path: root/doc/rfc/rfc8154.txt

Internet Engineering Task Force (IETF)                        C. Hellwig
Request for Comments: 8154                                      May 2017
Category: Standards Track
ISSN: 2070-1721


   Parallel NFS (pNFS) Small Computer System Interface (SCSI) Layout

Abstract

   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 Small Computer System Interface (SCSI)
   layout type is defined in this document as an extension to pNFS to
   allow the use of SCSI-based block storage devices.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8154.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
     1.2.  General Definitions . . . . . . . . . . . . . . . . . . .   4
     1.3.  Code Components Licensing Notice  . . . . . . . . . . . .   5
     1.4.  XDR Description . . . . . . . . . . . . . . . . . . . . .   5
   2.  SCSI Layout Description . . . . . . . . . . . . . . . . . . .   7
     2.1.  Background and Architecture . . . . . . . . . . . . . . .   7
     2.2.  layouttype4 . . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  GETDEVICEINFO . . . . . . . . . . . . . . . . . . . . . .   8
       2.3.1.  Volume Identification . . . . . . . . . . . . . . . .   8
       2.3.2.  Volume Topology . . . . . . . . . . . . . . . . . . .  10
     2.4.  Data Structures: Extents and Extent Lists . . . . . . . .  12
       2.4.1.  Layout Requests and Extent Lists  . . . . . . . . . .  15
       2.4.2.  Layout Commits  . . . . . . . . . . . . . . . . . . .  16
       2.4.3.  Layout Returns  . . . . . . . . . . . . . . . . . . .  17
       2.4.4.  Layout Revocation . . . . . . . . . . . . . . . . . .  17
       2.4.5.  Client Copy-on-Write Processing . . . . . . . . . . .  17
       2.4.6.  Extents Are Permissions . . . . . . . . . . . . . . .  18
       2.4.7.  Partial-Block Updates . . . . . . . . . . . . . . . .  19
       2.4.8.  End-of-File Processing  . . . . . . . . . . . . . . .  20
       2.4.9.  Layout Hints  . . . . . . . . . . . . . . . . . . . .  20
       2.4.10. Client Fencing  . . . . . . . . . . . . . . . . . . .  21
     2.5.  Crash Recovery Issues . . . . . . . . . . . . . . . . . .  22
     2.6.  Recalling Resources: CB_RECALL_ANY  . . . . . . . . . . .  23
     2.7.  Transient and Permanent Errors  . . . . . . . . . . . . .  23
     2.8.  Volatile Write Caches . . . . . . . . . . . . . . . . . .  24
   3.  Enforcing NFSv4 Semantics . . . . . . . . . . . . . . . . . .  24
     3.1.  Use of Open Stateids  . . . . . . . . . . . . . . . . . .  25
     3.2.  Enforcing Security Restrictions . . . . . . . . . . . . .  26
     3.3.  Enforcing Locking Restrictions  . . . . . . . . . . . . .  26
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .  28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  30














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1.  Introduction

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

        +-----------+
        |+-----------+                                 +-----------+
        ||+-----------+                                |           |
        |||           |       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 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 SCSI Architecture Model [SAM-5], e.g., the Fibre Channel
   Protocol (FCP), 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 sets 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 [RFC5663] and makes changes to the block
   layout type to provide a better pNFS layout protocol for SCSI-based
   storage devices.  Despite these changes, [RFC5663] remains the
   defining document for the existing block layout type. pNFS Block Disk
   Protection [RFC6688] is unnecessary in the context of the SCSI layout
   type because the new layout type provides mandatory disk access



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   protection as part of the layout type definition.  In contrast to
   [RFC5663], this document uses SCSI protocol features to provide
   reliable fencing by using 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.  This new layout type also
   optimizes the Input/Output (I/O) path by reducing the size of the
   LAYOUTCOMMIT payload.

   The above two paragraphs summarize the major functional differences
   from [RFC5663].  There are other minor differences, e.g., the "base"
   volume type in this specification is used instead of the "simple"
   volume type in [RFC5663], but there are no significant differences in
   the data structures that describe the volume topology above this
   level (Section 2.3.2) or in the data structures that describe extents
   (Section 2.4).

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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:  an octet, i.e., a datum exactly 8 bits in length.

   Client:  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 entity responsible for coordinating client access to a
      set of file systems and is identified by a server owner.

