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Versions: 00                                                            
NFSv4                                                         G. Goodson
Internet-Draft                                                    NetApp
Expires: April 10, 2006                                         B. Welch
                                                               B. Halevy
                                                                D. Black
                                                              A. Adamson
                                                         October 7, 2005

                         NFSv4 pNFS Extensions

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 10, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This Internet-Draft provides a description of the pNFS extension for

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   The key feature of the protocol extension is the ability for clients
   to perform read and write operations that go directly from the client
   to individual storage system elements without funneling all such
   accesses through a single file server.  Of course, the file server
   must provide sufficient coordination of the client I/O so that the
   file system retains its integrity.

   The extension adds operations that query and manage layout
   information that allows parallel I/O between clients and storage
   system elements.  The layouts are managed in a similar way to
   delegations in that they are associated with leases and can be
   recalled by the server, but layout information is independent of

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  General Definitions  . . . . . . . . . . . . . . . . . . . . .  7
     2.1   Metadata Server  . . . . . . . . . . . . . . . . . . . . .  7
     2.2   Client . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3   Storage Device . . . . . . . . . . . . . . . . . . . . . .  8
     2.4   Storage Protocol . . . . . . . . . . . . . . . . . . . . .  8
     2.5   Control Protocol . . . . . . . . . . . . . . . . . . . . .  8
     2.6   Metadata . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.7   Layout . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  pNFS protocol semantics  . . . . . . . . . . . . . . . . . . .  9
     3.1   Definitions  . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1   Layout Types . . . . . . . . . . . . . . . . . . . . .  9
       3.1.2   Layout Iomode  . . . . . . . . . . . . . . . . . . . . 10
       3.1.3   Layout Segments  . . . . . . . . . . . . . . . . . . . 10
       3.1.4   Device IDs . . . . . . . . . . . . . . . . . . . . . . 11
       3.1.5   Aggregation Schemes  . . . . . . . . . . . . . . . . . 12
     3.2   Guarantees Provided by Layouts . . . . . . . . . . . . . . 12
     3.3   Getting a Layout . . . . . . . . . . . . . . . . . . . . . 13
     3.4   Committing a Layout  . . . . . . . . . . . . . . . . . . . 14
       3.4.1   LAYOUTCOMMIT and mtime/atime/change  . . . . . . . . . 15
       3.4.2   LAYOUTCOMMIT and size  . . . . . . . . . . . . . . . . 15
       3.4.3   LAYOUTCOMMIT and layoutupdate  . . . . . . . . . . . . 16
     3.5   Recalling a Layout . . . . . . . . . . . . . . . . . . . . 17
       3.5.1   Basic Operation  . . . . . . . . . . . . . . . . . . . 17
       3.5.2   Recall Callback Robustness . . . . . . . . . . . . . . 18
       3.5.3   Recall/Return Sequencing . . . . . . . . . . . . . . . 19
     3.6   Metadata Server Write Propagation  . . . . . . . . . . . . 21
     3.7   Crash Recovery . . . . . . . . . . . . . . . . . . . . . . 21
       3.7.1   Leases . . . . . . . . . . . . . . . . . . . . . . . . 21
       3.7.2   Client Recovery  . . . . . . . . . . . . . . . . . . . 23
       3.7.3   Metadata Server Recovery . . . . . . . . . . . . . . . 23
       3.7.4   Storage Device Recovery  . . . . . . . . . . . . . . . 25
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     4.1   File Layout Security . . . . . . . . . . . . . . . . . . . 27
     4.2   Object Layout Security . . . . . . . . . . . . . . . . . . 27
     4.3   Block/Volume Layout Security . . . . . . . . . . . . . . . 29
   5.  The NFSv4 File Layout Type . . . . . . . . . . . . . . . . . . 29
     5.1   File Striping and Data Access  . . . . . . . . . . . . . . 29
       5.1.1   Sparse and Dense Storage Device Data Layouts . . . . . 31
       5.1.2   Metadata and Storage Device Roles  . . . . . . . . . . 32
       5.1.3   Device Multipathing  . . . . . . . . . . . . . . . . . 33
       5.1.4   Operations Issued to Storage Devices . . . . . . . . . 34
     5.2   Global Stateid Requirements  . . . . . . . . . . . . . . . 35
     5.3   The Layout Iomode  . . . . . . . . . . . . . . . . . . . . 35
     5.4   Storage Device State Propagation . . . . . . . . . . . . . 35
       5.4.1   Lock State Propagation . . . . . . . . . . . . . . . . 36

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       5.4.2   Open-mode Validation . . . . . . . . . . . . . . . . . 36
       5.4.3   File Attributes  . . . . . . . . . . . . . . . . . . . 37
     5.5   Storage Device Component File Size . . . . . . . . . . . . 38
     5.6   Crash Recovery Considerations  . . . . . . . . . . . . . . 38
     5.7   Security Considerations  . . . . . . . . . . . . . . . . . 39
     5.8   Alternate Approaches . . . . . . . . . . . . . . . . . . . 39
   6.  pNFS Typed Data Structures . . . . . . . . . . . . . . . . . . 40
     6.1   pnfs_layouttype4 . . . . . . . . . . . . . . . . . . . . . 40
     6.2   pnfs_deviceid4 . . . . . . . . . . . . . . . . . . . . . . 41
     6.3   pnfs_deviceaddr4 . . . . . . . . . . . . . . . . . . . . . 41
     6.4   pnfs_devlist_item4 . . . . . . . . . . . . . . . . . . . . 42
     6.5   pnfs_layout4 . . . . . . . . . . . . . . . . . . . . . . . 42
     6.6   pnfs_layoutupdate4 . . . . . . . . . . . . . . . . . . . . 43
     6.7   pnfs_layouthint4 . . . . . . . . . . . . . . . . . . . . . 43
     6.8   pnfs_layoutiomode4 . . . . . . . . . . . . . . . . . . . . 43
   7.  pNFS File Attributes . . . . . . . . . . . . . . . . . . . . . 44
     7.1   pnfs_layouttype4<> FS_LAYOUT_TYPES . . . . . . . . . . . . 44
     7.2   pnfs_layouttype4<> FILE_LAYOUT_TYPES . . . . . . . . . . . 44
     7.3   pnfs_layouthint4 FILE_LAYOUT_HINT  . . . . . . . . . . . . 44
     7.4   uint32_t FS_LAYOUT_PREFERRED_BLOCKSIZE . . . . . . . . . . 44
     7.5   uint32_t FS_LAYOUT_PREFERRED_ALIGNMENT . . . . . . . . . . 44
   8.  pNFS Error Definitions . . . . . . . . . . . . . . . . . . . . 45
   9.  pNFS Operations  . . . . . . . . . . . . . . . . . . . . . . . 45
     9.1   LAYOUTGET - Get Layout Information . . . . . . . . . . . . 46
     9.2   LAYOUTCOMMIT - Commit writes made using a layout . . . . . 48
     9.3   LAYOUTRETURN - Release Layout Information  . . . . . . . . 51
     9.4   GETDEVICEINFO - Get Device Information . . . . . . . . . . 53
     9.5   GETDEVICELIST - Get List of Devices  . . . . . . . . . . . 54
   10.   Callback Operations  . . . . . . . . . . . . . . . . . . . . 55
     10.1  CB_LAYOUTRECALL  . . . . . . . . . . . . . . . . . . . . . 56
     10.2  CB_SIZECHANGED . . . . . . . . . . . . . . . . . . . . . . 58
   11.   Layouts and Aggregation  . . . . . . . . . . . . . . . . . . 58
     11.1  Simple Map . . . . . . . . . . . . . . . . . . . . . . . . 59
     11.2  Block Extent Map . . . . . . . . . . . . . . . . . . . . . 59
     11.3  Striped Map (RAID 0) . . . . . . . . . . . . . . . . . . . 59
     11.4  Replicated Map . . . . . . . . . . . . . . . . . . . . . . 59
     11.5  Concatenated Map . . . . . . . . . . . . . . . . . . . . . 60
     11.6  Nested Map . . . . . . . . . . . . . . . . . . . . . . . . 60
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . . 60
     12.1  Normative References . . . . . . . . . . . . . . . . . . . 60
     12.2  Informative References . . . . . . . . . . . . . . . . . . 60
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 61
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 62
       Intellectual Property and Copyright Statements . . . . . . . . 63

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1.  Introduction

   The NFSv4 protocol [2] specifies the interaction between a client
   that accesses files and a server that provides access to files and is
   responsible for coordinating access by multiple clients.  As
   described in the pNFS problem statement, this requires that all
   access to a set of files exported by a single NFSv4 server be
   performed by that server; at high data rates the server may become a

   The parallel NFS (pNFS) extensions to NFSv4 allow data accesses to
   bypass this bottleneck by permitting direct client access to the
   storage devices containing the file data.  When file data for a
   single NFSv4 server is stored on multiple and/or higher throughput
   storage devices (by comparison to the server's throughput
   capability), the result can be significantly better file access
   performance.  The relationship among multiple clients, a single
   server, and multiple storage devices for pNFS (server and clients
   have access to all storage devices) is shown in this diagram:

       |+-----------+                                 +-----------+
       ||+-----------+                                |           |
       |||           |        NFSv4 + pNFS            |           |
       +||  Clients  |<------------------------------>|   Server  |
        +|           |                                |           |
         +-----------+                                |           |
              |||                                     +-----------+
              |||                                           |
              |||                                           |
              ||| Storage        +-----------+              |
              ||| Protocol       |+-----------+             |
              ||+----------------||+-----------+  Control|
              |+-----------------|||           |    Protocol|
              +------------------+||  Storage  |------------+
                                  +|  Devices  |

                                 Figure 1

   In this structure, the responsibility for coordination of file access
   by multiple clients is shared among the server, clients, and storage
   devices.  This is in contrast to NFSv4 without pNFS extensions, in
   which this is primarily the server's responsibility, some of which
   can be delegated to clients under strictly specified conditions.

   The pNFS extension to NFSv4 takes the form of new operations that
   manage data location information called a "layout".  The layout is

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   managed in a similar fashion as NFSv4 data delegations (e.g., they
   are recallable and revocable).  However, they are distinct
   abstractions and are manipulated with new operations that are
   described in Section 9.  When a client holds a layout, it has rights
   to access the data directly using the location information in the

   There are new attributes that describe general layout
   characteristics.  However, much of the required information cannot be
   managed solely within the attribute framework, because it will need
   to have a strictly limited term of validity, subject to invalidation
   by the server.  This requires the use of new operations to obtain,
   return, recall, and modify layouts, in addition to new attributes.

   This document specifies both the NFSv4 extensions required to
   distribute file access coordination between the server and its
   clients and a NFSv4 file storage protocol that may be used to access
   data stored on NFSv4 storage devices.

   Storage protocols used to access a variety of other storage devices
   are deliberately not specified here.  These might include:

   o  Block/volume protocols such as iSCSI ([4]), and FCP ([5]).  The
      block/volume protocol support can be independent of the addressing
      structure of the block/volume protocol used, allowing more than
      one protocol to access the same file data and enabling
      extensibility to other block/volume protocols.

   o  Object protocols such as OSD over iSCSI or Fibre Channel [6].

   o  Other storage protocols, including PVFS and other file systems
      that are in use in HPC environments.

   pNFS is designed to accommodate these protocols and be extensible to
   new classes of storage protocols that may be of interest.

   The distribution of file access coordination between the server and
   its clients increases the level of responsibility placed on clients.
   Clients are already responsible for ensuring that suitable access
   checks are made to cached data and that attributes are suitably
   propagated to the server.  Generally, a misbehaving client that hosts
   only a single-user can only impact files accessible to that single
   user.  Misbehavior by a client hosting multiple users may impact
   files accessible to all of its users.  NFSv4 delegations increase the
   level of client responsibility as a client that carries out actions
   requiring a delegation without obtaining that delegation will cause
   its user(s) to see unexpected and/or incorrect behavior.

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   Some uses of pNFS extend the responsibility of clients beyond
   delegations.  In some configurations, the storage devices cannot
   perform fine-grained access checks to ensure that clients are only
   performing accesses within the bounds permitted to them by the pNFS
   operations with the server (e.g., the checks may only be possible at
   file system granularity rather than file granularity).  In situations
   where this added responsibility placed on clients creates
   unacceptable security risks, pNFS configurations in which storage
   devices cannot perform fine-grained access checks SHOULD NOT be used.
   All pNFS server implementations MUST support NFSv4 access to any file
   accessible via pNFS in order to provide an interoperable means of
   file access in such situations.  See Section 4 on Security for
   further discussion.

   Finally, there are issues about how layouts interact with the
   existing NFSv4 abstractions of data delegations and byte range
   locking.  These issues, and others, are also discussed here.

2.  General Definitions

   This protocol extension partitions the NFSv4 file system protocol
   into two parts, the control path and the data path.  The control path
   is implemented by the extended (p)NFSv4 server.  When the file system
   being exported by (p)NFSv4 uses storage devices that are visible to
   clients over the network, the data path may be implemented by direct
   communication between the extended (p)NFSv4 file system client and
   the storage devices.  This leads to a few new terms used to describe
   the protocol extension and some clarifications of existing terms.

2.1  Metadata Server

   A pNFS "server" or "metadata server" is a server as defined by
   RFC3530 [2], which additionally provides support of the pNFS minor
   extension.  When using the pNFS NFSv4 minor extension, the metadata
   server may hold only the metadata associated with a file, while the
   data can be stored on the storage devices.  However, similar to
   NFSv4, data may also be written through the metadata server.  Note:
   directory data is always accessed through the metadata server.

2.2  Client

   A pNFS "client" is a client as defined by RFC3530 [2], with the
   addition of supporting the pNFS minor extension server protocol and
   with the addition of supporting at least one storage protocol for
   performing I/O directly to storage devices.

