.  The se_file_offset, se_length, and se_state fields for
   an extent returned from the server are valid for all extents.  In
   contrast, the interpretation of the se_storage_offset field depends
   on the value of se_state as follows (in increasing order):

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   PNFS_SCSI_READ_WRITE_DATA  means that se_storage_offset is valid, and
      points to valid/initialized data that can be read and written.

   PNFS_SCSI_READ_DATA  means that se_storage_offset is valid andpoints
      to valid/initialized data that can only be read.  Write operations
      are prohibited; the client may need to request a read-write

   PNFS_SCSI_INVALID_DATA  means that se_storage_offset is valid, but
      points to invalid un-initialized data.  This data must not be
      physically read from the disk until it has been initialized.  A
      read request for a PNFS_SCSI_INVALID_DATA extent must fill the
      user buffer with zeros, unless the extent is covered by a
      PNFS_SCSI_READ_DATA extent of a copy-on-write file system.  Write
      requests must write whole server-sized blocks to the disk; bytes
      not initialized by the user must be set to zero.  Any write to
      storage in a PNFS_SCSI_INVALID_DATA extent changes the written
      portion of the extent to PNFS_SCSI_READ_WRITE_DATA; the pNFS
      client is responsible for reporting this change via LAYOUTCOMMIT.

   PNFS_SCSI_NONE_DATA  means that se_storage_offset is not valid, and
      this extent may not be used to satisfy write requests.  Read
      requests may be satisfied by zero-filling as for
      returned by requests for readable extents; they are never returned
      if the request was for a writable extent.

   An extent list contains all relevant extents in increasing order of
   the se_file_offset of each extent; any ties are broken by increasing
   order of the extent state (se_state).

2.4.1.  Layout Requests and Extent Lists

   Each request for a layout specifies at least three parameters: file
   offset, desired size, and minimum size.  If the status of a request
   indicates success, the extent list returned must meet the following

   o  A request for a readable (but not writable) layout returns only
      PNFS_SCSI_READ_DATA or PNFS_SCSI_NONE_DATA extents (but not

   o  A request for a writable layout returns PNFS_SCSI_READ_WRITE_DATA
      extents).  It may also return PNFS_SCSI_READ_DATA extents only
      when the offset ranges in those extents are also covered by
      PNFS_SCSI_INVALID_DATA extents to permit writes.

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   o  The first extent in the list MUST contain the requested starting

   o  The total size of extents within the requested range MUST cover at
      least the minimum size.  One exception is allowed: the total size
      MAY be smaller if only readable extents were requested and EOF is

   o  Extents in the extent list MUST be logically contiguous for a
      read-only layout.  For a read-write layout, the set of writable
      extents (i.e., excluding PNFS_SCSI_READ_DATA extents) MUST be
      logically contiguous.  Every PNFS_SCSI_READ_DATA extent in a read-
      write layout MUST be covered by one or more PNFS_SCSI_INVALID_DATA
      extents.  This overlap of PNFS_SCSI_READ_DATA and
      PNFS_SCSI_INVALID_DATA extents is the only permitted extent

   o  Extents MUST be ordered in the list by starting offset, with
      extents in the case of equal se_file_offsets.

   If the minimum requested size, loga_minlength, is zero, this is an
   indication to the metadata server that the client desires any layout
   at offset loga_offset or less that the metadata server has "readily
   available".  Readily is subjective, and depends on the layout type
   and the pNFS server implementation.  For SCSI layout servers, readily
   available SHOULD be interpreted such that readable layouts are always
   available, even if some extents are in the PNFS_SCSI_NONE_DATA state.
   When processing requests for writable layouts, a layout is readily
   available if extents can be returned in the PNFS_SCSI_READ_WRITE_DATA

