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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5663

NFSv4 Working Group                                      David L. Black
Internet Draft                                         Stephen Fridella
Expires: June 2006                                      EMC Corporation
                                                      December 30, 2005



                         pNFS Block/Volume Layout
                    draft-ietf-nfsv4-pnfs-block-00.txt


Status of this Memo

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   This Internet-Draft will expire in April 2006.

Abstract

   Parallel NFS (pNFS) extends NFSv4 to allow clients to directly access
   file data on the storage used by the NFSv4 server.  This ability to
   bypass the server for data access can increase both performance and
   parallelism, but requires additional client functionality for data
   access, some of which is dependent on the class of storage used.  The
   main pNFS operations draft specifies storage-class-independent
   extensions to NFS; this draft specifies the additional extensions
   (primarily data structures) for use of pNFS with block and volume
   based storage.



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Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [RFC2119].

Table of Contents

   1. Introduction...................................................3
   2. Background and Architecture....................................3
      2.1. Data Structures: Extents, Extent Lists, and Volumes.......4
         2.1.1. Layout Requests and Extent Lists.....................8
         2.1.2. Layout Commits.......................................9
         2.1.3. Layout Returns.......................................9
         2.1.4. Client Copy-on-Write Processing.....................10
         2.1.5. Extents are Permissions.............................11
         2.1.6. End-of-file Processing..............................12
      2.2. Volume Identification....................................14
   3. New Operations Issues.........................................15
      3.1. Server Controlling Client Access to Block Devices........15
         3.1.1. Guarantees Provided by Layouts......................15
         3.1.2. I/O In-flight Issues................................15
         3.1.3. Crash Recovery Issues...............................15
      3.2. End-of-File (EOF) Handling issues........................16
         3.2.1. Truncation..........................................16
         3.2.2. Extension...........................................17
         3.2.3. Extension vs. Readable Layouts......................17
   4. Security Considerations.......................................18
   5. Conclusions...................................................19
   6. IANA Considerations...........................................19
   7. Revision History..............................................19
   8. Acknowledgments...............................................20
   9. References....................................................20
      9.1. Normative References.....................................20
      9.2. Informative References...................................20
   Author's Addresses...............................................21
   Intellectual Property Statement..................................21
   Disclaimer of Validity...........................................22
   Copyright Statement..............................................22
   Acknowledgment...................................................22









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1. Introduction

   Figure 1 shows the overall architecture of a pNFS system:

       +-----------+
       |+-----------+                                 +-----------+
       ||+-----------+                                |           |
       |||           |        NFSv4 + pNFS            |           |
       +||  Clients  |<------------------------------>|   Server  |
        +|           |                                |           |
         +-----------+                                |           |
              |||                                     +-----------+
              |||                                           |
              |||                                           |
              |||                +-----------+              |
              |||                |+-----------+             |
              ||+----------------||+-----------+            |
              |+-----------------|||           |            |
              +------------------+||  Storage  |------------+
                                  +|  Systems  |
                                   +-----------+

                        Figure 1 pNFS Architecture

   The overall approach is that pNFS-enhanced clients obtain sufficient
   information from the server to enable them to access the underlying
   storage (on the Storage Systems) directly.  See the pNFS portion of
   [NFSV4.1] for more details.  This draft is concerned with access from
   pNFS clients to Storage Systems over storage protocols based on
   blocks and volumes, such as the SCSI protocol family (e.g., parallel
   SCSI, FCP for Fibre Channel, iSCSI, SAS).  This class of storage is
   referred to as block/volume storage.  While the Server to Storage
   System protocol is not of concern for interoperability here, it will
   typically also be a block/volume protocol when clients use
   block/volume protocols.

2. Background and Architecture

   The fundamental storage abstraction supported by block/volume storage
   is a storage volume consisting of a sequential series of fixed size
   blocks.  This can be thought of as a logical disk; it may be realized
   by the Storage System as a physical disk, a portion of a physical
   disk or something more complex (e.g., concatenation, striping, RAID,
   and combinations thereof) involving multiple physical disks or
   portions thereof.



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   A pNFS layout for this block/volume class of storage is responsible
   for mapping from an NFS file (or portion of a file) to the blocks of
   storage volumes that contain the file.  The blocks are expressed as
   extents with 64 bit offsets and lengths using the existing NFSv4
   offset4 and length4 types.  Clients must be able to perform I/O to
   the block extents without affecting additional areas of storage
   (especially important for writes), therefore extents MUST be aligned
   to 512-byte boundaries, and SHOULD be aligned to the block size used
   by the NFSv4 server in managing the actual filesystem (4 kilobytes
   and 8 kilobytes are common block sizes).  This block size is
   available as an NFSv4 attribute - see Section 11.4 of [NFSV4.1].