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2.3  Storage Device

   This is a device, or server, that controls the file's data, but
   leaves other metadata management up to the metadata server.  A
   storage device could be another NFS server, or an Object Storage
   Device (OSD) or a block device accessed over a SAN (e.g., either
   FiberChannel or iSCSI SAN).  The goal of this extension is to allow
   direct communication between clients and storage devices.

2.4  Storage Protocol

   This is the protocol between the pNFS client and the storage device
   used to access the file data.  Three following types have been
   described: file protocols (e.g., NFSv4), object protocols (e.g.,
   OSD), and block/volume protocols (e.g., based on SCSI-block
   commands).  These protocols are in turn realizable over a variety of
   transport stacks.  We anticipate there will be variations on these
   storage protocols, including new protocols that are unknown at this
   time or experimental in nature.  The details of the storage protocols
   will be described in other documents so that pNFS clients can be
   written to use these storage protocols.  Use of NFSv4 itself as a
   file-based storage protocol is described in Section 5.

2.5  Control Protocol

   This is a protocol used by the exported file system between the
   server and storage devices.  Specification of such protocols is
   outside the scope of this draft.  Such control protocols would be
   used to control such activities as the allocation and deallocation of
   storage and the management of state required by the storage devices
   to perform client access control.  The control protocol should not be
   confused with protocols used to manage LUNs in a SAN and other
   sysadmin kinds of tasks.

   While the pNFS protocol allows for any control protocol, in practice
   the control protocol is closely related to the storage protocol.  For
   example, if the storage devices are NFS servers, then the protocol
   between the pNFS metadata server and the storage devices is likely to
   involve NFS operations.  Similarly, when object storage devices are
   used, the pNFS metadata server will likely use iSCSI/OSD commands to
   manipulate storage.

   However, this document does not mandate any particular control
   protocol.  Instead, it just describes the requirements on the control
   protocol for maintaining attributes like modify time, the change
   attribute, and the end-of-file position.

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2.6  Metadata

   This is information about a file, like its name, owner, where it
   stored, and so forth.  The information is managed by the exported
   file system server (metadata server).  Metadata also includes lower-
   level information like block addresses and indirect block pointers.
   Depending the storage protocol, block-level metadata may or may not
   be managed by the metadata server, but is instead managed by Object
   Storage Devices or other servers acting as a storage device.

2.7  Layout

   A layout defines how a file's data is organized on one or more
   storage devices.  There are many possible layout types.  They vary in
   the storage protocol used to access the data, and in the aggregation
   scheme that lays out the file data on the underlying storage devices.
   Layouts are described in more detail below.

3.  pNFS protocol semantics

   This section describes the semantics of the pNFS protocol extension
   to NFSv4; this is the protocol between the client and the metadata

3.1  Definitions

   This sub-section defines a number of terms necessary for describing
   layouts and their semantics.  In addition, it more precisely defines
   how layouts are identified and how they can be composed of smaller
   granularity layout segments.

3.1.1  Layout Types

   A layout describes the mapping of a file's data to the storage
   devices that hold the data.  A layout is said to belong to a specific
   "layout type" (see Section 6.1 for its RPC definition).  The layout
   type allows for variants to handle different storage protocols (e.g.,
   block/volume [7], object [8], and file [Section 5] layout types).  A
   metadata server, along with its control protocol, must support at
   least one layout type.  A private sub-range of the layout type name
   space is also defined.  Values from the private layout type range can
   be used for internal testing or experimentation.

   As an example, a file layout type could be an array of tuples (e.g.,
   deviceID, file_handle), along with a definition of how the data is
   stored across the devices (e.g., striping).  A block/volume layout
   might be an array of tuples that store <deviceID, block_number, block
   count> along with information about block size and the file offset of

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   the first block.  An object layout might be an array of tuples
   <deviceID, objectID> and an additional structure (i.e., the
   aggregation map) that defines how the logical byte sequence of the
   file data is serialized into the different objects.  Note, the actual
   layouts are more complex than these simple expository examples.

   This document defines a NFSv4 file layout type using a stripe-based
   aggregation scheme (see Section 5).  Adjunct specifications are being
   drafted that precisely define other layout formats (e.g., block/
   volume [7], and object [8] layouts) to allow interoperability among
   clients and metadata servers.

3.1.2  Layout Iomode

   The iomode indicates to the metadata server the client's intent to
   perform either READs (only) or a mixture of I/O possibly containing
   WRITEs as well as READs (i.e., READ/WRITE).  For certain layout
   types, it is useful for a client to specify this intent at LAYOUTGET
   time.  E.g., for block/volume based protocols, block allocation could
   occur when a READ/WRITE iomode is specified.  A special
   LAYOUTIOMODE_ANY iomode is defined and can only be used for
   LAYOUTRETURN and LAYOUTRECALL, not for LAYOUTGET.  It specifies that
   layouts pertaining to both READ and RW iomodes are being returned or
   recalled, respectively.

   A storage device may validate I/O with regards to the iomode; this is
   dependent upon storage device implementation.  Thus, if the client's
   layout iomode differs from the I/O being performed the storage device
   may reject the client's I/O with an error indicating a new layout
   with the correct I/O mode should be fetched.  E.g., if a client gets
   a layout with a READ iomode and performs a WRITE to a storage device,
   the storage device is allowed to reject that WRITE.

   The iomode does not conflict with OPEN share modes or lock requests;
   open mode checks and lock enforcement are always enforced, and are
   logically separate from the pNFS layout level.  As well, open modes
   and locks are the preferred method for restricting user access to
   data files.  E.g., an OPEN of read, deny-write does not conflict with
   a LAYOUTGET containing an iomode of READ/WRITE performed by another
   client.  Applications that depend on writing into the same file
   concurrently may use byte range locking to serialize their accesses.

3.1.3  Layout Segments

   Until this point, layouts have been defined in a fairly vague manner.
   A layout is more precisely identified by the following tuple:
   <ClientID, FH, layout type>; the FH refers to the FH of the file on
   the metadata server.  Note, layouts describe a file, not a byte-range

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   of a file.

   Since a layout that describes an entire file may be very large, there
   is a desire to manage layouts in smaller chunks that correspond to
   byte-ranges of the file.  For example, the entire layout need not be
   returned, recalled, or committed.  These chunks are called "layout
   segments" and are further identified by the byte-range they
   represent.  Layout operations require the identification of the
   layout segment (i.e., clientID, FH, layout type, and byte-range), as
   well as the iomode.  This structure allows clients and metadata
   servers to aggregate the results of layout operations into a singly
   maintained layout.

   It is important to define when layout segments overlap and/or
   conflict with each other.  For a layout segment to overlap another
   layout segment both segments must be of the same layout type,
   correspond to the same filehandle, and have the same iomode; in
   addition, the byte-ranges of the segments must overlap.  Layout
   segments conflict, when they overlap and differ in the content of the
   layout (i.e., the storage device/file mapping parameters differ).
   Note, differing iomodes do not lead to conflicting layouts.  It is
   permissible for layout segments with different iomodes, pertaining to
   the same byte range, to be held by the same client.

3.1.4  Device IDs

   The "deviceID" is a short name for a storage device.  In practice, a
   significant amount of information may be required to fully identify a
   storage device.  Instead of embedding all that information in a
   layout, a level of indirection is used.  Layouts embed device IDs,
   and a new operation (GETDEVICEINFO) is used to retrieve the complete
   identity information about the storage device according to its layout
   type.  For example, the identity of a file server or object server
   could be an IP address and port.  The identity of a block device
   could be a volume label.  Due to multipath connectivity in a SAN
   environment, agreement on a volume label is considered the reliable
   way to locate a particular storage device.

   The device ID is qualified by the layout type and unique per file
   system (FSID).  This allows different layout drivers to generate
   device IDs without the need for co-ordination.  In addition to
   GETDEVICEINFO, another operation, GETDEVICELIST, has been added to
   allow clients to fetch the mappings of multiple storage devices
   attached to a metadata server.

   Clients cannot expect the mapping between device ID and storage
   device address to persist across server reboots, hence a client MUST
   fetch new mappings on startup or upon detection of a metadata server

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   reboot unless it can revalidate its existing mappings.  Not all
   layout types support such revalidation, and the means of doing so is
   layout specific.  If data are reorganized from a storage device with
   a given device ID to a different storage device (i.e., if the mapping
   between storage device and data changes), the layout describing the
   data MUST be recalled rather than assigning the new storage device to
   the old device ID.

3.1.5  Aggregation Schemes

   Aggregation schemes can describe layouts like simple one-to-one
   mapping, concatenation, and striping.  A general aggregation scheme
   allows nested maps so that more complex layouts can be compactly
   described.  The canonical aggregation type for this extension is
   striping, which allows a client to access storage devices in
   parallel.  Even a one-to-one mapping is useful for a file server that
   wishes to distribute its load among a set of other file servers.

3.2  Guarantees Provided by Layouts

   Layouts delegate to the client the ability to access data out of
   band.  The layout guarantees the holder that the layout will be
   recalled when the state encapsulated by the layout becomes invalid
   (e.g., through some operation that directly or indirectly modifies
   the layout) or, possibly, when a conflicting layout is requested, as
   determined by the layout's iomode.  When a layout is recalled, and
   then returned by the client, the client retains the ability to access
   file data with normal NFSv4 I/O operations through the metadata
   server.  Only the right to do I/O out-of-band is affected.

   Holding a layout does not guarantee that a user of the layout has the
   rights to access the data represented by the layout.  All user access
   rights MUST be obtained through the appropriate open, lock, and
   access operations (i.e., those that would be used in the absence of
   pNFS).  However, if a valid layout for a file is not held by the
   client, the storage device should reject all I/Os to that file's byte
   range that originate from that client.  In summary, layouts and
   ordinary file access controls are independent.  The act of modifying
   a file for which a layout is held, does not necessarily conflict with
   the holding of the layout that describes the file being modified.
   However, with certain layout types (e.g., block/volume layouts), the
   layout's iomode must agree with the type of I/O being performed.

   Depending upon the layout type and storage protocol in use, storage
   device access permissions may be granted by LAYOUTGET and may be
   encoded within the type specific layout.  If access permissions are
   encoded within the layout, the metadata server must recall the layout
   when those permissions become invalid for any reason; for example

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   when a file becomes unwritable or inaccessible to a client.  Note,
   clients are still required to perform the appropriate access
   operations as described above (e.g., open and lock ops).  The degree
   to which it is possible for the client to circumvent these access
   operations must be clearly addressed by the individual layout type
   documents, as well as the consequences of doing so.  In addition,
   these documents must be clear about the requirements and non-
   requirements for the checking performed by the server.

   If the pNFS metadata server supports mandatory byte range locks then
   byte range locks must behave as specified by the NFSv4 protocol, as
   observed by users of files.  If a storage device is unable to
   restrict access by a pNFS client who does not hold a required
   mandatory byte range lock then the metadata server must not grant
   layouts to a client, for that storage device, that permits any access
   that conflicts with a mandatory byte range lock held by another
   client.  In this scenario, it is also necessary for the metadata
   server to ensure that byte range locks are not granted to a client if
   any other client holds a conflicting layout; in this case all
   conflicting layouts must be recalled and returned before the lock
   request can be granted.  This requires the pNFS server to understand
   the capabilities of its storage devices.

3.3  Getting a Layout

   A client obtains a layout through a new operation, LAYOUTGET.  The
   metadata server will give out layouts of a particular type (e.g.,
   block/volume, object, or file) and aggregation as requested by the
   client.  The client selects an appropriate layout type which the
   server supports and the client is prepared to use.  The layout
   returned to the client may not line up exactly with the requested
   byte range.  A field within the LAYOUTGET request, "minlength",
   specifies the minimum overlap that MUST exist between the requested
   layout and the layout returned by the metadata server.  The
   "minlength" field should specify a size of at least one.  A metadata
   server may give-out multiple overlapping, non-conflicting layout
   segments to the same client in response to a LAYOUTGET.

   There is no implied ordering between getting a layout and performing
   a file OPEN.  For example, a layout may first be retrieved by placing
   a LAYOUTGET operation in the same compound as the initial file OPEN.
   Once the layout has been retrieved, it can be held across multiple
   OPEN and CLOSE sequences.

   The storage protocol used by the client to access the data on the
   storage device is determined by the layout's type.  The client needs
   to select a "layout driver" that understands how to interpret and use
   that layout.  The API used by the client to talk to its drivers is

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   outside the scope of the pNFS extension.  The storage protocol
   between the client's layout driver and the actual storage is covered
   by other protocols specifications such as iSCSI (block storage), OSD
   (object storage) or NFS (file storage).

   Although, the metadata server is in control of the layout for a file,
   the pNFS client can provide hints to the server when a file is opened
   or created about preferred layout type and aggregation scheme.  The
   pNFS extension introduces a LAYOUT_HINT attribute that the client can
   set at creation time to provide a hint to the server for new files.
   It is suggested that this attribute be set as one of the initial
   attributes to OPEN when creating a new file.  Setting this attribute
   separately, after the file has been created could make it difficult,
   or impossible, for the server implementation to comply.