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2.4.2.  Layout Commits

      /// /* SCSI layout specific type for lou_body */
      /// struct pnfs_scsi_range4 {
      ///     offset4      sr_file_offset;   /* starting byte offset
      ///                                       in the file */
      ///     length4      sr_length;        /* size in bytes */
      /// };
      /// struct pnfs_scsi_layoutupdate4 {
      ///     pnfs_scsi_range4 slu_commit_list<>;
      ///                                    /* list of extents which
      ///                                     * now contain valid data.
      ///                                     */
      /// };

   The "pnfs_scsi_layoutupdate4" structure is used by the client as the
   SCSI layout specific argument in a LAYOUTCOMMIT operation.  The
   "slu_commit_list" field is a list covering regions of the file layout
   that were previously in the PNFS_SCSI_INVALID_DATA state, but have
   been written by the client and should now be considered in the
   PNFS_SCSI_READ_WRITE_DATA state.  The extents in the commit list MUST
   be disjoint and MUST be sorted by sr_file_offset.  Implementors
   should be aware that a server may be unable to commit regions at a
   granularity smaller than a file-system block (typically 4 KB or 8
   KB).  As noted above, the block-size that the server uses is
   available as an NFSv4 attribute, and any extents included in the
   "slu_commit_list" MUST be aligned to this granularity and have a size
   that is a multiple of this granularity.  If the client believes that
   its actions have moved the end-of-file into the middle of a block
   being committed, the client MUST write zeroes from the end-of-file to
   the end of that block before committing the block.  Failure to do so
   may result in junk (un-initialized data) appearing in that area if
   the file is subsequently extended by moving the end-of-file.

2.4.3.  Layout Returns

   The LAYOUTRETURN operation is done without any SCSI layout specific
   data.  When the LAYOUTRETURN operation specifies a
   LAYOUTRETURN4_FILE_return type, then the layoutreturn_file4 data
   structure specifies the region of the file layout that is no longer
   needed by the client.  The opaque "lrf_body" field of the
   "layoutreturn_file4" data structure MUST have length zero.  A
   LAYOUTRETURN operation represents an explicit release of resources by
   the client, usually done for the purpose of avoiding unnecessary
   CB_LAYOUTRECALL operations in the future.  The client may return

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   disjoint regions of the file by using multiple LAYOUTRETURN
   operations within a single COMPOUND operation.

   Note that the SCSI layout supports unilateral layout revocation.
   When a layout is unilaterally revoked by the server, usually due to
   the client's lease time expiring, or a delegation being recalled, or
   the client failing to return a layout in a timely manner, it is
   important for the sake of correctness that any in-flight I/Os that
   the client issued before the layout was revoked are rejected at the
   storage.  For the SCSI protocol, this is possible by fencing a client
   with an expired layout timer from the physical storage.  Note,
   however, that the granularity of this operation can only be at the
   host/LU level.  Thus, if one of a client's layouts is unilaterally
   revoked by the server, it will effectively render useless *all* of
   the client's layouts for files located on the storage units
   comprising the logical volume.  This may render useless the client's
   layouts for files in other file systems.

2.4.4.  Client Copy-on-Write Processing

   Copy-on-write is a mechanism used to support file and/or file system
   snapshots.  When writing to unaligned regions, or to regions smaller
   than a file system block, the writer must copy the portions of the
   original file data to a new location on disk.  This behavior can
   either be implemented on the client or the server.  The paragraphs
   below describe how a pNFS SCSI layout client implements access to a
   file that requires copy-on-write semantics.

   extent types in combination with the allowed overlap of
   allows copy-on-write processing to be done by pNFS clients.  In
   classic NFS, this operation would be done by the server.  Since pNFS
   enables clients to do direct block access, it is useful for clients
   to participate in copy-on-write operations.  All SCSI pNFS clients
   MUST support this copy-on-write processing.