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








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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:

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

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

    sh extract.sh < spec.txt > 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 RFC 8154.
       ///  * Please reproduce this note if possible.
       ///  */
       /// /*
       ///  * Copyright (c) 2017 IETF Trust and the persons
       ///  * identified as authors of the code.  All rights reserved.
       ///  *






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       ///  * 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.
       ///  *
       ///  *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
       ///  *   AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
       ///  *   WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
       ///  *   IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
       ///  *   FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
       ///  *   EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
       ///  *   LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
       ///  *   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
       ///  *   NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
       ///  *   SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
       ///  *   INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
       ///  *   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
       ///  *   OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
       ///  *   IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
       ///  *   ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
       ///  */
       ///
       /// /*
       ///  *      nfs4_scsi_layout_prot.x
       ///  */
       ///
       /// %#include "nfsv41.h"
       ///










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2.  SCSI Layout Description

2.1.  Background and Architecture

   The fundamental storage model supported by SCSI storage devices is a
   logical unit (LU) consisting of a sequential series of fixed-size
   blocks.  Logical units used as devices for NFS SCSI layouts, and the
   SCSI initiators used for the pNFS metadata server and clients, MUST
   support SCSI persistent reservations as defined in [SPC4].

   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 logical block size boundaries of the underlying logical units
   (typically 512 or 4096 bytes).  For complex volume topologies, the
   server MUST ensure extents are aligned to the logical block size
   boundaries of the largest logical block size in the volume topology.

   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 uninitialized 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 uninitialized
   storage for the same range in a layout.  Reads are initially
   performed on the read-only storage, with writes going to the
   uninitialized storage.  After the first write that initializes the
   uninitialized 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 in Section 4 ("Security
   Considerations").








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   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 MUST NOT be
   used.

2.2.  layouttype4

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

        enum layouttype4 {
            LAYOUT4_NFSV4_1_FILES   = 1,
            LAYOUT4_OSD2_OBJECTS    = 2,
            LAYOUT4_BLOCK_VOLUME    = 3,
            LAYOUT4_SCSI            = 5
        };

   This document defines the 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.  GETDEVICEINFO

2.3.1.  Volume Identification

   SCSI targets implementing [SPC4] export unique LU names for each LU
   through the Device Identification Vital Product Data (VPD) page (page
   code 0x83), which can be obtained using the INQUIRY command with the
   Enable VPD (EVPD) bit set to one.  This document uses a subset of
   this information to identify LUs backing pNFS SCSI layouts.  The





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   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 Section 7.7.3 of
       [SPC4], as explicitly listed in the "pnfs_scsi_designator_type"
       enumeration:

          PS_DESIGNATOR_T10 - based on T10 vendor ID

          PS_DESIGNATOR_EUI64 - based on EUI-64

          PS_DESIGNATOR_NAA - Network Address Authority (NAA)

          PS_DESIGNATOR_NAME - SCSI name string

   3.  Any other association or designator type MUST NOT be used.  Use
       of T10 vendor IDs is discouraged when one of the other types can
       be used.

   The "CODE SET" VPD page field is stored in the "sbv_code_set" field
   of the "pnfs_scsi_base_volume_info4" data structure, the "DESIGNATOR
   TYPE" is stored in "sbv_designator_type", and the DESIGNATOR is
   stored in "sbv_designator".  Due to the use of an 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 [SPC4] 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.  Thus, NFS clients MUST check all the descriptors
   for a possible match to "sbv_code_set", "sbv_designator_type", and
   "sbv_designator".

   Storage devices such as storage arrays can have multiple physical
   network interfaces 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 2.4.10 for more details on the use
   of persistent reservations.




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2.3.2.  Volume Topology

   The pNFS SCSI layout volume topology is expressed in terms of the
   volume types described below.  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 */
   /// };
   ///

   /// /*
   ///  * Code sets from SPC-4.
   ///  */
   /// enum pnfs_scsi_code_set {
   ///     PS_CODE_SET_BINARY     = 1,
   ///     PS_CODE_SET_ASCII      = 2,
   ///     PS_CODE_SET_UTF8       = 3
   /// };
   ///
   /// /*
   ///  * Designator types taken from SPC-4.
   ///  *
   ///  * Other values are allocated in SPC-4 but are 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;



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   ///     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
   ///                                       that are concatenated */
   /// };

   ///
   /// struct pnfs_scsi_stripe_volume_info4 {
   ///     length4  ssv_stripe_unit;      /* size of stripe in bytes */
   ///     uint32_t ssv_volumes<>;        /* array indices of
   ///                                       volumes that 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;
   /// };
   ///









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   /// /* 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_deviceaddr4" data structure is returned by the server
   as the storage-protocol-specific opaque field "da_addr_body" in the
   "device_addr4" data structure by a successful GETDEVICEINFO operation
   [RFC5661].