3.4  Committing a Layout

   Due to the nature of the protocol, the file attributes, and data
   location mapping (e.g., which offsets store data vs. store holes)
   that exist on the metadata storage device may become inconsistent in
   relation to the data stored on the storage devices; e.g., when WRITEs
   occur before a layout has been committed (e.g., between a LAYOUTGET
   and a LAYOUTCOMMIT).  Thus, it is necessary to occasionally re-sync
   this state and make it visible to other clients through the metadata

   The LAYOUTCOMMIT operation is responsible for committing a modified
   layout segment to the metadata server.  Note: the data should be
   written and committed to the appropriate storage devices before the
   LAYOUTCOMMIT occurs.  Note, if the data is being written
   asynchronously through the metadata server a COMMIT to the metadata
   server is required to sync the data and make it visible on the
   storage devices (see Section 3.6 for more details).  The scope of
   this operation depends on the storage protocol in use.  For block/
   volume-based layouts, it may require updating the block list that
   comprises the file and committing this layout to stable storage.
   While, for file-layouts it requires some synchronization of
   attributes between the metadata and storage devices (i.e., mainly the
   size attribute; EOF).  It is important to note that the level of
   synchronization is from the point of view of the client who issued
   the LAYOUTCOMMIT.  The updated state on the metadata server need only
   reflect the state as of the client's last operation previous to the
   LAYOUTCOMMIT, it need not reflect a globally synchronized state
   (e.g., other clients may be performing, or may have performed I/O
   since the client's last operation and the LAYOUTCOMMIT).

   The control protocol is free to synchronize the attributes before it
   receives a LAYOUTCOMMIT, however upon successful completion of a

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   LAYOUTCOMMIT, state that exists on the metadata server that describes
   the file MUST be in sync with the state existing on the storage
   devices that comprise that file as of the issuing client's last
   operation.  Thus, a client that queries the size of a file between a
   WRITE to a storage device and the LAYOUTCOMMIT may observe a size
   that does not reflects the actual data written.

3.4.1  LAYOUTCOMMIT and mtime/atime/change

   The change attribute and the modify/access times may be updated, by
   the server, at LAYOUTCOMMIT time; since for some layout types, the
   change attribute and atime/mtime can not be updated by the
   appropriate I/O operation performed at a storage device.  The
   arguments to LAYOUTCOMMIT allow the client to provide suggested
   access and modify time values to the server.  Again, depending upon
   the layout type, these client provided values may or may not be used.
   The server should sanity check the client provided values before they
   are used.  For example, the server should ensure that time does not
   flow backwards.  According to the NFSv4 specification, The client
   always has the option to set these attributes through an explicit
   SETATTR operation.

   As mentioned, for some layout protocols the change attribute and
   mtime/atime may be updated at or after the time the I/O occurred
   (e.g., if the storage device is able to communicate these attributes
   to the metadata server).  If, upon receiving a LAYOUTCOMMIT, the
   server implementation is able to determine that the file did not
   change since the last time the change attribute was updated (e.g., no
   WRITEs or over-writes occurred), the implementation need not update
   the change attribute; file-based protocols may have enough state to
   make this determination or may update the change attribute upon each
   file modification.  This also applies for mtime and atime; if the
   server implementation is able to determine that the file has not been
   modified since the last mtime update, the server need not update
   mtime at LAYOUTCOMMIT time.  Once LAYOUTCOMMIT completes, the new
   change attribute and mtime/atime should be visible if that file was
   modified since the latest previous LAYOUTCOMMIT or LAYOUTGET.

3.4.2  LAYOUTCOMMIT and size

   The file's size may be updated at LAYOUTCOMMIT time as well.  The
   LAYOUTCOMMIT operation contains an argument that indicates the last
   byte offset to which the client wrote ("last_write_offset").  Note:
   for this offset to be viewed as a file size it must be incremented by
   one byte (e.g., a write to offset 0 would map into a file size of 1,
   but the last write offset is 0).  The metadata server may do one of
   the following:

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   1.  It may update the file's size based on the last write offset.
       However, to the extent possible, the metadata server should
       sanity check any value to which the file's size is going to be
       set.  E.g., it must not truncate the file based on the client
       presenting a smaller last write offset than the file's current

   2.  If it has sufficient other knowledge of file size (e.g., by
       querying the storage devices through the control protocol), it
       may ignore the client provided argument and use the query-derived

   3.  It may use the last write offset as a hint, subject to correction
       when other information is available as above.

   The method chosen to update the file's size will depend on the
   storage device's and/or the control protocol's implementation.  For
   example, if the storage devices are block devices with no knowledge
   of file size, the metadata server must rely on the client to set the
   size appropriately.  A new size flag and length are also returned in
   the results of a LAYOUTCOMMIT.  This union indicates whether a new
   size was set, and to what length it was set.  If a new size is set as
   a result of LAYOUTCOMMIT, then the metadata server must reply with
   the new size.  As well, if the size is updated, the metadata server
   in conjunction with the control protocol SHOULD ensure that the new
   size is reflected by the storage devices immediately upon return of
   the LAYOUTCOMMIT operation; e.g., a READ up to the new file size
   should succeed on the storage devices (assuming no intervening
   truncations).  Again, if the client wants to explicitly zero-extend
   or truncate a file, SETATTR must be used; it need not be used when
   simply writing past EOF.

   Since client layout holders may be unaware of changes made to the
   file's size, through LAYOUTCOMMIT or SETATTR, by other clients, an
   additional callback/notification has been added for pNFS.
   CB_SIZECHANGED is a notification that the metadata server sends to
   layout holders to notify them of a change in file size.  This is
   preferred over issuing CB_LAYOUTRECALL to each of the layout holders.

3.4.3  LAYOUTCOMMIT and layoutupdate

   The LAYOUTCOMMIT operation contains a "layoutupdate" argument.  This
   argument is a layout type specific structure.  The structure can be
   used to pass arbitrary layout type specific information from the
   client to the metadata server at LAYOUTCOMMIT time.  For example, if
   using a block/volume layout, the client can indicate to the metadata
   server which reserved or allocated blocks it used and which it did
   not.  The "layoutupdate" structure need not be the same structure as

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   the layout returned by LAYOUTGET.  The structure is defined by the
   layout type and is opaque to LAYOUTCOMMIT.

3.5  Recalling a Layout

3.5.1  Basic Operation

   Since a layout protects a client's access to a file via a direct
   client-storage-device path, a layout need only be recalled when it is
   semantically unable to serve this function.  Typically, this occurs
   when the layout no longer encapsulates the true location of the file
   over the byte range it represents.  Any operation or action (e.g.,
   server driven restriping or load balancing) that changes the layout
   will result in a recall of the layout.  A layout is recalled by the
   CB_LAYOUTRECALL callback operation (see Section 10.1).  This callback
   can either recall a layout segment identified by a byte range, or all
   the layouts associated with a file system (FSID).  However, there is
   no single operation to return all layouts associated with an FSID;
   multiple layout segments may be returned in a single compound
   operation.  Section 3.5.3 discusses sequencing issues surrounding the
   getting, returning, and recalling of layouts.

   The iomode is also specified when recalling a layout or layout
   segment.  Generally, the iomode in the recall request must match the
   layout, or segment, being returned; e.g., a recall with an iomode of
   RW should cause the client to only return RW layout segments (not R
   segments).  However, a special LAYOUTIOMODE_ANY enumeration is
   defined to enable recalling a layout of any type (i.e., the client
   must return both read-only and read/write layouts).

   A REMOVE operation may cause the metadata server to recall the layout
   to prevent the client from accessing a non-existent file and to
   reclaim state stored on the client.  Since a REMOVE may be delayed
   until the last close of the file has occurred, the recall may also be
   delayed until this time.  As well, once the file has been removed,
   after the last reference, the client SHOULD no longer be able to
   perform I/O using the layout (e.g., with file-based layouts an error
   such as ESTALE could be returned).

   Although, the pNFS extension does not alter the caching capabilities
   of clients, or their semantics, it recognizes that some clients may
   perform more aggressive write-behind caching to optimize the benefits
   provided by pNFS.  However, write-behind caching may impact the
   latency in returning a layout in response to a CB_LAYOUTRECALL; just
   as caching impacts DELEGRETURN with regards to data delegations.
   Client implementations should limit the amount of dirty data they
   have outstanding at any one time.  Server implementations may fence
   clients from performing direct I/O to the storage devices if they

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   perceive that the client is taking too long to return a layout once
   recalled.  A server may be able to monitor client progress by
   watching client I/Os or by observing LAYOUTRETURNs of sub-portions of
   the recalled layout.  The server can also limit the amount of dirty
   data to be flushed to storage devices by limiting the byte ranges
   covered in the layouts it gives out.

   Once a layout has been returned, the client MUST NOT issue I/Os to
   the storage devices for the file, byte range, and iomode represented
   by the returned layout.  If a client does issue an I/O to a storage
   device for which it does not hold a layout, the storage device SHOULD
   reject the I/O.

3.5.2  Recall Callback Robustness

   For simplicity, the discussion thus far has assumed that pNFS client
   state for a file exactly matches the pNFS server state for that file
   and client regarding layout ranges and permissions.  This assumption
   leads to the implicit assumption that any callback results in a
   LAYOUTRETURN or set of LAYOUTRETURNs that exactly match the range in
   the callback, since both client and server agree about the state
   being maintained.  However, it can be useful if this assumption does
   not always hold.  For example:

   o  It may be useful for clients to be able to discard layout
      information without calling LAYOUTRETURN.  If conflicts that
      require callbacks are very rare, and a server can use a multi-file
      callback to recover per-client resources (e.g., via a FSID recall,
      or a multi-file recall within a single compound), the result may
      be significantly less client-server pNFS traffic.

   o  It may be similarly useful for servers to enhance information
      about what layout ranges are held by a client beyond what a client
      actually holds.  In the extreme, a server could manage conflicts
      on a per-file basis, only issuing whole-file callbacks even though
      clients may request and be granted sub-file ranges.

   o  As well, the synchronized state assumption is not robust to minor
      errors.  A more robust design would allow for divergence between
      client and server and the ability to recover.  It is vital that a
      client not assign itself layout permissions beyond what the server
      has granted and that the server not forget layout permissions that
      have been granted in order to avoid errors.  On the other hand, if
      a server believes that a client holds a layout segment that the
      client does not know about, it's useful for the client to be able
      to issue the LAYOUTRETURN that the server is expecting in response
      to a recall.

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   Thus, in light of the above, it is useful for a server to be able to
   issue callbacks for layout ranges it has not granted to a client, and
   for a client to return ranges it does not hold.  A pNFS client must
   always return layout segments that comprise the full range specified
   by the recall.  Note, the full recalled layout range need not be
   returned as part of a single operation, but may be returned in
   segments.  This allows the client to stage the flushing of dirty
   data, layout commits, and returns.  Also, it indicates to the
   metadata server that the client is making progress.

   In order to ensure client/server convergence on the layout state, the
   final LAYOUTRETURN operation in a sequence of returns for a
   particular recall, SHOULD specify the entire range being recalled,
   even if layout segments pertaining to partial ranges were previously
   returned.  In addition, if the client holds no layout segment that
   overlaps the range being recalled, the client should return the
   NFS4ERR_NOMATCHING_LAYOUT error code.  This allows the server to
   update its view of the client's layout state.

3.5.3  Recall/Return Sequencing

   As with other stateful operations, pNFS requires the correct
   sequencing of layout operations.  This proposal assumes that sessions
   will precede or accompany pNFS into NFSv4.x and thus, pNFS will
   require the use of sessions.  If the sessions proposal does not
   precede pNFS, then this proposal needs to be modified to provide for
   the correct sequencing of pNFS layout operations.  Also, this
   specification is reliant on the sessions protocol to provide the
   correct sequencing between regular operations and callbacks.  It is
   the server's responsibility to avoid inconsistencies regarding the
   layouts it hands out and the client's responsibility to properly
   serialize its layout requests.

   One critical issue with operation sequencing concerns callbacks.  The
   protocol must defend against races between the reply to a LAYOUTGET
   operation and a subsequent CB_LAYOUTRECALL.  It MUST NOT be possible
   for a client to process the CB_LAYOUTRECALL for a layout that it has
   not received in a reply message to a LAYOUTGET.  Client Side Considerations

   Consider a pNFS client that has issued a LAYOUTGET and then receives
   an overlapping recall callback for the same file.  There are two
   possibilities, which the client cannot distinguish when the callback

   1.  The server processed the LAYOUTGET before issuing the recall, so
       the LAYOUTGET response is in flight, and must be waited for

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       because it may be carrying layout info that will need to be
       returned to deal with the recall callback.

   2.  The server issued the callback before receiving the LAYOUTGET.
       The server will not respond to the LAYOUTGET until the recall
       callback is processed.

   This can cause deadlock, as the client must wait for the LAYOUTGET
   response before processing the recall in the first case, but that
   response will not arrive until after the recall is processed in the
   second case.  This deadlock can be avoided by adhering to the
   following requirements:

   o  A LAYOUTGET MUST be rejected with an error (i.e.,
      NFS4ERR_RECALLCONFLICT) if there's an overlapping outstanding
      recall callback to the same client

   o  When processing a recall, the client MUST wait for a response to
      all conflicting outstanding LAYOUTGETs before performing any
      RETURN that could be affected by any such response.

   o  The client SHOULD wait for responses to all operations required to
      complete a recall before sending any LAYOUTGETs that would
      conflict with the recall because the server is likely to return
      errors for them.

   Now the client can wait for the LAYOUTGET response, as it will be
   received in both cases.  Server Side Considerations

   Consider a related situation from the pNFS server's point of view.
   The server has issued a recall callback and receives an overlapping
   LAYOUTGET for the same file before the LAYOUTRETURN(s) that respond
   to the recall callback.  Again, there are two cases:

   1.  The client issued the LAYOUTGET before processing the recall

   2.  The client issued the LAYOUTGET after processing the recall
       callback, but it arrived before the LAYOUTRETURN that completed
       that processing.

   The simplest approach is to always reject the overlapping LAYOUTGET.
   The client has two ways to avoid this result - it can issue the
   LAYOUTGET as a subsequent element of a COMPOUND containing the
   LAYOUTRETURN that completes the recall callback, or it can wait for
   the response to that LAYOUTRETURN.