   When a client wishes to write data covered by a PNFS_SCSI_READ_DATA
   extent, it MUST have requested a writable layout from the server;
   that layout will contain PNFS_SCSI_INVALID_DATA extents to cover all
   the data ranges of that layout's PNFS_SCSI_READ_DATA extents.  More
   precisely, for any se_file_offset range covered by one or more
   PNFS_SCSI_READ_DATA extents in a writable layout, the server MUST
   include one or more PNFS_SCSI_INVALID_DATA extents in the layout that
   cover the same se_file_offset range.  When performing a write to such
   an area of a layout, the client MUST effectively copy the data from
   the PNFS_SCSI_READ_DATA extent for any partial blocks of
   se_file_offset and range, merge in the changes to be written, and

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   write the result to the PNFS_SCSI_INVALID_DATA extent for the blocks
   for that se_file_offset and range.  That is, if entire blocks of data
   are to be overwritten by an operation, the corresponding
   PNFS_SCSI_READ_DATA blocks need not be fetched, but any partial-
   block writes must be merged with data fetched via PNFS_SCSI_READ_DATA
   extents before storing the result via PNFS_SCSI_INVALID_DATA extents.
   For the purposes of this discussion, "entire blocks" and "partial
   blocks" refer to the server's file-system block size.  Storing of
   data in a PNFS_SCSI_INVALID_DATA extent converts the written portion
   extent; all subsequent reads MUST be performed from this extent; the
   corresponding portion of the PNFS_SCSI_READ_DATA extent MUST NOT be
   used after storing data in a PNFS_SCSI_INVALID_DATA extent.  If a
   client writes only a portion of an extent, the extent may be split at
   block aligned boundaries.

   When a client wishes to write data to a PNFS_SCSI_INVALID_DATA extent
   that is not covered by a PNFS_SCSI_READ_DATA extent, it MUST treat
   this write identically to a write to a file not involved with copy-
   on-write semantics.  Thus, data must be written in at least block-
   sized increments, aligned to multiples of block-sized offsets, and
   unwritten portions of blocks must be zero filled.

2.4.5.  Extents are Permissions

   Layout extents returned to pNFS clients grant permission to read or
   write; PNFS_SCSI_READ_DATA and PNFS_SCSI_NONE_DATA are read-only
   as zeros, any write converts it to PNFS_SCSI_READ_WRITE_DATA).  This
   is the only means a client has of obtaining permission to perform
   direct I/O to storage devices; a pNFS client MUST NOT perform direct
   I/O operations that are not permitted by an extent held by the
   client.  Client adherence to this rule places the pNFS server in
   control of potentially conflicting storage device operations,
   enabling the server to determine what does conflict and how to avoid
   conflicts by granting and recalling extents to/from clients.

   SCSI storage devices do not provide byte granularity access and can
   only perform read and write operations atomically on a block
   granularity, and thus require read-modify-write cycles to write data
   smaller than the block size.  Overlapping concurrent read and write
   operations to the same data thus will cause the read to return a
   mixture of before-write and after-write data.  Additionally, data
   corruption can occur if the underlying storage is striped and the
   operations complete in different orders on different stripes.  When
   there are multiple clients who wish to access the same data, a pNFS
   server MUST avoid these conflicts by implementing a concurrency

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   control policy of single writer XOR multiple readers for a given data

   If a client makes a layout request that conflicts with an existing
   layout delegation, the request will be rejected with the error
   NFS4ERR_LAYOUTTRYLATER.  This client is then expected to retry the
   request after a short interval.  During this interval, the server
   SHOULD recall the conflicting portion of the layout delegation from
   the client that currently holds it.  This reject-and-retry approach
   does not prevent client starvation when there is contention for the
   layout of a particular file.  For this reason, a pNFS server SHOULD
   implement a mechanism to prevent starvation.  One possibility is that
   the server can maintain a queue of rejected layout requests.  Each
   new layout request can be checked to see if it conflicts with a
   previous rejected request, and if so, the newer request can be
   rejected.  Once the original requesting client retries its request,
   its entry in the rejected request queue can be cleared, or the entry
   in the rejected request queue can be removed when it reaches a
   certain age.