   As noted above, all "device_addr4" data structures eventually resolve
   to a set of volumes of type PNFS_SCSI_VOLUME_BASE.  Complicated
   volume hierarchies may be composed of dozens of volumes, each with
   several 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
   GETDEVICEINFO request.

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 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.












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   /// 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.  The client MUST
   ///                                       not read from this
   ///                                       space. */
   ///     PNFS_SCSI_NONE_DATA      = 3   /* the location is invalid.
   ///                                       It is a hole in the file.
   ///                                       The client MUST NOT read
   ///                                       from or write to this
   ///                                       space. */
   /// };

   ///
   /// struct pnfs_scsi_extent4 {
   ///     deviceid4    se_vol_id;         /* id of the 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 that make up this
   ///                                       layout */
   /// };
   ///






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   The SCSI layout consists of a list of extents that map the regions of
   the file to locations on a volume.  The "se_storage_offset" field
   within each extent identifies a location on the volume specified by
   the "se_vol_id" field in the extent.  The "se_vol_id" itself is
   shorthand for the whole topology of the volume on which the file is
   stored.  The client is responsible for translating this volume-
   relative offset into an offset on the appropriate underlying SCSI LU.

   Each extent maps a 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):

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

   PNFS_SCSI_READ_DATA
      "se_storage_offset" is valid and points to valid/initialized data
      that can only be read.  Write operations are prohibited.

   PNFS_SCSI_INVALID_DATA
      "se_storage_offset" is valid but points to invalid, uninitialized
      data.  This data MUST not be 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
      "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 PNFS_SCSI_INVALID_DATA.  PNFS_SCSI_NONE_DATA
      extents MAY be 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).







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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
   criteria:

   o  A request for a readable (but not writable) layout MUST return
      either PNFS_SCSI_READ_DATA or PNFS_SCSI_NONE_DATA extents.  It
      SHALL NOT return PNFS_SCSI_INVALID_DATA or
      PNFS_SCSI_READ_WRITE_DATA extents.

   o  A request for a writable layout MUST return
      PNFS_SCSI_READ_WRITE_DATA or PNFS_SCSI_INVALID_DATA extents, and
      it MAY return additional PNFS_SCSI_READ_DATA extents for ranges
      covered by PNFS_SCSI_INVALID_DATA extents to allow client-side
      copy-on-write operations.  A request for a writable layout SHALL
      NOT return PNFS_SCSI_NONE_DATA extents.

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

   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
      encountered.

   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
      overlap.

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

   According to [RFC5661], 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".  Given the lack of a
   clear definition of this phrase, in the context of the SCSI layout






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   type, when loga_minlength is zero, the metadata server SHOULD do the
   following:

   o  when processing requests for readable layouts, return all such
      layouts, even if some extents are in the PNFS_SCSI_NONE_DATA
      state.

   o  when processing requests for writable layouts, return extents that
      can be returned in the PNFS_SCSI_READ_WRITE_DATA state.

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 that
     ///                                     * now contain valid data.
     ///                                     */
     /// };

   The "pnfs_scsi_layoutupdate4" data 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.  Since the block in question
   is in state PNFS_SCSI_INVALID_DATA, byte ranges not written SHOULD be
   filled with zeros.  This applies even if it appears that the area
   being written is beyond what the client believes to be the end of
   file.






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2.4.3.  Layout Returns

   A LAYOUTRETURN operation represents an explicit release of resources
   by the client.  This MAY be done in response to a CB_LAYOUTRECALL or
   before any recall, in order to avoid a future CB_LAYOUTRECALL.  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 LAYOUTRETURN operation is done without any data specific to the
   SCSI layout.  The opaque "lrf_body" field of the "layoutreturn_file4"
   data structure MUST have length zero.

2.4.4.  Layout Revocation

   Layouts MAY be unilaterally revoked by the server due to the client's
   lease time expiring or the client failing to return a layout that has
   been recalled in a timely manner.  For the SCSI layout type, this is
   accomplished by fencing off the client from access to storage as
   described in Section 2.4.10.  When this is done, it is necessary that
   all I/Os issued by the fenced-off client be rejected by the storage.
   This includes any in-flight I/Os that the client issued before the
   layout was revoked.

   Note 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 volume.  This may render useless the client's layouts for files
   in other file systems.  See Section 2.4.10.5 for a discussion of
   recovery from fencing.

2.4.5.  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 be
   implemented either 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.