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   This leads to a more general problem; in the absence of a callback if
   a client issues concurrent overlapping LAYOUTGET and LAYOUTRETURN
   operations, it is possible for the server to process them in either
   order.  Again, a client must take the appropriate precautions in
   serializing its actions.

   [ASIDE: HighRoad forbids a client from doing this, as the per-file
   layout stateid will cause one of the two operations to be rejected
   with a stale layout stateid.  This approach is simpler and produces
   better results by comparison to allowing concurrent operations, at
   least for this sort of conflict case, because server execution of
   operations in an order not anticipated by the client may produce
   results that are not useful to the client (e.g., if a LAYOUTRETURN is
   followed by a concurrent overlapping LAYOUTGET, but executed in the
   other order, the client will not retain layout extents for the
   overlapping range).]

3.6  Metadata Server Write Propagation

   Asynchronous writes written through the metadata server may be
   propagated lazily to the storage devices.  For data written
   asynchronously through the metadata server, a client performing a
   read at the appropriate storage device is not guaranteed to see the
   newly written data until a COMMIT occurs at the metadata server.
   While the write is pending, reads to the storage device can give out
   either the old data, the new data, or a mixture thereof.  After
   either a synchronous write completes, or a COMMIT is received (for
   asynchronously written data), the metadata server must ensure that
   storage devices give out the new data and that the data has been
   written to stable storage.  If the server implements its storage in
   any way such that it cannot obey these constraints, then it must
   recall the layouts to prevent reads being done that cannot be handled

3.7  Crash Recovery

   Crash recovery is complicated due to the distributed nature of the
   pNFS protocol.  In general, crash recovery for layouts is similar to
   crash recovery for delegations in the base NFSv4 protocol.  However,
   the client's ability to perform I/O without contacting the metadata
   server introduces subtleties that must be handled correctly if file
   system corruption is to be avoided.

3.7.1  Leases

   The layout lease period plays a critical role in crash recovery.
   Depending on the capabilities of the storage protocol, it is crucial
   that the client is able to maintain an accurate layout lease timer to

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   ensure that I/Os are not issued to storage devices after expiration
   of the layout lease period.  In order for the client to do so, it
   must know which operations renew a lease.  Lease Renewal

   The current NFSv4 specification allows for implicit lease renewals to
   occur upon receiving an I/O. However, due to the distributed pNFS
   architecture, implicit lease renewals are limited to operations
   performed at the metadata server; this includes I/O performed through
   the metadata server.  So, a client must not assume that READ and
   WRITE I/O to storage devices implicitly renew lease state.

   If sessions are required for pNFS, as has been suggested, then the
   SEQUENCE operation is to be used to explicitly renew leases.  It is
   proposed that the SEQUENCE operation be extended to return all the
   specific information that RENEW does, but not as an error as RENEW
   returns it.  Since, when using session, beginning each compound with
   the SEQUENCE op allows renews to be performed without an additional
   operation and without an additional request.  Again, the client must
   not rely on any operation to the storage devices to renew a lease.
   Using the SEQUENCE operation for renewals, simplifies the client's
   perception of lease renewal.  Client Lease Timer

   Depending on the storage protocol and layout type in use, it may be
   crucial that the client not issue I/Os to storage devices if the
   corresponding layout's lease has expired.  Doing so may lead to file
   system corruption if the layout has been given out and used by
   another client.  In order to prevent this, the client must maintain
   an accurate lease timer for all layouts held.  RFC3530 has the
   following to say regarding the maintenance of a client lease timer:

      ...the client must track operations which will renew the lease
      period.  Using the time that each such request was sent and the
      time that the corresponding reply was received, the client should
      bound the time that the corresponding renewal could have occurred
      on the server and thus determine if it is possible that a lease
      period expiration could have occurred.

   To be conservative, the client should start its lease timer based on
   the time that the it issued the operation to the metadata server,
   rather than based on the time of the response.

   It is also necessary to take propagation delay into account when
   requesting a renewal of the lease:

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      ...the client should subtract it from lease times (e.g., if the
      client estimates the one-way propagation delay as 200 msec, then
      it can assume that the lease is already 200 msec old when it gets
      it).  In addition, it will take another 200 msec to get a response
      back to the server.  So the client must send a lock renewal or
      write data back to the server 400 msec before the lease would

   Thus, the client must be aware of the one-way propagation delay and
   should issue renewals well in advance of lease expiration.  Clients,
   to the extent possible, should try not to issue I/Os that may extend
   past the lease expiration time period.  However, since this is not
   always possible, the storage protocol must be able to protect against
   the effects of inflight I/Os, as is discussed later.

3.7.2  Client Recovery

   Client recovery for layouts works in much the same way as NFSv4
   client recovery works for other lock/delegation state.  When an NFSv4
   client reboots, it will lose all information about the layouts that
   it previously owned.  There are two methods by which the server can
   reclaim these resources and allow otherwise conflicting layouts to be
   provided to other clients.

   The first is through the expiry of the client's lease.  If the client
   recovery time is longer than the lease period, the client's lease
   will expire and the server will know that state may be released. for
   layouts the server may release the state immediately upon lease
   expiry or it may allow the layout to persist awaiting possible lease
   revival, as long as there are no conflicting requests.

   On the other hand, the client may recover in less time than it takes
   for the lease period to expire.  In such a case, the client will
   contact the server through the standard SETCLIENTID protocol.  The
   server will find that the client's id matches the id of the previous
   client invocation, but that the verifier is different.  The server
   uses this as a signal to release all the state associated with the
   client's previous invocation.

3.7.3  Metadata Server Recovery

   The server recovery case is slightly more complex.  In general, the
   recovery process again follows the standard NFSv4 recovery model: the
   client will discover that the metadata server has rebooted when it
   receives an unexpected STALE_STATEID or STALE_CLIENTID reply from the
   server; it will then proceed to try to reclaim its previous
   delegations during the server's recovery grace period.  However,
   layouts are not reclaimable in the same sense as data delegations;

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   there is no reclaim bit, thus no guarantee of continuity between the
   previous and new layout.  This is not necessarily required since a
   layout is not required to perform I/O; I/O can always be performed
   through the metadata server.

   [NOTE: there is no reclaim bit for getting a layout.  Thus, in the
   case of reclaiming an old layout obtained through LAYOUTGET, there is
   no guarantee of continuity.  If a reclaim bit existed a block/volume
   layout type might be happier knowing it got the layout back with the
   assurance of continuity.  However, this would require the metadata
   server trusting the client in telling it the exact layout it had
   (i.e., the full block-list); however, divergence is avoided by having
   the server tell the client what is contained within the layout.]

   If the client has dirty data that it needs to write out, or an
   outstanding LAYOUTCOMMIT, the client should try to obtain a new
   layout segment covering the byte range covered by the previous layout
   segment.  However, the client might not not get the same layout
   segment it had.  The range might be different or it might get the
   same range but the content of the layout might be different.  For
   example, if using a block/volume-based layout, the blocks
   provisionally assigned by the layout might be different, in which
   case the client will have to write the corresponding blocks again; in
   the interest of simplicity, the client might decide to always write
   them again.  Alternatively, the client might be unable to obtain a
   new layout and thus, must write the data using normal NFSv4 through
   the metadata server.

   There is an important safety concern associated with layouts that
   does not come into play in the standard NFSv4 case.  If a standard
   NFSv4 client makes use of a stale delegation, while reading, the
   consequence could be to deliver stale data to an application.  If
   writing, using a stale delegation or a stale state stateid for an
   open or lock would result in the rejection of the client's write with
   the appropriate stale stateid error.

   However, the pNFS layout enables the client to directly access the
   file system storage---if this access is not properly managed by the
   NFSv4 server the client can potentially corrupt the file system data
   or metadata.  Thus, it is vitally important that the client discover
   that the metadata server has rebooted, and that the client stops
   using stale layouts before the metadata server gives them away to
   other clients.  To ensure this, the client must be implemented so
   that layouts are never used to access the storage after the client's
   lease timer has expired.  It is crucial that clients have precise
   knowledge of the lease periods of their layouts.  For specific
   details on lease renewal and client lease timers, see Section 3.7.1.

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   The prohibition on using stale layouts applies to all layout related
   accesses, especially the flushing of dirty data to the storage
   devices.  If the client's lease timer expires because the client
   could not contact the server for any reason, the client MUST
   immediately stop using the layout until the server can be contacted
   and the layout can be officially recovered or reclaimed.  However,
   this is only part of the solution.  It is also necessary to deal with
   the consequences of I/Os already in flight.

   The issue of the effects of I/Os started before lease expiration and
   possibly continuing through lease expiration is the responsibility of
   the data storage protocol and as such is layout type specific.  There
   are two approaches the data storage protocol can take.  The protocol
   may adopt a global solution which prevents all I/Os from being
   executed after the lease expiration and thus is safe against a client
   who issues I/Os after lease expiration.  This is the preferred
   solution and the solution used by NFSv4 file based layouts (see
   Section 5.6); as well, the object storage device protocol allows
   storage to fence clients after lease expiration.  Alternatively, the
   storage protocol may rely on proper client operation and only deal
   with the effects of lingering I/Os.  These solutions may impact the
   client layout-driver, the metadata server layout-driver, and the
   control protocol.

3.7.4  Storage Device Recovery

   Storage device crash recovery is mostly dependent upon the layout
   type in use.  However, there are a few general techniques a client
   can use if it discovers a storage device has crashed while holding
   asynchronously written, non-committed, data.  First and foremost, it
   is important to realize that the client is the only one who has the
   information necessary to recover asynchronously written data; since,
   it holds the dirty data and most probably nobody else does.  Second,
   the best solution is for the client to err on the side or caution and
   attempt to re-write the dirty data through another path.

   The client, rather than hold the asynchronously written data
   indefinitely, is encouraged to, and can make sure that the data is
   written by using other paths to that data.  The client may write the
   data to the metadata server, either synchronously or asynchronously
   with a subsequent COMMIT.  Once it does this, there is no need to
   wait for the original storage device.  In the event that the data
   range to be committed is transferred to a different storage device,
   as indicated in a new layout, the client may write to that storage
   device.  Once the data has been committed at that storage device,
   either through a synchronous write or through a commit to that
   storage device (e.g., through the NFSv4 COMMIT operation for the
   NFSv4 file layout), the client should consider the transfer of

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   responsibility for the data to the new server as strong evidence that
   this is the intended and most effective method for the client to get
   the data written.  In either case, once the write is on stable
   storage (through either the storage device or metadata server), there
   is no need to continue either attempting to commit or attempting to
   synchronously write the data to the original storage device or wait
   for that storage device to become available.  That storage device may
   never be visible to the client again.

   This approach does have a "lingering write" problem, similar to
   regular NFSv4.  Suppose a WRITE is issued to a storage device for
   which no response is received.  The client breaks the connection,
   trying to re-establish a new one, and gets a recall of the layout.
   The client issues the I/O for the dirty data through an alternative
   path, for example, through the metadata server and it succeeds.  The
   client then goes on to perform additional writes that all succeed.
   If at some time later, the original write to the storage device
   succeeds, data inconsistency could result.  The same problem can
   occur in regular NFSv4.  For example, a WRITE is held in a switch for
   some period of time while other writes are issued and replied to, if
   the original WRITE finally succeeds, the same issues can occur.
   However, this is solved by sessions in NFSv4.x.

4.  Security Considerations

   The pNFS extension partitions the NFSv4 file system protocol into two
   parts, the control path and the data path (i.e., storage protocol).
   The control path contains all the new operations described by this
   extension; all existing NFSv4 security mechanisms and features apply
   to the control path.  The combination of components in a pNFS system
   (see Figure 1) is required to preserve the security properties of
   NFSv4 with respect to an entity accessing data via a client,
   including security countermeasures to defend against threats that
   NFSv4 provides defenses for in environments where these threats are
   considered significant.

   In some cases, the security countermeasures for connections to
   storage devices may take the form of physical isolation or a
   recommendation not to use pNFS in an environment.  For example, it is
   currently infeasible to provide confidentiality protection for some
   storage device access protocols to protect against eavesdropping; in
   environments where eavesdropping on such protocols is of sufficient
   concern to require countermeasures, physical isolation of the
   communication channel (e.g., via direct connection from client(s) to
   storage device(s)) and/or a decision to forego use of pNFS (e.g., and
   fall back to NFSv4) may be appropriate courses of action.

   In full generality where communication with storage devices is

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   subject to the same threats as client-server communication, the
   protocols used for that communication need to provide security
   mechanisms comparable to those available via RPSEC_GSS for NFSv4.
   Many situations in which pNFS is likely to be used will not be
   subject to the overall threat profile for which NFSv4 is required to
   provide countermeasures.

   pNFS implementations MUST NOT remove NFSv4's access controls.  The
   combination of clients, storage devices, and the server are
   responsible for ensuring that all client to storage device file data
   access respects NFSv4 ACLs and file open modes.  This entails
   performing both of these checks on every access in the client, the
   storage device, or both.  If a pNFS configuration performs these
   checks only in the client, the risk of a misbehaving client obtaining
   unauthorized access is an important consideration in determining when
   it is appropriate to use such a pNFS configuration.  Such
   configurations SHOULD NOT be used when client- only access checks do
   not provide sufficient assurance that NFSv4 access control is being
   applied correctly.

   The following subsections describe security considerations
   specifically applicable to each of the three major storage device
   protocol types supported for pNFS.

   [Requiring strict equivalence to NFSv4 security mechanisms is the
   wrong approach.  Will need to lay down a set of statements that each
   protocol has to make starting with access check location/properties.]