   NFSv4 supports mandatory locks and share reservations.  These are
   mechanisms that clients can use to restrict the set of I/O operations
   that are permissible to other clients.  Since all I/O operations
   ultimately arrive at the NFSv4 server for processing, the server is
   in a position to enforce these restrictions.  However, with pNFS
   layouts, I/Os will be issued from the clients that hold the layouts
   directly to the storage devices that host the data.  These devices
   have no knowledge of files, mandatory locks, or share reservations,
   and are not in a position to enforce such restrictions.  For this
   reason the NFSv4 server MUST NOT grant layouts that conflict with
   mandatory locks or share reservations.  Further, if a conflicting
   mandatory lock request or a conflicting open request arrives at the
   server, the server MUST recall the part of the layout in conflict
   with the request before granting the request.

2.4.6.  End-of-file Processing

   The end-of-file location can be changed in two ways: implicitly as
   the result of a WRITE or LAYOUTCOMMIT beyond the current end-of-file,
   or explicitly as the result of a SETATTR request.  Typically, when a
   file is truncated by an NFSv4 client via the SETATTR call, the server
   frees any disk blocks belonging to the file that are beyond the new
   end-of-file byte, and MUST write zeros to the portion of the new end-
   of-file block beyond the new end-of-file byte.  These actions render
   any pNFS layouts that refer to the blocks that are freed or written
   semantically invalid.  Therefore, the server MUST recall from clients
   the portions of any pNFS layouts that refer to blocks that will be
   freed or written by the server before processing the truncate

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   request.  These recalls may take time to complete; as explained in
   [RFC5661], if the server cannot respond to the client SETATTR request
   in a reasonable amount of time, it SHOULD reply to the client with
   the error NFS4ERR_DELAY.

   Blocks in the PNFS_SCSI_INVALID_DATA state that lie beyond the new
   end-of-file block present a special case.  The server has reserved
   these blocks for use by a pNFS client with a writable layout for the
   file, but the client has yet to commit the blocks, and they are not
   yet a part of the file mapping on disk.  The server MAY free these
   blocks while processing the SETATTR request.  If so, the server MUST
   recall any layouts from pNFS clients that refer to the blocks before
   processing the truncate.  If the server does not free the
   PNFS_SCSI_INVALID_DATA blocks while processing the SETATTR request,
   it need not recall layouts that refer only to the

   When a file is extended implicitly by a WRITE or LAYOUTCOMMIT beyond
   the current end-of-file, or extended explicitly by a SETATTR request,
   the server need not recall any portions of any pNFS layouts.

2.4.7.  Layout Hints

   The SETATTR operation supports a layout hint attribute [RFC5661].
   Clients MUST NOT set a layout hint with a layout type (the loh_type
   field) of LAYOUT4_SCSI_VOLUME.

2.4.8.  Client Fencing

   The pNFS SCSI protocol must handle situations in which a system
   failure, typically a network connectivity issue, requires the server
   to unilaterally revoke extents from one client in order to transfer
   the extents to another client.  The pNFS server implementation MUST
   ensure that when resources are transferred to another client, they
   are not used by the client originally owning them, and this must be
   ensured against any possible combination of partitions and delays
   among all of the participants to the protocol (server, storage and

   The pNFS SCSI protocol implements fencing using Persistent
   Reservations (PRs), similar to the fencing method used by existing
   shared disk file systems.  By placing a PR of type "Exclusive Access
   - All Registrants" on each SCSI LU exported to pNFS clients the MDS
   prevents access from any client that does not have an outstanding
   device device ID that gives the client a reservation key to access
   the LU, and allows the MDS to revoke access to the logic unit at any

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   To allow fencing individual systems, each system must use a unique
   Persistent Reservation key.  [SPC3] does not specify a way to
   generate keys.  This document assigns the burden to generate unique
   keys to the MDS, which must generate a key for itself before
   exporting a volume, and a key for each client that accesses a scsi
   layout volumes.  Individuals keys for each volume that a client can
   access are permitted but not required.  PRs - MDS Registration and Reservation