   Distinguishing the PNFS_SCSI_READ_WRITE_DATA and PNFS_SCSI_READ_DATA
   extent types in combination with the allowed overlap of
   PNFS_SCSI_READ_DATA extents with PNFS_SCSI_INVALID_DATA extents
   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



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   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
   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 block size of the server's file system.  Storing
   of data in a PNFS_SCSI_INVALID_DATA extent converts the written
   portion of the PNFS_SCSI_INVALID_DATA extent to a
   PNFS_SCSI_READ_WRITE_DATA 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 and aligned to multiples of block-sized offsets, and
   unwritten portions of blocks MUST be zero filled.

2.4.6.  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
   (PNFS_SCSI_NONE_DATA reads as zeros), and PNFS_SCSI_READ_WRITE_DATA
   and PNFS_SCSI_INVALID_DATA are read-write (PNFS_SCSI_INVALID_DATA
   reads 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



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   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.

   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 they 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.

2.4.7.  Partial-Block Updates

   SCSI storage devices do not provide byte granularity access and can
   only perform read and write operations atomically on a block
   granularity.  Writes to SCSI storage devices thus require read-
   modify-write cycles to write data that is smaller than the block size
   or that is otherwise not block aligned.  Write operations from
   multiple clients to the same block can thus lead to data corruption
   even if the byte range written by the applications does not overlap.
   When there are multiple clients who wish to access the same block, a





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   pNFS server MUST avoid these conflicts by implementing a concurrency
   control policy of single writer XOR multiple readers for a given data
   block.

2.4.8.  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
   semantically invalid any pNFS layouts that refer to the blocks that
   are freed or written.  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 effecting the file truncation.
   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
   PNFS_SCSI_INVALID_DATA blocks.

   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.

2.4.9.  Layout Hints

   The layout hint attribute specified in [RFC5661] is not supported by
   the SCSI layout, and the pNFS server MUST reject setting a layout
   hint attribute with a loh_type value of LAYOUT4_SCSI_VOLUME during
   OPEN or SETATTR operations.  On a file system only supporting the
   SCSI layout, a server MUST NOT report the layout_hint attribute in
   the supported_attrs attribute.





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2.4.10.  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 a client after the client fails
   to respond to a CB_LAYOUTRECALL request.  This is implemented by
   fencing off a non-responding client from access to the storage
   device.

   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
   - Registrants Only" on each SCSI LU exported to pNFS clients, the MDS
   prevents access from any client that does not have an outstanding
   device ID that gives the client a reservation key to access the LU
   and allows the MDS to revoke access to the logical unit at any time.

2.4.10.1.  PRs -- Key Generation

   To allow fencing individual systems, each system MUST use a unique
   persistent reservation key.  [SPC4] 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 SCSI
   layout volumes.  Individuals keys for each volume that a client can
   access are permitted but not required.

2.4.10.2.  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 - Registrants Only).  To make sure all I_T nexuses (see
   Section 3.1.45 of [SAM-5]) 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 target port,
   and it MUST perform registration for each initiator port.

2.4.10.3.  PRs -- Client Registration

   Before performing the first I/O 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



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   returned in sbv_pr_key.  To make sure all I_T nexuses 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 target port, and it MUST perform registration 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.

2.4.10.4.  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-
   responding client, and the "TYPE" field set to 8h (Exclusive Access -
   Registrants Only).

   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 [SPC4] for details.

2.4.10.5.  Client Recovery after a Fence Action

   A client that detects a "RESERVATION CONFLICT" SCSI status (I/O
   error) 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 Section 8.4 ("Crash Recovery") of the NFSv4.1
   specification [RFC5661].  This document contains additional crash
   recovery material specific only to the SCSI layout.




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   When the server crashes while the client holds a writable layout, 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.

   The NFSv4.1 specification [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.

   The error NFS4ERR_RECALLCONFLICT indicates that the server has
   recently issued a CB_LAYOUTRECALL to the requesting client, making it
   necessary for the client to respond to the recall before processing
   the layout request.  A client can wait for that recall to be received
   and processed, or it can retry as NFS4ERR_TRYLATER, as described
   below.




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   The error NFS4ERR_TRYLATER is used to indicate that the server cannot
   immediately grant the layout to the client.  This may be due to
   constraints on writable sharing of blocks by multiple clients or to a
   conflict with a recallable lock (e.g., a delegation).  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 abandon the attempt to get a layout and perform READ and WRITE
   operations by sending them 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 in the process of migrating the file from secondary
   storage, there is a conflicting lock that would prevent the layout
   from being granted, or any other reason causes the server to be
   unable to supply the layout.  As a result of receiving
   NFS4ERR_LAYOUTUNAVAILABLE, the client SHOULD abandon the attempt to
   get a layout and perform READ and WRITE operations by sending them to
   the MDS.  It is expected that a client will not cache the file's
   layoutunavailable state forever.  In particular, when the file is
   closed or opened by the client, issuing a new LAYOUTGET is
   appropriate.