4.1  File Layout Security

   A NFSv4 file layout type is defined in Section 5; see Section 5.7 for
   additional security considerations and details.  In summary, the
   NFSv4 file layout type requires that all I/O access checks MUST be
   performed by the storage devices, as defined by the NFSv4
   specification.  If another file layout type is being used, additional
   access checks may be required.  But in all cases, the access control
   performed by the storage devices must be at least as strict as that
   specified by the NFSv4 protocol.

4.2  Object Layout Security

   The object storage protocol MUST implement the security aspects
   described in version 1 of the T10 OSD protocol definition [6].  The
   remainder of this section gives an overview of the security mechanism
   described in that standard.  The goal is to give the reader a basic
   understanding of the object security model.  Any discrepancies
   between this text and the actual standard are obviously to be
   resolved in favor of the OSD standard.

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   The object storage protocol relies on a cryptographically secure
   capability to control accesses at the object storage devices.
   Capabilities are generated by the metadata server, returned to the
   client, and used by the client as described below to authenticate
   their requests to the Object Storage Device (OSD).  Capabilities
   therefore achieve the required access and open mode checking.  They
   allow the file server to define and check a policy (e.g., open mode)
   and the OSD to check and enforce that policy without knowing the
   details (e.g., user IDs and ACLs).  Since capabilities are tied to
   layouts, and since they are used to enforce access control, the
   server should recall layouts and revoke capabilities when the file
   ACL or mode changes in order to signal the clients.

   Each capability is specific to a particular object, an operation on
   that object, a byte range w/in the object, and has an explicit
   expiration time.  The capabilities are signed with a secret key that
   is shared by the object storage devices (OSD) and the metadata
   managers. clients do not have device keys so they are unable to forge
   capabilities.  The the following sketch of the algorithm should help
   the reader understand the basic model.

   LAYOUTGET returns

     {CapKey = MAC<SecretKey>(CapArgs), CapArgs}

   The client uses CapKey to sign all the requests it issues for that
   object using the respective CapArgs.  In other words, the CapArgs
   appears in the request to the storage device, and that request is
   signed with the CapKey as follows:

     ReqMAC = MAC<CapKey>(Req, Nonceln)

   The following is sent to the OSD: {CapArgs, Req, Nonceln, ReqMAC}.
   The OSD uses the SecretKey it shares with the metadata server to
   compare the ReqMAC the client sent with a locally computed

     MAC<MAC<SecretKey>(CapArgs)>(Req, Nonceln)

   and if they match the OSD assumes that the capabilities came from an
   authentic metadata server and allows access to the object, as allowed
   by the CapArgs.  Therefore, if the server LAYOUTGET reply, holding
   CapKey and CapArgs, is snooped by another client, it can be used to
   generate valid OSD requests (within the CapArgs access restriction).

   To provide the required privacy requirements for the capabilities
   returned by LAYOUTGET, the GSS-API can be used, e.g. by using a
   session key known to the file server and to the client to encrypt the
   whole layout or parts of it.  Two general ways to provide privacy in

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   the absence of GSS-API that are independent of NFSv4 are either an
   isolated network such as a VLAN or a secure channel provided by

4.3  Block/Volume Layout Security

   As typically used, block/volume protocols rely on clients to enforce
   file access checks since the storage devices are generally unaware of
   the files they are storing and in particular are unaware of which
   blocks belongs to which file.  In such environments, the physical
   addresses of blocks are exported to pNFS clients via layouts.  An
   alternative method of block/volume protocol use is for the storage
   devices to export virtualized block addresses, which do reflect the
   files to which blocks belong.  These virtual block addresses are
   exported to pNFS clients via layouts.  This allows the storage device
   to make appropriate access checks, while mapping virtual block
   addresses to physical block addresses.

   In environments where access control is important and client-only
   access checks provide insufficient assurance of access control
   enforcement (e.g., there is concern about a malicious of
   malfunctioning client skipping the access checks) and where physical
   block addresses are exported to clients, the storage devices will
   generally be unable to compensate for these client deficiencies.

   In such threat environments, block/volume protocols SHOULD NOT be
   used with pNFS, unless the storage device is able to implement the
   appropriate access checks, via use of virtualized block addresses, or
   other means.  NFSv4 without pNFS or pNFS with a different type of
   storage protocol would be a more suitable means to access files in
   such environments.  Storage-device/protocol-specific methods (e.g.
   LUN masking/mapping) may be available to prevent malicious or high-
   risk clients from directly accessing storage devices.

5.  The NFSv4 File Layout Type

   This section describes the semantics and format of NFSv4 file-based

5.1  File Striping and Data Access

   The file layout type describes a method for striping data across
   multiple devices.  The data for each stripe unit is stored within an
   NFSv4 file located on a particular storage device.  The structures
   used to describe the stripe layout are as follows:

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    enum stripetype4 {
           STRIPE_SPARSE = 1,
           STRIPE_DENSE = 2

    struct nfsv4_file_layouthint {
            stripetype4             stripe_type;
            length4                 stripe_unit;
            uint32_t                stripe_width;

    struct nfsv4_file_layout {                   /* Per data stripe */
           pnfs_deviceid4          dev_id<>;
           nfs_fh4                 fh;

    struct nfsv4_file_layouttype4 {              /* Per file */
           stripetype4             stripe_type;
           length4                 stripe_unit;
           length4                 file_size;
           nfsv4_file_layout       dev_list<>;

   The file layout specifies an ordered array of <deviceID, filehandle>
   tuples, as well as the stripe size, type of stripe layout (discussed
   a little later), and the file's current size as of LAYOUTGET time.
   The filehandle, "fh", identifies the file on a storage device
   identified by "dev_id", that holds a particular stripe of the file.
   The "dev_id" array can be used for multipathing and is discussed
   further in Section 5.1.3.  The stripe width is determined by the
   stripe unit size multiplied by the number of devices in the dev_list.
   The stripe held by <dev_id, fh> is determined by that tuples position
   within the device list, "dev_list".  For example, consider a dev_list
   consisting of the following <dev_id, fh> pairs:

   <(1,0x12), (2,0x13), (1,0x15)> and stripe_unit = 32KB

   The stripe width is 32KB * 3 devices = 96KB.  The first entry
   specifies that on device 1 in the data file with filehandle 0x12
   holds the first 32KB of data (and every 32KB stripe beginning where
   the file's offset % 96KB == 0).

   Devices may be repeated multiple times within the device list array;
   this is shown where storage device 1 holds both the first and third
   stripe of data.  Filehandles can only be repeated if a sparse stripe
   type is used.  Data is striped across the devices in the order listed
   in the device list array in increments of the stripe size.  A data
   file stored on a storage device MUST map to a single file as defined

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   by the metadata server; i.e., data from two files as viewed by the
   metadata server MUST NOT be stored within the same data file on any
   storage device.

   The "stripe_type" field specifies how the data is laid out within the
   data file on a storage device.  It allows for two different data
   layouts: sparse and dense or packed.  The stripe type determines the
   calculation that must be made to map the client visible file offset
   to the offset within the data file located on the storage device.

   The layout hint structure is described in more detail in Section 6.7.
   It is used, by the client, as by the FILE_LAYOUT_HINT attribute to
   specify the type of layout to be used for a newly created file.

5.1.1  Sparse and Dense Storage Device Data Layouts

   The stripe_type field allows for two storage device data file
   representations.  Example sparse and dense storage device data
   layouts are illustrated below:

    Sparse file-layout (stripe_unit = 4KB)

    Is represented by the following file layout on the storage devices:

        Offset  ID:0    ID:1   ID:2
        0       +--+    +--+   +--+                 +--+  indicates a
                |//|    |  |   |  |                 |//|  stripe that
        4KB     +--+    +--+   +--+                 +--+  contains data
                |  |    |//|   |  |
        8KB     +--+    +--+   +--+
                |  |    |  |   |//|
        12KB    +--+    +--+   +--+
                |//|    |  |   |  |
        16KB    +--+    +--+   +--+
                |  |    |//|   |  |
                +--+    +--+   +--+

   The sparse file-layout has holes for the byte ranges not exported by
   that storage device.  This allows clients to access data using the
   real offset into the file, regardless of the storage device's
   position within the stripe.  However, if a client writes to one of
   the holes (e.g., offset 4-12KB on device 1), then an error MUST be
   returned by the storage device.  This requires that the storage
   device have knowledge of the layout for each file.

   When using a sparse layout, the offset into the storage device data
   file is the same as the offset into the main file.

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    Dense/packed file-layout (stripe_unit = 4KB)

    Is represented by the following file layout on the storage devices:

        Offset  ID:0    ID:1   ID:2
        0       +--+    +--+   +--+
                |//|    |//|   |//|
        4KB     +--+    +--+   +--+
                |//|    |//|   |//|
        8KB     +--+    +--+   +--+
                |//|    |//|   |//|
        12KB    +--+    +--+   +--+
                |//|    |//|   |//|
        16KB    +--+    +--+   +--+
                |//|    |//|   |//|
                +--+    +--+   +--+

   The dense or packed file-layout does not leave holes on the storage
   devices.  Each stripe unit is spread across the storage devices.  As
   such, the storage devices need not know the file's layout since the
   client is allowed to write to any offset.

   The calculation to determine the byte offset within the data file for
   dense storage device layouts is:

     stripe_width = stripe_unit * N; where N = |dev_list|
     dev_offset = floor(file_offset / stripe_width) * stripe_unit +
                  file_offset % stripe_unit

   Regardless of the storage device data file layout, the calculation to
   determine the index into the device array is the same:

     dev_idx = floor(file_offset / stripe_unit) mod N

   Section 5.5 describe the semantics for dealing with reads to holes
   within the striped file.  This is of particular concern, since each
   individual component stripe file (i.e., the component of the striped
   file that lives on a particular storage device) may be of different
   length.  Thus, clients may experience 'short' reads when reading off
   the end of one of these component files.

5.1.2  Metadata and Storage Device Roles

   In many cases, the metadata server and the storage device will be
   separate pieces of physical hardware.  The specification text is
   written as if that were always case.  However, it can be the case
   that the same physical hardware is used to implement both a metadata

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   and storage device and in this case, the specification text's
   references to these two entities are to be understood as referring to
   the same physical hardware implementing two distinct roles and it is
   important that it be clearly understood on behalf of which role the
   hardware is executing at any given time.

   Two sub-cases can be distinguished.  In the first sub-case, the same
   physical hardware is used to implement both a metadata and data
   server in which each role is addressed through a distinct network
   interface (e.g., IP addresses for the metadata server and storage
   device are distinct).  As long as the storage device address is
   obtained from the layout and is distinct from the metadata server's
   address, using the device ID therein to obtain the appropriate
   storage device address, it is always clear, for any given request, to
   what role it is directed, based on the destination IP address.

   However, it may also be the case that even though the metadata server
   and storage device are distinct from one client's point of view, the
   roles may be reversed according to another client's point of view.
   For example, in the cluster file system model a metadata server to
   one client, may be a storage device to another client.  Thus, it is
   safer to always mark the filehandle so that operations addressed to
   storage devices can be distinguished.

   The second sub-case is where both the metadata and storage device
   have the same network address.  This requires us to make the
   distinction as to which role each request is directed, on a another
   basis.  Since the network address is the same, the request is
   understood as being directed at one or the other, based on the
   filehandle of the first current filehandle value for the request.  If
   the first current file handle is one derived from a layout (i.e., it
   is specified within the layout) (and it is recommended that these be
   distinguishable), then the request is to be considered as executed by
   a storage device.  Otherwise, the operation is to be understood as
   executed by the metadata server.

   If a current filehandle is set that is inconsistent with the role to
   which it is directed, then the error NFS4ERR_BADHANDLE should result.
   For example, if a request is directed at the storage device, because
   the first current handle is from a layout, any attempt to set the
   current filehandle to be a value not from a layout should be
   rejected.  Similarly, if the first current file handle was for a
   value not from a layout, a subsequent attempt to set the current file
   handle to a value obtained from a layout should be rejected.

5.1.3  Device Multipathing

   The NFSv4 file layout supports multipathing to 'equivalent' devices.

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   Device-level multipathing is primarily of use in the case of a data
   server failure --- it allows the client to switch to another storage
   device that is exporting the same data stripe, without having to
   contact the metadata server for a new layout.

   To support device multipathing, an array of device IDs is encoded
   within the data stripe portion of the file's layout.  This array
   represents an ordered list of devices where the first element has the
   highest priority.  Each device in the list MUST be 'equivalent' to
   every other device in the list and each device must be attempted in
   the order specified.

   Equivalent devices MUST export the same system image (e.g., the
   stateids and filehandles that they use are the same) and must provide
   the same consistency guarantees.  Two equivalent storage devices must
   also have sufficient connections to the storage, such that writing to
   one storage device is equivalent to writing to another, this also
   applies to reading.  Also, if multiple copies of the same data exist,
   reading from one must provide access to all existing copies.  As
   such, it is unlikely that multipathing will provide additional
   benefit in the case of an I/O error.

   [NOTE: the error cases in which a client is expected to attempt an
   equivalent storage device should be specified.]

5.1.4  Operations Issued to Storage Devices

   Clients MUST use the filehandle described within the layout when
   accessing data on the storage devices.  When using the layout's
   filehandle, the client MUST only issue READ, WRITE, PUTFH, COMMIT,
   and NULL operations to the storage device associated with that
   filehandle.  If a client issues an operation other than those
   specified above, using the filehandle and storage device listed in
   the client's layout, that storage device SHOULD return an error to
   the client.  The client MUST follow the instruction implied by the
   layout (i.e., which filehandles to use on which devices).  As
   described in Section 3.2, a client MUST NOT issue I/Os to storage
   devices for which it does not hold a valid layout.  The storage
   devices may reject such requests.