   The pNFS operation for requesting a layout (LAYOUTGET) includes the
   "pnfs_layoutiomode4 iomode" argument which indicates whether the
   requested layout is for read-only use or read-write use.  A read-only
   layout may contain holes that are read as zero, whereas a read-write
   layout will contain allocated, but uninitialized storage in those
   holes (read as zero, can be written by client).  This draft also
   supports client participation in copy on write by providing both
   read-only and uninitialized storage for the same range in a layout.
   Reads are initially performed on the read-only storage, with writes
   going to the uninitialized storage.  After the first write that
   initializes the uninitialized storage, all reads are performed to
   that now-initialized writeable storage, and the corresponding read-
   only storage is no longer used.

2.1. Data Structures: Extents, Extent Lists, and Volumes

   A pNFS block layout is a list of extents within a flat array of 512-
   byte data blocks known as a volume.  A volume may correspond to a
   single logical unit in a SAN, or a more complex aggregation of
   multiple logical units.  The block layout describes both the topology
   of the volume as well as the individual block extents on the volume
   that make up the file.  Each individual extent MUST be at least 512-
   byte aligned.














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   enum extentState4 {

     READ_WRITE_DATA  = 0, /* the data located by this extent is valid
                              for reading and writing. */

     READ_DATA = 1,        /* the data located by this extent is valid
                              for reading only; it may not be written.
                              */

     INVALID_DATA = 2,     /* the location is valid; the data is
                              invalid. It is a newly (pre-) allocated
                              extent. There is physical space on the
                              volume. */

     NONE_DATA = 3,        /* the location is invalid. It is a hole in
                              the file. There is no physical space on
                              the volume. */

   };

   struct pnfs_block_extent {

     offset4         offset;          /* the starting offset in the
                                         file */

     length4         length;          /* the size of the extent */

     offset4         storage_offset;  /* the starting offset in the
                                         volume */

     extentState4    es;              /* the state of this extent */

   };

   enum pnfs_block_volume_type {

      VOLUME_SIMPLE = 0,      /* volume maps to a single LU */

      VOLUME_SLICE = 1,       /* volume is a slice of another volume */

      VOLUME_CONCAT = 2,      /* volume is a concatenation of multiple
                                 volumes */

      VOLUME_STRIPE = 3,      /* volume is striped across multiple
                                 volumes */

   };


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   struct pnfs_block_slice_volume_info {

      offset4          start;   /* block-offset of the start of the
                                    slice */

      length4          length;  /* length of slice in blocks */

      pnfs_block_volume volume; /* volume which is sliced */

   };

   struct pnfs_block_concat_volume_info {

      pnfs_block_volume volumes<>;  /* volumes which are concatenated */

   };

   struct pnfs_block_stripe_volume_info {

      length4           stripe_unit;   /* size of stripe */

      pnfs_block_volume volumes<>;     /* volumes which are striped
                                         across*/

   };



   union pnfs_block_volume switch (pnfs_block_volume_type type) {

         case VOLUME_SIMPLE:

               pnfs_deviceid4 volume_ID;

         case VOLUME_SLICE:

               pnfs_block_slice_volume_info slice_info;

         case VOLUME_CONCAT:

               pnfs_block_concat_volume_info concat_info;

         case VOLUME_STRIPE:

               pnfs_block_stripe_volume_info stripe_info;

   };


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   struct pnfs_block_layout {

      pnfs_block_volume volume;        /* topology of the volume on
                                         which file is stored. */

      pnfs_block_extent extents<>;     /* extents which make up this
                                         layout. */

   };

   The block layout consists of information describing the topology of
   the logical volume on which the file is stored, followed by a list of
   extents which map the logical regions of the file to physical
   locations on the volume.  The "pnfs_block_volume" union is a
   recursive structure that allows arbitrarily complex nested volume
   structures to be encoded.  The types of aggregations that are allowed
   are stripes, concatenations, and slices.  The base case is a volume
   which maps simply to one logical unit in the SAN, identified by the
   "pnfs_deviceid4" (see discussion of volume identification, section
   2.2 below).  The "storage_offset" field within each extent identifies
   a location on the logical volume described by the "volume" field in
   the layout.  The client is responsible for translating this logical
   offset into an offset on the appropriate underlying SAN logical unit.

   Each extent maps a logical region of the file onto a portion of the
   specified logical volume.  The file_offset, extent_length, and es
   fields for an extent returned from the server are always valid. The
   interpretation of the storage_offset field depends on the value of es
   as follows:

   o  READ_WRITE_DATA means that storage_offset is valid, and points to
      valid/initialized data that can be read and written.

   o  READ_DATA means that storage_offset is valid and points to valid/
      initialized data which can only be read.  Write operations are
      prohibited; the client may need to request a read-write layout.

   o  INVALID_DATA means that storage_offset is valid, but points to
      invalid uninitialized data. This data must not be physically read
      from the disk until it has been initialized.  A read request for
      an INVALID_DATA extent must fill the user buffer with zeros. Write
      requests must write whole server-sized blocks to the disk with
      bytes not initialized by the user must be set to zero.  Any write
      to storage in an INVALID_DATA extent changes the written portion
      of the extent to READ_WRITE_DATA; the pNFS client is responsible
      for reporting this change via LAYOUTCOMMIT.