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 implementing [SBC3] should indicate a
   volatile write cache by setting the Write Cache Enable (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 SYNCHRONIZE CACHE commands.

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



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   they currently hold (e.g., 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 I/O 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 that 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:

   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 I/O operation
   unless it possesses both:

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

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

   As a result, if the equivalent of I/O 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 I/O using READs and WRITEs
   that do not use pNFS and are directed to the MDS.



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   When open stateids are revoked, due to lease expiration or any form
   of administrative revocation, the server MUST recall all layouts that
   allow I/O 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 I/O
   is the same as that opening the file.  The server is responsible for
   checking that the I/O mode requested by the OPEN is allowed for the
   principal doing the OPEN.  If the correct sort of I/O is done on
   behalf of the same principal, then the security restriction is
   thereby enforced.

   If I/O is done by a principal different from the one that opened the
   file, the client SHOULD send the I/O 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 3.1.  Since performing I/O requires a valid open
   stateid, an I/O that violates an existing share reservation would
   only be possible when the server allows conflicting open stateids to
   exist.

   The nature of the SCSI layout type is that 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 I/O is pending, lockowner B requests a mandatory byte-
   range lock for a byte range potentially overlapping the pending I/O.
   In such a situation, the lock request cannot be granted while the I/O
   is pending.  In a non-pNFS environment, the server would have to wait
   for pending I/O before granting the mandatory byte-range lock.  In
   the pNFS environment, the server does not issue the I/O 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 I/O 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 that overlap a currently locked area with



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   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; 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 and 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
   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 using IP-based storage protocols such as iSCSI, IPsec should be
   used as outlined in [RFC3723] and updated in [RFC7146].

   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, and 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.





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5.  IANA Considerations

   IANA has assigned a new pNFS layout type in the "pNFS Layout Types
   Registry" as follows:

    Layout Type Name: LAYOUT4_SCSI
    Value:            0x00000005
    RFC:              RFC 8154
    How:              L
    Minor Versions:   1

6.  Normative References

   [LEGAL]    IETF Trust, "Legal Provisions Relating to IETF Documents",
              March 2015, <http://trustee.ietf.org/docs/
              IETF-Trust-License-Policy.pdf>.

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

   [RFC3723]  Aboba, B., Tseng, J., Walker, J., Rangan, V., and F.
              Travostino, "Securing Block Storage Protocols over IP",
              RFC 3723, DOI 10.17487/RFC3723, April 2004,
              <http://www.rfc-editor.org/info/rfc3723>.

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

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

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

   [RFC5663]  Black, D., Fridella, S., and J. Glasgow, "Parallel NFS
              (pNFS) Block/Volume Layout", RFC 5663,
              DOI 10.17487/RFC5663, January 2010,
              <http://www.rfc-editor.org/info/rfc5663>.





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   [RFC6688]  Black, D., Ed., Glasgow, J., and S. Faibish, "Parallel NFS
              (pNFS) Block Disk Protection", RFC 6688,
              DOI 10.17487/RFC6688, July 2012,
              <http://www.rfc-editor.org/info/rfc6688>.

   [RFC7143]  Chadalapaka, M., Satran, J., Meth, K., and D. Black,
              "Internet Small Computer System Interface (iSCSI) Protocol
              (Consolidated)", RFC 7143, DOI 10.17487/RFC7143, April
              2014, <http://www.rfc-editor.org/info/rfc7143>.

   [RFC7146]  Black, D. and P. Koning, "Securing Block Storage Protocols
              over IP: RFC 3723 Requirements Update for IPsec v3",
              RFC 7146, DOI 10.17487/RFC7146, April 2014,
              <http://www.rfc-editor.org/info/rfc7146>.

   [SAM-5]    INCITS Technical Committee T10, "Information Technology -
              SCSI Architecture Model - 5 (SAM-5)", ANSI
              INCITS 515-2016, 2016.

   [SAS3]     INCITS Technical Committee T10, "Information technology -
              Serial Attached SCSI-3 (SAS-3)", ANSI INCITS 519-2014,
              ISO/IEC 14776-154, 2014.

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

   [SPC4]     INCITS Technical Committee T10, "Information Technology -
              SCSI Primary Commands - 4 (SPC-4)", ANSI INCITS 513-2015,
              2015.

Acknowledgments

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

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

   David Noveck provided ample feedback to various drafts of this
   document, wrote the section on enforcing NFSv4 semantics, and rewrote
   various sections to better catch the intent.






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Author's Address

   Christoph Hellwig

   Email: hch@lst.de














































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