   GETATTR and SETATTR MUST be directed to the metadata server.  In the
   case of a SETATTR of the size attribute, the control protocol is
   responsible for propagating size updates/truncations to the storage
   devices.  In the case of extending WRITEs to the storage devices, the
   new size must be visible on the metadata server once a LAYOUTCOMMIT
   has completed (see Section 3.4.2).  Section 5.5, describes the
   mechanism by which the client is to handle storage device file's that
   do not reflect the metadata server's size.

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5.2  Global Stateid Requirements

   Note, there are no stateids returned embedded within the layout.  The
   client MUST use the stateid representing open or lock state as
   returned by an earlier metadata operation (e.g., OPEN, LOCK), or a
   special stateid to perform I/O on the storage devices, as in regular
   NFSv4.  Special stateid usage for I/O is subject to the NFSv4
   protocol specification.  The stateid used for I/O MUST have the same
   effect and be subject to the same validation on storage device as it
   would if the I/O was being performed on the metadata server itself in
   the absence of pNFS.  This has the implication that stateids are
   globally valid on both the metadata and storage devices.  This
   requires the metadata server to propagate changes in lock and open
   state to the storage devices, so that the storage devices can
   validate I/O accesses.  This is discussed further in Section 5.4.
   Depending on when stateids are propagated, the existence of a valid
   stateid on the storage device may act as proof of a valid layout.

   [NOTE: a number of proposals have been made that have the possibility
   of limiting the amount of validation performed by the storage device,
   if any of these proposals are accepted or obtain consensus, the
   global stateid requirement can be revisited.]

5.3  The Layout Iomode

   The layout iomode need not used by the metadata server when servicing
   NFSv4 file-based layouts, although in some circumstances it may be
   useful to use.  For example, if the server implementation supports
   reading from read-only replicas or mirrors, it would be useful for
   the server to return a layout enabling the client to do so.  As such,
   the client should set the iomode based on its intent to read or write
   the data.  The client may default to an iomode of READ/WRITE
   (LAYOUTIOMODE_RW).  The iomode need not be checked by the storage
   devices when clients perform I/O. However, the storage devices SHOULD
   still validate that the client holds a valid layout and return an
   error if the client does not.

5.4  Storage Device State Propagation

   Since the metadata server, which handles lock and open-mode state
   changes, as well as ACLs, may not be collocated with the storage
   devices where I/O access are validated, as such, the server
   implementation MUST take care of propagating changes of this state to
   the storage devices.  Once the propagation to the storage devices is
   complete, the full effect of those changes must be in effect at the
   storage devices.  However, some state changes need not be propagated
   immediately, although all changes SHOULD be propagated promptly.
   These state propagations have an impact on the design of the control

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   protocol, even though the control protocol is outside of the scope of
   this specification.  Immediate propagation refers to the synchronous
   propagation of state from the metadata server to the storage
   device(s); the propagation must be complete before returning to the

5.4.1  Lock State Propagation

   Mandatory locks MUST be made effective at the storage devices before
   the request that establishes them returns to the caller.  Thus,
   mandatory lock state MUST be synchronously propagated to the storage
   devices.  On the other hand, since advisory lock state is not used
   for checking I/O accesses at the storage devices, there is no
   semantic reason for propagating advisory lock state to the storage
   devices.  However, since all lock, unlock, open downgrades and
   upgrades affect the sequence ID stored within the stateid, the
   stateid changes which may cause difficulty if this state is not
   propagated.  Thus, when a client uses a stateid on a storage device
   for I/O with a newer sequence number than the one the storage device
   has, the storage device should query the metadata server and get any
   pending updates to that stateid.  This allows stateid sequence number
   changes to be propagated lazily, on-demand.

   [NOTE: With the reliance on the sessions protocol, there is no real
   need for sequence ID portion of the stateid to be validated on I/O
   accesses.  It is proposed that the seq.  ID checking is obsoleted.]

   Since updates to advisory locks neither confer nor remove privileges,
   these changes need not be propagated immediately, and may not need to
   be propagated promptly.  The updates to advisory locks need only be
   propagated when the storage device needs to resolve a question about
   a stateid.  In fact, if byte-range locking is not mandatory (i.e., is
   advisory) the clients are advised not to use the lock-based stateids
   for I/O at all.  The stateids returned by open are sufficient and
   eliminate overhead for this kind of state propagation.

5.4.2  Open-mode Validation

   Open-mode validation MUST be performed against the open mode(s) held
   by the storage devices.  However, the server implementation may not
   always require the immediate propagation of changes.  Reduction in
   access because of CLOSEs or DOWNGRADEs do not have to be propagated
   immediately, but SHOULD be propagated promptly; whereas changes due
   to revocation MUST be propagated immediately.  On the other hand,
   changes that expand access (e.g., new OPEN's and upgrades) don't have
   to be propagated immediately but the storage device SHOULD NOT reject
   a request because of mode issues without making sure that the upgrade
   is not in flight.

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5.4.3  File Attributes

   Since the SETATTR operation has the ability to modify state that is
   visible on both the metadata and storage devices (e.g., the size),
   care must be taken to ensure that the resultant state across the set
   of storage devices is consistent; especially when truncating or
   growing the file.

   As described earlier, the LAYOUTCOMMIT operation is used to ensure
   that the metadata is synced with changes made to the storage devices.
   For the file-based protocol, it is necessary to re-sync state such as
   the size attribute, and the setting of mtime/atime.  See Section 3.4
   for a full description of the semantics regarding LAYOUTCOMMIT and
   attribute synchronization.  It should be noted, that by using a file-
   based layout type, it is possible to synchronize this state before
   LAYOUTCOMMIT occurs.  For example, the control protocol can be used
   to query the attributes present on the storage devices.

   Any changes to file attributes that control authorization or access
   as reflected by ACCESS calls or READs and WRITEs on the metadata
   server, MUST be propagated to the storage devices for enforcement on
   READ and WRITE I/O calls.  If the changes made on the metadata server
   result in more restrictive access permissions for any user, those
   changes MUST be propagated to the storage devices synchronously.

   Recall that the NFSv4 protocol [2] specifies that:

      ...since the NFS version 4 protocol does not impose any
      requirement that READs and WRITEs issued for an open file have the
      same credentials as the OPEN itself, the server still must do
      appropriate access checking on the READs and WRITEs themselves.

   This also includes changes to ACLs.  The propagation of access right
   changes due to changes in ACLs may be asynchronous only if the server
   implementation is able to determine that the updated ACL is not more
   restrictive for any user specified in the old ACL.  Due to the
   relative infrequency of ACL updates, it is suggested that all changes
   be propagated synchronously.

   [NOTE: it has been suggested that the NFSv4 specification is in error
   with regard to allowing principles other than those used for OPEN to
   be used for file I/O. If changes within a minor version alter the
   behavior of NFSv4 with regard to OPEN principals and stateids some
   access control checking at the storage device can be made less
   expensive. pNFS should be altered to take full advantage of these

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5.5  Storage Device Component File Size

   A potential problem exists when a component data file on a particular
   storage device is grown past EOF; the problem exists for both dense
   and sparse layouts.  Imagine the following scenario: a client creates
   a new file (size == 0) and writes to byte 128KB; the client then
   seeks to the beginning of the file and reads byte 100.  The client
   should receive 0s back as a result of the read.  However, if the read
   falls on a different storage device to the client's original write,
   the storage device servicing the READ may still believe that the
   file's size is at 0 and return no data with the EOF flag set.  The
   storage device can only return 0s if it knows that the file's size
   has been extended.  This would require the immediate propagation of
   the file's size to all storage devices, which is potentially very
   costly, instead, another approach as outlined below.

   First, the file's size is returned within the layout by LAYOUTGET.
   This size must reflect the latest size at the metadata server as set
   by the most recent of either the last LAYOUTCOMMIT or SETATTR;
   however, it may be more recent.  Second, if a client performs a read
   that is returned short (i.e., is fully within the file's size, but
   the storage device indicates EOF and returns partial or no data), the
   client must assume that it is a hole and substitute 0s for the data
   not read up until its known local file size.  If a client extends the
   file, it must update its local file size.  Third, if the metadata
   server receives a SETATTR of the size or a LAYOUTCOMMIT that alters
   the file's size, the metadata server must send out CB_SIZECHANGED
   messages with the new size to clients holding layouts; it need not
   send a notification to the client that performed the operation that
   resulted in the size changing).  Upon reception of the CB_SIZECHANGED
   notification, clients must update their local size for that file.  As
   well, if a new file size is returned as a result to LAYOUTCOMMIT, the
   client must update their local file size.

5.6  Crash Recovery Considerations

   As described in Section 3.7, the layout type specific storage
   protocol is responsible for handling the effects of I/Os started
   before lease expiration, extending through lease expiration.  The
   NFSv4 file layout type prevents all I/Os from being executed after
   lease expiration, without relying on a precise client lease timer and
   without requiring storage devices to maintain lease timers.

   It works as follows.  In the presence of sessions, each compound
   begins with a SEQUENCE operation that contains the "clientID".  On
   the storage device, the clientID can be used to validate that the
   client has a valid layout for the I/O being performed, if it does
   not, the I/O is rejected.  Before the metadata server takes any

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   action to invalidate a layout given out by a previous instance, it
   must make sure that all layouts from that previous instance are
   invalidated at the storage devices.  Note: it is sufficient to
   invalidate the stateids associated with the layout only if special
   stateids are not being used for I/O at the storage devices, otherwise
   the layout itself must be invalidated.

   This means that a metadata server may not restripe a file until it
   has contacted all of the storage devices to invalidate the layouts
   from the previous instance nor may it give out locks that conflict
   with locks embodied by the stateids associated with any layout from
   the previous instance without either doing a specific invalidation
   (as it would have to do anyway) or doing a global storage device

5.7  Security Considerations

   The NFSv4 file layout type MUST adhere to the security considerations
   outlined in Section 4.  More specifically, storage devices must make
   all of the required access checks on each READ or WRITE I/O as
   determined by the NFSv4 protocol [2].  This impacts the control
   protocol and the propagation of state from the metadata server to the
   storage devices; see Section 5.4 for more details.

5.8  Alternate Approaches

   Two alternate approaches exist for file-based layouts and the method
   used by clients to obtain stateids used for I/O. Both approaches
   embed stateids within the layout.

   However, before examining these approaches it is important to
   understand the distinction between clients and owners.  Delegations
   belong to clients, while locks (e.g., record and share reservations)
   are held by owners which in turn belong to a specific client.  As
   such, delegations can only protect against inter-client conflicts,
   not intra-client conflicts.  Layouts are held by clients and SHOULD
   NOT be associated with state held by owners.  Therefore, if stateids
   used for data access are embedded within a layout, these stateids can
   only act as delegation stateids, protecting against inter-client
   conflicts; stateids pertaining to an owner can not be embedded within
   the layout.  This has the implication that the client MUST arbitrate
   among all intra-client conflicts (e.g., arbitrating among lock
   requests by different processes) before issuing pNFS operations.
   Using the stateids stored within the layout, storage devices can only
   arbitrate between clients (not owners).

   The first alternate approach is to do away with global stateids,
   stateids returned by OPEN/LOCK that are valid on the metadata server

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   and storage devices, and use only stateids embedded within the
   layout.  This approach has the drawback that the stateids used for
   I/O access can not be validated against per owner state, since they
   are only associated with the client holding the layout.  It breaks
   the semantics of tieing a stateid used for I/O to an open instance.
   This has the implication that clients must delegate per owner lock
   and open requests internally, rather than push the work onto the
   storage devices.  The storage devices can still arbitrate and enforce
   inter-client lock and open state.

   The second approach is a hybrid approach.  This approach allows for
   stateids to be embedded with the layout, but also allows for the
   possibility of global stateids.  If the stateid embedded within the
   layout is a special stateid of all zeros, then the stateid referring
   to the last successful OPEN/LOCK should be used.  This approach is
   recommended if it is decided that using NFSv4 as a control protocol
   is required.

   This proposal suggests the global stateid approach due to the cleaner
   semantics it provides regarding the relationship between stateids
   used for I/O and their corresponding open instance or lock state.
   However, it does have a profound impact on the control protocol's
   implementation and the state propagation that is required (as
   described in Section 5.4).

6.  pNFS Typed Data Structures

6.1  pnfs_layouttype4

     enum pnfs_layouttype4 {
            LAYOUT_NFSV4_FILES = 1,
            LAYOUT_OSD2_OBJECTS = 2,
            LAYOUT_BLOCK_VOLUME = 3

   A layout type specifies the layout being used.  The implication is
   that clients have "layout drivers" that support one or more layout
   types.  The file server advertises the layout types it supports
   through the LAYOUT_TYPES file system attribute.  A client asks for
   layouts of a particular type in LAYOUTGET, and passes those layouts
   to its layout driver.  The set of well known layout types must be
   defined.  As well, a private range of layout types is to be defined
   by this document.  This would allow custom installations to introduce
   new layout types.

   [OPEN ISSUE: Determine private range of layout types]

   New layout types must be specified in RFCs approved by the IESG

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   before becoming part of the pNFS specification.

   The LAYOUT_NFSV4_FILES enumeration specifies that the NFSv4 file
   layout type is to be used.  The LAYOUT_OSD2_OBJECTS enumeration
   specifies that the object layout, as defined in [8], is to be used.
   Similarly, the LAYOUT_BLOCK_VOLUME enumeration that the block/volume
   layout, as defined in [7], is to be used.