   Before returning a PNFS_SCSI_VOLUME_BASE volume to the client, the
   MDS needs to prepare the volume for fencing using PRs.  This is done
   by registering the reservation generated for the MDS with the device
   using the "PERSISTENT RESERVE OUT" command with a service action of
   "REGISTER", followed by a "PERSISTENT RESERVE OUT" command, with a
   service action of "RESERVE" and the type field set to 8h (Exclusive
   Access - All Registrants).  To make sure all I_T nexuses are
   registered, the MDS SHOULD set the "All Target Ports" (ALL_TG_PT) bit
   when registering the key, or otherwise ensure the registration is
   performed for each initiator port.  PRs - Client Registration

   Before performing the first IO to a device returned from a
   GETDEVICEINFO operation the client will register the registration key
   returned in sbv_pr_key with the storage device by issuing a
   "PERSISTENT RESERVE OUT" command with a service action of REGISTER
   with the "SERVICE ACTION RESERVATION KEY" set to the reservation key
   returned in sbv_pr_key.  To make sure all I_T nexus are registered,
   the client SHOULD set the "All Target Ports" (ALL_TG_PT) bit when
   registering the key, or otherwise ensure the registration is
   performed for each initiator port.

   When a client stops using a device earlier returned by GETDEVICEINFO
   it MUST unregister the earlier registered key by issuing a
   "PERSISTENT RESERVE OUT" command with a service action of "REGISTER"
   with the "RESERVATION KEY" set to the earlier registered reservation
   key.  PRs - Fencing Action

   In case of a non-responding client the MDS fences the client by
   issuing a "PERSISTENT RESERVE OUT" command with the service action
   set to "PREEMPT" or "PREEMPT AND ABORT", the reservation key field
   set to the server's reservation key, the service action reservation
   key field set to the reservation key associated with the non-

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   responding client, and the type field set to 8h (Exclusive Access -
   All Registrants).

   After the MDS preempts a client, all client I/O to the LU fails.  The
   client should at this point return any layout that refers to the
   device ID that points to the LU.  Note that the client can
   distinguish I/O errors due to fencing from other errors based on the
   "RESERVATION CONFLICT" SCSI status.  Refer to [SPC3] for details.  Client Recovery After a Fence Action

   A client that detects a "RESERVATION CONFLICT" SCSI status on the
   storage devices MUST commit all layouts that use the storage device
   through the MDS, return all outstanding layouts for the device,
   forget the device ID and unregister the reservation key.  Future
   GETDEVICEINFO calls may refer to the storage device again, in which
   case the client will perform a new registration based on the key
   provided (via sbv_pr_key) at that time.

2.5.  Crash Recovery Issues

   A critical requirement in crash recovery is that both the client and
   the server know when the other has failed.  Additionally, it is
   required that a client sees a consistent view of data across server
   restarts.  These requirements and a full discussion of crash recovery
   issues are covered in the "Crash Recovery" section of the NFSv41
   specification [RFC5661].  This document contains additional crash
   recovery material specific only to the SCSI layout.

   When the server crashes while the client holds a writable layout, and
   the client has written data to blocks covered by the layout, and the
   blocks are still in the PNFS_SCSI_INVALID_DATA state, the client has
   two options for recovery.  If the data that has been written to these
   blocks is still cached by the client, the client can simply re-write
   the data via NFSv4, once the server has come back online.  However,
   if the data is no longer in the client's cache, the client MUST NOT
   attempt to source the data from the data servers.  Instead, it should
   attempt to commit the blocks in question to the server during the
   server's recovery grace period, by sending a LAYOUTCOMMIT with the
   "loca_reclaim" flag set to true.  This process is described in detail
   in Section 18.42.4 of [RFC5661].