6.2  pnfs_deviceid4

     typedef uint64_t pnfs_deviceid4;       /* 64-bit device ID */

   Layout information includes device IDs that specify a storage device
   through a compact handle.  Addressing and type information is
   obtained with the GETDEVICEINFO operation.  A client must not assume
   that device IDs are valid across metadata server reboots.  The device
   ID is qualified by the layout type and are unique per file system
   (FSID).  This allows different layout drivers to generate device IDs
   without the need for co-ordination.  See Section 3.1.4 for more

6.3  pnfs_deviceaddr4

     struct pnfs_netaddr4 {
              string           r_netid<>;   /* network ID */
              string           r_addr<>;    /* universal address */

     union pnfs_deviceaddr4 switch (pnfs_layouttype4 layout_type) {
            case LAYOUT_NFSV4_FILES:
                   pnfs_netaddr4    netaddr;
                   opaque           device_addr<>; /* Other layouts */

   The device address is used to set up a communication channel with the
   storage device.  Different layout types will require different types
   of structures to define how they communicate with storage devices.
   The union is switched on the layout type.

   Currently, the only device address defined is that for the NFSv4 file
   layout, which identifies a storage device by network IP address and
   port number.  This is sufficient for the clients to communicate with
   the NFSv4 storage devices, and may also be sufficient for object-
   based storage drivers to communicate with OSDs.  The other device
   address we expect to support is a SCSI volume identifier.  The final
   protocol specification will detail the allowed values for device_type
   and the format of their associated location information.

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   [NOTE: other device addresses will be added as the respective
   specifications mature.  It has been suggested that a separate
   device_type enumeration is used as a switch to the pnfs_deviceaddr4
   structure (e.g., if multiple types of addresses exist for the same
   layout type).  Until such a time as a real case is made and the
   respective layout types have matured, the device address structure
   will be left as is.]

6.4  pnfs_devlist_item4

     struct pnfs_devlist_item4 {
            pnfs_deviceid4          id;
            pnfs_deviceaddr4        addr;

   An array of these values is returned by the GETDEVICELIST operation.
   They define the set of devices associated with a file system.

6.5  pnfs_layout4

     union pnfs_layoutdata4 switch (pnfs_layouttype4 layout_type) {
            case LAYOUT_NFSV4_FILES:
                   nfsv4_file_layouttype4 file_layout;
                   opaque           layout_data<>;

     struct pnfs_layout4 {
            offset4                 offset;
            length4                 length;
            pnfs_layoutiomode4      iomode;
            pnfs_layoutdata4        layout;

   The pnfs_layout4 structure defines a layout for a file.  The
   pnfs_layoutdata4 union contains the portion of the layout specific to
   the layout type.  Currently, only the NFSv4 file layout type is
   defined; see Section 5.1 for its definition.  Since layouts are sub-
   dividable, the offset and length together with the file's filehandle,
   the clientid, iomode, and layout type, identifies the layout.

   [OPEN ISSUE: there is a discussion of moving the striping
   information, or more generally the "aggregation scheme", up to the
   generic layout level.  This creates a two-layer system where the top
   level is a switch on different data placement layouts, and the next
   level down is a switch on different data storage types.  This lets
   different layouts (e.g., striping or mirroring or redundant servers)
   to be layered over different storage devices.  This would move

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   geometry information out of nfsv4_file_layouttype4 and up into a
   generic pnfs_striped_layout type that would specify a set of
   pnfs_deviceid4 and pnfs_devicetype4 to use for storage.  Instead of
   nfsv4_file_layouttype4, there would be pnfs_nfsv4_devicetype4.]

6.6  pnfs_layoutupdate4

     union pnfs_layoutupdate4 switch (pnfs_layouttype4 layout_type) {
            case LAYOUT_NFSV4_FILES:
                   opaque           layout_data<>;

   The pnfs_layoutupdate4 structure is used by the client to return
   'updated' layout information to the metadata server at LAYOUTCOMMIT
   time.  This provides a channel to pass layout type specific
   information back to the metadata server.  E.g., for block/volume
   layout types this could include the list of reserved blocks that were
   written.  The contents of the structure are determined by the layout
   type and are defined in their context.

6.7  pnfs_layouthint4

     union pnfs_layouthint4 switch (pnfs_layouttype4 layout_type) {
            case LAYOUT_NFSV4_FILES:
                   nfsv4_file_layouthint layout_hint;
                   opaque                layout_hint_data<>;

   The pnfs_layouthint4 structure is used by the client to pass in a
   hint about the type of layout it would like created for a particular
   file.  It is the structure specified by the FILE_LAYOUT_HINT
   attribute described below.  The metadata server may ignore the hint,
   or may selectively ignore fields within the hint.  This hint should
   be provided at create time as part of the initial attributes within
   OPEN.  The "nfsv4_file_layouthint" structure is defined in
   Section 5.1.

6.8  pnfs_layoutiomode4

     enum pnfs_layoutiomode4 {
             LAYOUTIOMODE_READ          = 1,
             LAYOUTIOMODE_RW            = 2,
             LAYOUTIOMODE_ANY           = 3

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   The iomode specifies whether the client intends to read or write
   (with the possibility of reading) the data represented by the layout.
   The ANY iomode MUST NOT be used for LAYOUTGET, however, it can be
   used for LAYOUTRETURN and LAYOUTRECALL.  The ANY iomode specifies
   that layouts pertaining to both READ and RW iomodes are being
   returned or recalled, respectively.  The metadata server's use of the
   iomode may depend on the layout type being used.  The storage devices
   may validate I/O accesses against the iomode and reject invalid

7.  pNFS File Attributes

7.1  pnfs_layouttype4<> FS_LAYOUT_TYPES

   This attribute applies to a file system and indicates what layout
   types are supported by the file system.  We expect this attribute to
   be queried when a client encounters a new fsid.  This attribute is
   used by the client to determine if it has applicable layout drivers.

7.2  pnfs_layouttype4<> FILE_LAYOUT_TYPES

   This attribute indicates the particular layout type(s) used for a
   file.  This is for informational purposes only.  The client needs to
   use the LAYOUTGET operation in order to get enough information (e.g.,
   specific device information) in order to perform I/O.

7.3  pnfs_layouthint4 FILE_LAYOUT_HINT

   This attribute may be set on newly created files to influence the
   metadata server's choice for the file's layout.  It is suggested that
   this attribute is set as one of the initial attributes within the
   OPEN call.  The metadata server may ignore this attribute.  This
   attribute is a sub-set of the layout structure returned by LAYOUTGET.
   For example, instead of specifying particular devices, this would be
   used to suggest the stripe width of a file.  It is up to the server
   implementation to determine which fields within the layout it uses.

   [OPEN ISSUE: it has been suggested that the HINT is a well defined
   type other than pnfs_layoutdata4, similar to pnfs_layoutupdate4.]


   This attribute is a file system wide attribute and indicates the
   preferred block size for direct storage device access.


   This attribute is a file system wide attribute and indicates the

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   preferred alignment for direct storage device access.

8.  pNFS Error Definitions

   NFS4ERR_BADLAYOUT Layout specified is invalid.

   NFS4ERR_BADIOMODE Layout iomode is invalid.

   NFS4ERR_LAYOUTUNAVAILABLE Layouts are not available for the file or
      its containing file system.

   NFS4ERR_LAYOUTTRYLATER Layouts are temporarily unavailable for the
      file, client should retry later.

   NFS4ERR_NOMATCHING_LAYOUT Client has no matching layout (segment) to

   NFS4ERR_RECALLCONFLICT Layout is unavailable due to a conflicting
      LAYOUTRECALL that is in progress.

   NFS4ERR_UNKNOWN_LAYOUTTYPE Layout type is unknown.

9.  pNFS Operations

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9.1  LAYOUTGET - Get Layout Information


     (cfh), clientid, layout_type, iomode, offset, length,
     minlength, maxcount -> layout


     struct LAYOUTGET4args {
             /* CURRENT_FH: file */
             clientid4               clientid;
             pnfs_layouttype4        layout_type;
             pnfs_layoutiomode4      iomode;
             offset4                 offset;
             length4                 length;
             length4                 minlength;
             count4                  maxcount;


     struct LAYOUTGET4resok {
             pnfs_layout4            layout;

     union LAYOUTGET4res switch (nfsstat4 status) {
             case NFS4_OK:
                     LAYOUTGET4resok resok4;


   Requests a layout for reading or writing (and reading) the file given
   by the filehandle at the byte range specified by offset and length.
   Layouts are identified by the clientid, filehandle, and layout type.
   The use of the iomode depends upon the layout type, but should
   reflect the client's data access intent.

   The LAYOUTGET operation returns layout information for the specified
   byte range, a layout segment.  To get a layout segment from a
   specific offset through the end-of-file, regardless of the file's
   length, a length field with all bits set to 1 (one) should be used.
   If the length is zero, or if a length which is not all bits set to
   one is specified, and length when added to the offset exceeds the
   maximum 64-bit unsigned integer value, the error NFS4ERR_INVAL will

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   The "minlength" field specifies the minimum size overlap with the
   requested offset and length that is to be returned.  If this
   requirement cannot be met, no layout must be returned; the error
   NFS4ERR_LAYOUTTRYLATER can be returned.

   The "maxcount" field specifies the maximum layout size (in bytes)
   that the client can handle.  If the size of the layout structure
   exceeds the size specified by maxcount, the metadata server will
   return the NFS4ERR_TOOSMALL error.

   As well, the metadata server may adjust the range of the returned
   layout segment based on striping patterns and usage implied by the
   iomode.  The client must be prepared to get a layout that does not
   line up exactly with their request; there MUST be at least an overlap
   of "minlength" between the layout returned by the server and the
   client's request, or the server SHOULD reject the request.  See
   Section 3.3 for more details.

   The metadata server may also return a layout segment with an iomode
   other than that requested by the client.  If it does so, it must
   ensure that the iomode is more permissive than the iomode requested.
   E.g., this allows an implementation to upgrade read-only requests to
   read/write requests at its discretion, within the limits of the
   layout type specific protocol.  An iomode of either LAYOUTIOMODE_READ
   or LAYOUTIOMODE_RW must be returned.

   The format of the returned layout is specific to the underlying file
   system.  Layout types other than the NFSv4 file layout type should be
   specified outside of this document.

   If layouts are not supported for the requested file or its containing
   file system the server SHOULD return NFS4ERR_LAYOUTUNAVAILABLE.  If
   the layout type is not supported, the metadata server should return
   NFS4ERR_UNKNOWN_LAYOUTTYPE.  If layouts are supported but no layout
   matches the client provided layout identification, the server should
   return NFS4ERR_BADLAYOUT.  If an invalid iomode is specified, or an
   iomode of LAYOUTIOMODE_ANY is specified, the server should return

   If the layout for the file is unavailable due to transient
   conditions, e.g. file sharing prohibits layouts, the server must

   If the layout request is rejected due to an overlapping layout
   recall, the server must return NFS4ERR_RECALLCONFLICT.  See
   Section 3.5.3 for details.

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   If the layout conflicts with a mandatory byte range lock held on the
   file, and if the storage devices have no method of enforcing
   mandatory locks, other than through the restriction of layouts, the
   metadata server should return NFS4ERR_LOCKED.

   On success, the current filehandle retains its value.


   Typically, LAYOUTGET will be called as part of a compound RPC after
   an OPEN operation and results in the client having location
   information for the file; a client may also hold a layout across
   multiple OPENs.  The client specifies a layout type that limits what
   kind of layout the server will return.  This prevents servers from
   issuing layouts that are unusable by the client.



9.2  LAYOUTCOMMIT - Commit writes made using a layout

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     (cfh), clientid, offset, length, last_write_offset,
     time_modify, time_access, layoutupdate -> newsize


     union newtime4 switch (bool timechanged) {
             case TRUE:
                     nfstime4           time;
             case FALSE:

     union newsize4 switch (bool sizechanged) {
             case TRUE:
                     length4            size;
             case FALSE:

     struct LAYOUTCOMMIT4args {
             /* CURRENT_FH: file */
             clientid4               clientid;
             offset4                 offset;
             length4                 length;
             length4                 last_write_offset;
             newtime4                time_modify;
             newtime4                time_access;
             pnfs_layoutupdate4      layoutupdate;


     struct LAYOUTCOMMIT4resok {
            newsize4                 newsize;

     union LAYOUTCOMMIT4res switch (nfsstat4 status) {
             case NFS4_OK:
                     LAYOUTCOMMIT4resok  resok4;


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   Commits changes in the layout segment represented by the current
   filehandle, clientid, and byte range.  Since layouts are sub-
   dividable, a smaller portion of a layout, retrieved via LAYOUTGET,
   may be committed.  The region being committed is specified through
   the byte range (length and offset).  Note: the "layoutupdate"
   structure does not include the length and offset, as they are already
   specified in the arguments.

   The LAYOUTCOMMIT operation indicates that the client has completed
   writes using a layout obtained by a previous LAYOUTGET.  The client
   may have only written a subset of the data range it previously
   requested.  LAYOUTCOMMIT allows it to commit or discard provisionally
   allocated space and to update the server with a new end of file.  The
   layout referenced by LAYOUTCOMMIT is still valid after the operation
   completes and can be continued to be referenced by the clientid,
   filehandle, byte range, and layout type.

   The "last_write_offset" field specifies the offset of the last byte
   written by the client previous to the LAYOUTCOMMIT.  Note: this value
   is never equal to the file's size (at most it is one byte less than
   the file's size).  The metadata server may use this information to
   determine whether the file's size needs to be updated.  If the
   metadata server updates the file's size as the result of the
   LAYOUTCOMMIT operation, it must return the new size as part of the

   The "time_modify" and "time_access" fields allow the client to
   suggest times it would like the metadata server to set.  The metadata
   server may use these time values or it may use the time of the
   LAYOUTCOMMIT operation to set these time values.  If the metadata
   server uses the client provided times, it should sanity check the
   values (e.g., to ensure time does not flow backwards).  If the client
   wants to force the metadata server to set an exact time, the client
   should use a SETATTR operation in a compound right after
   LAYOUTCOMMIT.  See Section 3.4 for more details.  If the new client
   desires the resultant mtime or atime, it should issue a GETATTR
   following the LAYOUTCOMMIT; e.g., later in the same compound.

   The "layoutupdate" argument to LAYOUTCOMMIT provides a mechanism for
   a client to provide layout specific updates to the metadata server.
   For example, the layout update can describe what regions of the
   original layout have been used and what regions can be deallocated.
   There is no NFSv4 file layout specific layoutupdate structure.

   The layout information is more verbose for block devices than for
   objects and files because the latter hide the details of block
   allocation behind their storage protocols.  At the minimum, the
   client needs to communicate changes to the end of file location back

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   to the server, and, if desired, its view of the file modify and
   access time.  For block/volume layouts, it needs to specify precisely
   which blocks have been used.

   If the layout identified in the arguments does not exist, the error
   NFS4ERR_BADLAYOUT is returned.  The layout being committed may also
   be rejected if it does not correspond to an existing layout with an
   iomode of RW.

   On success, the current filehandle retains its value.



9.3  LAYOUTRETURN - Release Layout Information


     (cfh), clientid, offset, length, iomode, layout_type -> -


     struct LAYOUTRETURN4args {
             /* CURRENT_FH: file */
             clientid4               clientid;
             offset4                 offset;
             length4                 length;
             pnfs_layoutiomode4      iomode;
             pnfs_layouttype4        layout_type;


     struct LAYOUTRETURN4res {
             nfsstat4        status;


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   Returns the layout segment represented by the current filehandle,
   clientid, byte range, iomode, and layout type.  After this call, the
   client MUST NOT use the layout and the associated storage protocol to
   access the file data.  The layout being returned may be a subdivision
   of a layout previously fetched through LAYOUTGET.  As well, it may be
   a subset or superset of a layout specified by CB_LAYOUTRECALL.
   However, if it is a subset, the recall is not complete until the full
   byte range has been returned.  It is also permissible, and no error
   should result, for a client to return a byte range covering a layout
   it does not hold.  If the length is all 1s, the layout covers the
   range from offset to EOF.  An iomode of ANY specifies that all
   layouts that match the other arguments to LAYOUTRETURN (i.e.,
   clientid, byte range, and type) are being returned.

   Layouts may be returned when recalled or voluntarily (i.e., before
   the server has recalled them).  In either case the client must
   properly propagate state changed under the context of the layout to
   storage or to the server before returning the layout.

   If a client fails to return a layout in a timely manner, then the
   metadata server should use its control protocol with the storage
   devices to fence the client from accessing the data referenced by the
   layout.  See Section 3.5 for more details.

   If the layout identified in the arguments does not exist, the error
   NFS4ERR_BADLAYOUT is returned.  If a layout exists, but the iomode
   does not match, NFS4ERR_BADIOMODE is returned.

   On success, the current filehandle retains its value.

   [OPEN ISSUE: Should LAYOUTRETURN be modified to handle FSID



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9.4  GETDEVICEINFO - Get Device Information


     (cfh), device_id, layout_type, maxcount -> device_addr


     struct GETDEVICEINFO4args {
             /* CURRENT_FH: file */
             pnfs_deviceid4                  device_id;
             pnfs_layouttype4                layout_type;
             count4                          maxcount;


     struct GETDEVICEINFO4resok {
             pnfs_deviceaddr4                device_addr;

     union GETDEVICEINFO4res switch (nfsstat4 status) {
             case NFS4_OK:
                     GETDEVICEINFO4resok     resok4;


   Returns device type and device address information for a specified
   device.  The returned device_addr includes a type that indicates how
   to interpret the addressing information for that device.  The current
   filehandle (cfh) is used to identify the file system; device IDs are
   unique per file system (FSID) and are qualified by the layout type.

   See Section 3.1.4 for more details on device ID assignment.

   If the size of the device address exceeds maxcount bytes, the
   metadata server will return the error NFS4ERR_TOOSMALL.  If an
   invalid device ID is given, the metadata server will respond with



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9.5   GETDEVICELIST - Get List of Devices


     (cfh), layout_type, maxcount, cookie, cookieverf ->
     cookie, cookieverf, device_addrs<>


     struct GETDEVICELIST4args {
             /* CURRENT_FH: file */
             pnfs_layouttype4                layout_type;
             count4                          maxcount;
             nfs_cookie4                     cookie;
             verifier4                       cookieverf;


     struct GETDEVICELIST4resok {
             nfs_cookie4                     cookie;
             verifier4                       cookieverf;
             pnfs_devlist_item4              device_addrs<>;

     union GETDEVICELIST4res switch (nfsstat4 status) {
             case NFS4_OK:
                     GETDEVICELIST4resok     resok4;


   In some applications, especially SAN environments, it is convenient
   to find out about all the devices associated with a file system.
   This lets a client determine if it has access to these devices, e.g.,
   at mount time.

   This operation returns an array of items (pnfs_devlist_item4) that
   establish the association between the short pnfs_deviceid4 and the
   addressing information for that device, for a particular layout type.
   This operation may not be able to fetch all device information at
   once, thus it uses a cookie based approach, similar to READDIR, to
   fetch additional device information (see [2], section 14.2.24).  As

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   in GETDEVICEINFO, the current filehandle (cfh) is used to identify
   the file system.

   As in GETDEVICEINFO, maxcount specifies the maximum number of bytes
   to return.  If the metadata server is unable to return a single
   device address, it will return the error NFS4ERR_TOOSMALL.  If an
   invalid device ID is given, the metadata server will respond with



10.  Callback Operations

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     layout_type, iomode, layoutrecall -> -


     enum layoutrecall_type4 {
             RECALL_FILE = 1,
             RECALL_FSID = 2

     struct layoutrecall_file4 {
             nfs_fh4         fh;
             offset4         offset;
             length4         length;

     union layoutrecall4 switch(layoutrecall_type4 recalltype) {
             case RECALL_FILE:
                     layoutrecall_file4 layout;
             case RECALL_FSID:
                     fsid4              fsid;

     struct CB_LAYOUTRECALLargs {
             pnfs_layouttype4        layout_type;
             pnfs_layoutiomode4      iomode;
             layoutrecall4           layoutrecall;


     struct CB_LAYOUTRECALLres {
             nfsstat4        status;


   The CB_LAYOUTRECALL operation is used to begin the process of
   recalling a layout, a portion thereof, or all layouts pertaining to a
   particular file system (FSID).  If RECALL_FILE is specified, the
   offset and length fields specify the portion of the layout to be
   returned.  The iomode specifies the set of layouts to be returned.
   An iomode of ANY specifies that all matching layouts, regardless of
   iomode, must be returned; otherwise, only layouts that exactly match
   the iomode must be returned.

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   If RECALL_FSID is specified, the fsid specifies the file system for
   which any outstanding layouts must be returned.  Layouts are returned
   through the LAYOUTRETURN operation.

   If the client does not hold any layout segment either matching or
   overlapping with the requested layout, it returns
   NFS4ERR_NOMATCHING_LAYOUT.  If a length of all 1s is specified then
   the layout corresponding to the byte range from "offset" to the end-
   of-file MUST be returned.


   The client should reply to the callback immediately.  Replying does
   not complete the recall except when an error is returned.  The recall
   is not complete until the layout(s) are returned using a

   The client should complete any in-flight I/O operations using the
   recalled layout(s) before returning it/them via LAYOUTRETURN.  If the
   client has buffered dirty data, it may chose to write it directly to
   storage before calling LAYOUTRETURN, or to write it later using
   normal NFSv4 WRITE operations to the metadata server.

   If dirty data is flushed while the layout is held, the client must
   still issue LAYOUTCOMMIT operations at the appropriate time,
   especially before issuing the LAYOUTRETURN.  If a large amount of
   dirty data is outstanding, the client may issue LAYOUTRETURNs for
   portions of the layout being recalled; this allows the server to
   monitor the client's progress and adherence to the callback.
   However, the last LAYOUTRETURN in a sequence of returns, SHOULD
   specify the full range being recalled (see Section 3.5.2 for



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     fh, size -> -


     struct CB_SIZECHANGEDargs {
             nfs_fh4         fh;
             length4         size;


     struct CB_SIZECHANGEDres {
             nfsstat4        status;


   The CB_SIZECHANGED operation is used to notify the client that the
   size pertaining to the filehandle associated with "fh", has changed.
   The new size is specified.  Upon reception of this notification
   callback, the client should update its internal size for the file.
   If the layout being held for the file is of the NFSv4 file layout
   type, then the size field within that layout should be updated (see
   Section 5.5).  For other layout types see Section 3.4.2 for more

   If the handle specified is not one for which the client holds a
   layout, an NFS4ERR_BADHANDLE error is returned.



11.  Layouts and Aggregation

   This section describes several aggregation schemes in a semi-formal
   way to provide context for layout formats.  These definitions will be
   formalized in other protocols.  However, the set of understood types
   is part of this protocol in order to provide for basic

   The layout descriptions include (deviceID, objectID) tuples that
   identify some storage object on some storage device.  The addressing

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   formation associated with the deviceID is obtained with
   GETDEVICEINFO.  The interpretation of the objectID depends on the
   storage protocol.  The objectID could be a filehandle for an NFSv4
   storage device.  It could be a OSD object ID for an object server.
   The layout for a block device generally includes additional block map
   information to enumerate blocks or extents that are part of the

11.1  Simple Map

   The data is located on a single storage device.  In this case the
   file server can act as the front end for several storage devices and
   distribute files among them.  Each file is limited in its size and
   performance characteristics by a single storage device.  The simple
   map consists of (deviceID, objectID).

11.2  Block Extent Map

   The data is located on a LUN in the SAN.  The layout consists of an
   array of (deviceID, blockID, offset, length) tuples.  Each entry
   describes a block extent.

11.3  Striped Map (RAID 0)

   The data is striped across storage devices.  The parameters of the
   stripe include the number of storage devices (N) and the size of each
   stripe unit (U).  A full stripe of data is N * U bytes.  The stripe
   map consists of an ordered list of (deviceID, objectID) tuples and
   the parameter value for U. The first stripe unit (the first U bytes)
   are stored on the first (deviceID, objectID), the second stripe unit
   on the second (deviceID, objectID) and so forth until the first
   complete stripe.  The data layout then wraps around so that byte
   (N*U) of the file is stored on the first (deviceID, objectID) in the
   list, but starting at offset U within that object.  The striped
   layout allows a client to read or write to the component objects in
   parallel to achieve high bandwidth.

   The striped map for a block device would be slightly different.  The
   map is an ordered list of (deviceID, blockID, blocksize), where the
   deviceID is rotated among a set of devices to achieve striping.

11.4  Replicated Map

   The file data is replicated on N storage devices.  The map consists
   of N (deviceID, objectID) tuples.  When data is written using this
   map, it should be written to N objects in parallel.  When data is
   read, any component object can be used.

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   This map type is controversial because it highlights the issues with
   error recovery.  Those issues get interesting with any scheme that
   employs redundancy.  The handling of errors (e.g., only a subset of
   replicas get updated) is outside the scope of this protocol
   extension.  Instead, it is a function of the storage protocol and the
   metadata control protocol.

11.5  Concatenated Map

   The map consists of an ordered set of N (deviceID, objectID, size)
   tuples.  Each successive tuple describes the next segment of the

11.6  Nested Map

   The nested map is used to compose more complex maps out of simpler
   ones.  The map format is an ordered set of M sub-maps, each submap
   applies to a byte range within the file and has its own type such as
   the ones introduced above.  Any level of nesting is allowed in order
   to build up complex aggregation schemes.

12.  References

12.1  Normative References

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

   [2]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
        C., Eisler, M., and D. Noveck, "Network File System (NFS)
        version 4 Protocol", RFC 3530, April 2003.

   [3]  Gibson, G., "pNFS Problem Statement", July 2004, <ftp://

12.2  Informative References

   [4]  Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E.
        Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
        RFC 3720, April 2004.

   [5]  Snively, R., "Fibre Channel Protocol for SCSI, 2nd Version
        (FCP-2)", ANSI/INCITS 350-2003, Oct 2003.

   [6]  Weber, R., "Object-Based Storage Device Commands (OSD)", ANSI/
        INCITS 400-2004, July 2004,

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   [7]  Black, D., "pNFS Block/Volume Layout", July 2005, <ftp://

   [8]  Zelenka, J., Welch, B., and B. Halevy, "Object-based pNFS
        Operations", July 2005, <ftp://www.ietf.org/internet-drafts/

Authors' Addresses

   Garth Goodson
   Network Appliance
   495 E. Java Dr
   Sunnyvale, CA  94089

   Phone: +1-408-822-6847
   Email: goodson@netapp.com

   Brent Welch
   Panasas, Inc.
   6520 Kaiser Drive
   Fremont, CA  95444

   Phone: +1-650-608-7770
   Email: welch@panasas.com
   URI:   http://www.panasas.com/

   Benny Halevy
   Panasas, Inc.
   1501 Reedsdale St., #400
   Pittsburgh, PA  15233

   Phone: +1-412-323-3500
   Email: bhalevy@panasas.com
   URI:   http://www.panasas.com/

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   David L. Black
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748

   Phone: +1-508-293-7953
   Email: black_david@emc.com

   Andy Adamson
   CITI University of Michigan
   519 W. William
   Ann Arbor, MI  48103-4943

   Phone: +1-734-764-9465
   Email: andros@umich.edu

Appendix A.  Acknowledgments

   Many members of the pNFS informal working group have helped
   considerably.  The authors would like to thank Gary Grider, Peter
   Corbett, Dave Noveck, and Peter Honeyman.  This work is inspired by
   the NASD and OSD work done by Garth Gibson.  Gary Grider of the
   national labs (LANL) has been a champion of high-performance parallel

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