2.6.  Recalling Resources: CB_RECALL_ANY

   The server may decide that it cannot hold all of the state for
   layouts without running out of resources.  In such a case, it is free
   to recall individual layouts using CB_LAYOUTRECALL to reduce the
   load, or it may choose to request that the client return any layout.

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   The NFSv4.1 spec [RFC5661] defines the following types:

      const RCA4_TYPE_MASK_BLK_LAYOUT = 4;

      struct CB_RECALL_ANY4args {
             uint32_t      craa_objects_to_keep;
             bitmap4       craa_type_mask;

   When the server sends a CB_RECALL_ANY request to a client specifying
   the RCA4_TYPE_MASK_BLK_LAYOUT bit in craa_type_mask, the client
   should immediately respond with NFS4_OK, and then asynchronously
   return complete file layouts until the number of files with layouts
   cached on the client is less than craa_object_to_keep.

2.7.  Transient and Permanent Errors

   The server may respond to LAYOUTGET with a variety of error statuses.
   These errors can convey transient conditions or more permanent
   conditions that are unlikely to be resolved soon.

   are used to indicate that the server cannot immediately grant the
   layout to the client.  In the former case, this is because the server
   has recently issued a CB_LAYOUTRECALL to the requesting client,
   whereas in the case of NFS4ERR_TRYLATER, the server cannot grant the
   request possibly due to sharing conflicts with other clients.  In
   either case, a reasonable approach for the client is to wait several
   milliseconds and retry the request.  The client SHOULD track the
   number of retries, and if forward progress is not made, the client
   SHOULD send the READ or WRITE operation directly to the server.

   The error NFS4ERR_LAYOUTUNAVAILABLE may be returned by the server if
   layouts are not supported for the requested file or its containing
   file system.  The server may also return this error code if the
   server is the progress of migrating the file from secondary storage,
   or for any other reason that causes the server to be unable to supply
   the layout.  As a result of receiving NFS4ERR_LAYOUTUNAVAILABLE, the
   client SHOULD send future READ and WRITE requests directly to the
   server.  It is expected that a client will not cache the file's
   layoutunavailable state forever, particular if the file is closed,
   and thus eventually, the client MAY reissue a LAYOUTGET operation.

2.8.  Volatile write caches

   Many storage devices implement volatile write caches that require an
   explicit flush to persist the data from write operations to stable
   storage.  Storage devices implemeting [SBC3] should indicated a

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   volatile write cache by setting the WCE bit to 1 in the Caching mode
   page.  When a volatile write cache is used, the pNFS server must
   ensure the volatile write cache has been committed to stable storage
   before the LAYOUTCOMMIT operation returns by using one of the

3.  Enforcing NFSv4 Semantics

   The functionality provided by SCSI Persistent Reservations makes it
   possible for the MDS to control access by individual client machines
   to specific LUs.  Individual client machines may be allowed to or
   prevented from reading or writing to certain block devices.  Finer-
   grained access control methods are not generally available.

   For this reason, certain responsibilities for enforcing NFSv4
   semantics, including security and locking, are delegated to pNFS
   clients when SCSI layouts are being used.  The metadata server's role
   is to only grant layouts appropriately and the pNFS clients have to
   be trusted to only perform accesses allowed by the layout extents
   they currently hold (e.g., and not access storage for files on which
   a layout extent is not held).  In general, the server will not be
   able to prevent a client that holds a layout for a file from
   accessing parts of the physical disk not covered by the layout.
   Similarly, the server will not be able to prevent a client from
   accessing blocks covered by a layout that it has already returned.
   The pNFS client must respect the layout model for this mapping type
   to appropriately respect NFSv4 semantics.

   Furthermore, there is no way for the storage to determine the
   specific NFSv4 entity (principal, openowner, lockowner) on whose
   behalf the IO operation is being done.  This fact may limit the
   functionality to be supported and require the pNFS client to
   implement server policies other than those describable by layouts.
   In cases in which layouts previously granted become invalid, the
   server has the option of recalling them.  In situations in which
   communication difficulties prevent this from happening, layouts may
   be revoked by the server.  This revocation is accompanied by changes
   in persistent reservation which have the effect of preventing SCSI
   access to the LUs in question by the client.

3.1.  Use of Open Stateids

   The effective implementation of these NFSv4 semantic constraints is
   complicated by the different granularities of the actors for the
   different types of the functionality to be enforced:

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   o  To enforce security constraints for particular principals.

   o  To enforce locking constraints for particular owners (openowners
      and lockowners)

   Fundamental to enforcing both of these sorts of constraints is the
   principle that a pNFS client must not issue a SCSI IO operation
   unless it possesses both:

   o  A valid open stateid for the file in question, performing the IO
      that allows IO of the type in question, which is associated with
      the openowner and principal on whose behalf the IO is to be done.

   o  A valid layout stateid for the file in question that covers the
      byte range on which the IO is to be done and that allows IO of
      that type to be done.

   As a result, if the equivalent of IO with an anonymous or write-
   bypass stateid is to be done, it MUST NOT by done using the pNFS SCSI
   layout type.  The client MAY attempt such IO using READs and WRITEs
   that do not use pNFS and are directed to the MDS.

   When open stateids are revoked, due to lease expiration or any form
   of administrative revocation, the server MUST recall all layouts that
   allow IO to be done on any of the files for which open revocation
   happens.  When there is a failure to successfully return those
   layouts, the client MUST be fenced.

3.2.  Enforcing Security Restrictions

   The restriction noted above provides adequate enforcement of
   appropriate security restriction when the principal issuing the IO is
   the same as that opening the file.  The server is responsible for
   checking that the IO mode requested by the open is allowed for the
   principal doing the OPEN.  If the correct sort of IO is done on
   behalf of the same principal, then the security restriction is
   thereby enforced.

   If IO is done by a principal different from the one that opened the
   file, the client SHOULD send the IO to be performed by the metadata
   server rather than doing it directly to the storage device.

3.3.  Enforcing Locking Restrictions

   Mandatory enforcement of whole-file locking by means of share
   reservations is provided when the pNFS client obeys the requirement
   set forth in Section 2.1 above.  Since performing IO requires a valid
   open stateid an IO that violates an existing share reservation would

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   only be possible when the server allows conflicting open stateids to

   The nature of the SCSI layout type is such implementation/enforcement
   of mandatory byte-range locks is very difficult.  Given that layouts
   are granted to clients rather than owners, the pNFS client is in no
   position to successfully arbitrate among multiple lockowners on the
   same client.  Suppose lockowner A is doing a write and, while the IO
   is pending, lockowner B requests a mandatory byte-range for a byte
   range potentially overlapping the pending IO.  In such a situation,
   the lock request cannot be granted while the IO is pending.  In a
   non-pNFS environment, the server would have to wait for pending IO
   before granting the mandatory byte-range lock.  In the pNFS
   environment the server does not issue the IO and is thus in no
   position to wait for its completion.  The server may recall such
   layouts but in doing so, it has no way of distinguishing those being
   used by lockowners A and B, making it difficult to allow B to perform
   IO while forbidding A from doing so.  Given this fact, the MDS need
   to successfully recall all layouts that overlap the range being
   locked before returning a successful response to the LOCK request.
   While the lock is in effect, the server SHOULD respond to requests
   for layouts which overlap a currently locked area with
   NFS4ERR_LAYOUTUNAVAILABLE.  To simplify the required logic a server
   MAY do this for all layout requests on the file in question as long
   as there are any byte-range locks in effect.

   Given these difficulties it may be difficult for servers supporting
   mandatory byte-range locks to also support SCSI layouts.  Servers can
   support advisory byte-range locks instead.  The NFSv4 protocol
   currently has no way of determining whether byte-range lock support
   on a particular file system will be mandatory or advisory, except by
   trying operation which would conflict if mandatory locking is in
   effect.  Therefore, to avoid confusion, servers SHOULD NOT switch
   between mandatory and advisory byte-range locking based on whether
   any SCSI layouts have been obtained or whether a client that has
   obtained a SCSI layout has requested a byte-range lock.

4.  Security Considerations

   Access to SCSI storage devices is logically at a lower layer of the
   I/O stack than NFSv4, and hence NFSv4 security is not directly
   applicable to protocols that access such storage directly.  Depending
   on the protocol, some of the security mechanisms provided by NFSv4
   (e.g., encryption, cryptographic integrity) may not be available or
   may be provided via different means.  At one extreme, pNFS with SCSI
   layouts can be used with storage access protocols (e.g., serial
   attached SCSI ([SAS3]) that provide essentially no security

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   functionality.  At the other extreme, pNFS may be used with storage
   protocols such as iSCSI ([RFC7143]) that can provide significant
   security functionality.  It is the responsibility of those
   administering and deploying pNFS with a SCSI storage access protocol
   to ensure that appropriate protection is provided to that protocol
   (physical security is a common means for protocols not based on IP).
   In environments where the security requirements for the storage
   protocol cannot be met, pNFS SCSI layouts SHOULD NOT be used.

   When security is available for a storage protocol, it is generally at
   a different granularity and with a different notion of identity than
   NFSv4 (e.g., NFSv4 controls user access to files, iSCSI controls
   initiator access to volumes).  The responsibility for enforcing
   appropriate correspondences between these security layers is placed
   upon the pNFS client.  As with the issues in the first paragraph of
   this section, in environments where the security requirements are
   such that client-side protection from access to storage outside of
   the layout is not sufficient, pNFS SCSI layouts SHOULD NOT be used.

5.  IANA Considerations

   IANA is requested to assign a new pNFS layout type in the pNFS Layout
   Types Registry as follows (the value 5 is suggested): Layout Type
   Name: LAYOUT4_SCSI Value: 0x00000005 RFC: RFCTBD10 How: L (new layout
   type) Minor Versions: 1

6.  Normative References

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

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

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

   [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.

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   [RFC5663]  Black, D., Ed., Fridella, S., Ed., and J. Glasgow, Ed.,
              "Parallel NFS (pNFS) Block/Volume Layout", RFC 5663,
              January 2010.

   [RFC6688]  Black, D., Ed., Glasgow, J., and S. Faibish, "Parallel NFS
              (pNFS) Block Disk Protection", RFC 6688, July 2012.

   [RFC7143]  Chadalapaka, M., Meth, K., and D. Black, "Internet Small
              Computer System Interface (iSCSI) Protocol
              (Consolidated)", RFC RFC7143, April 2014.

   [SAM-4]    INCITS Technical Committee T10, "SCSI Architecture Model -
              4 (SAM-4)", ANSI INCITS 447-2008, ISO/IEC 14776-414, 2008.

   [SAS3]     INCITS Technical Committee T10, "Serial Attached Scsi-3",
              ANSI INCITS ANSI INCITS 519-2014, ISO/IEC 14776-154, 2014.

   [SBC3]     INCITS Technical Committee T10, "SCSI Block Commands-3",
              ANSI INCITS INCITS 514-2014, ISO/IEC 14776-323, 2014.

   [SPC3]     INCITS Technical Committee T10, "SCSI Primary Commands-3",
              ANSI INCITS 408-2005, ISO/IEC 14776-453, 2005.

Appendix A.  Acknowledgments

   Large parts of this document were copied verbatim, and others were
   inspired by [RFC5663].  Thank to David Black, Stephen Fridella and
   Jason Glasgow for their work on the pNFS block/volume layout

   David Black, Robert Elliott and Tom Haynes provided a throughout
   review of early drafts of this document, and their input lead to the
   current form of the document.

   David Noveck provided ample feedback to earlier drafts of this
   document and wrote the section on enforcing NFSv4 semantics.

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]

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Author's Address

   Christoph Hellwig


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