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

   The extent list lists all relevant extents in increasing order of the
   file_offset of each extent; any ties are broken by increasing order
   of the extent state (es).

2.1.1. Layout Requests and Extent Lists

   Each request for a layout specifies at least three parameters:
   offset, desired size, and minimum size (the desired size is missing
   from the operations draft - see Section 3).  If the status of a
   request indicates success, the extent list returned must meet the
   following criteria:

   o  A request for a readable (but not writeable) layout returns only
      READ_DATA or NONE_DATA extents (but not INVALID_DATA or
      READ_WRITE_DATA extents).

   o  A request for a writeable layout returns READ_WRITE_DATA or
      INVALID_DATA extents (but not NONE_DATA extents).  It may also
      return READ_DATA extents only when the offset ranges in those
      extents are also covered by INVALID_DATA extents to permit writes.

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

   o  The total size of extents in the extent list MUST cover at least
      the minimum size and no more than the desired size.  One exception
      is allowed: the total size MAY be smaller if only readable extents
      were requested and EOF is encountered.

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

   o  Extents MUST be ordered in the list by starting offset, with
      READ_DATA extents preceding INVALID_DATA extents in the case of
      equal file_offsets.





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2.1.2. Layout Commits

   struct pnfs_block_layoutupdate {

     pnfs_block_extent commit_list;   /* list of extents to which now
                                         contain valid data. */

     bool              make_version;  /* client requests server to
                                         create copy-on-write image of
                                         this file. */

   }

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

   The "make_version" field of the structure is a flag that the client
   may set to request that the server create a copy-on-write image of
   the file (see section 2.1.4, below).  In anticipation of this
   operation the client which sets the "make_version" flag in the
   LAYOUTCOMMIT operation should immediately mark all extents in the
   layout that is possesses as state READ_DATA.  Future writes to the
   file require a new LAYOUTGET operation to the server with an "iomode"
   set to LAYOUTIOMODE_RW.

2.1.3. Layout Returns

   struct pnfs_block_layoutreturn {

     pnfs_block_extent rel_list;      /* list of extents the client
                                         will no longer use. */

   }


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   The "rel_list" field is an extent list covering regions of the file
   layout that are no longer needed by the client.  Including extents in
   the "rel_list" for a LAYOUTRETURN operation represents an explicit
   release of resources by the client, usually done for the purpose of
   avoiding unnecessary CB_LAYOUTRECALL operations in the future.

2.1.4. Client Copy-on-Write Processing

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

   When a client wishes to write data covered by a READ_DATA extent, it
   MUST have requested a writable layout from the server; that layout
   will contain INVALID_DATA extents to cover all the data ranges of
   that layout's READ_DATA extents. More precisely, for any file_offset
   range covered by one or more READ_DATA extents in a writable layout,
   the server MUST include one or more INVALID_DATA extents in the
   layout that cover the same file_offset range. The client MUST
   logically copy the data from the READ_DATA extent for any partial
   blocks of file_offset and range, merge in the changes to be written,
   and write the result to the INVALID_DATA extent for the blocks for
   that file_offset and range. That is, if entire blocks of data are to
   be overwritten by an operation, the corresponding READ_DATA blocks
   need not be fetched, but any partial-block writes must be merged with
   data fetched via READ_DATA extents before storing the result via
   INVALID_DATA extents.  For the purposes of this discussion, "entire
   blocks" and "partial blocks" refer to the server's file-system block
   size.  Storing of data in an INVALID_DATA extent converts the written
   portion of the INVALID_DATA extent to a READ_WRITE_DATA extent; all
   subsequent reads MUST be performed from this extent; the
   corresponding portion of the READ_DATA extent MUST NOT be used after
   storing data in an INVALID_DATA extent.

   In the LAYOUTCOMMIT operation that normally sends updated layout
   information back to the server, for writable data, some INVALID_DATA
   extents may be committed as READ_WRITE_DATA extents, signifying that
   the storage at the corresponding storage_offset values has been
   stored into and is now to be considered as valid data to be read.
   READ_DATA extents need not be sent to the server. For extents that
   the client receives via LAYOUTGET as INVALID_DATA and returns via
   LAYOUTCOMMIT as READ_WRITE_DATA, the server will understand that the



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   READ_DATA mapping for that extent is no longer valid or necessary for
   that file.

2.1.5. Extents are Permissions

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

   Block/volume class storage devices are not required to perform read
   and write operations atomically.  Overlapping concurrent read and
   write operations to the same data may cause the read to return a
   mixture of before-write and after-write data.  Overlapping write
   operations can be worse, as the result could be a mixture of data
   from the two write operations; this can be particularly nasty if the
   underlying storage is striped and the operations complete in
   different orders on different stripes.  A pNFS server can avoid these
   conflicts by implementing a single writer XOR multiple readers
   concurrency control policy when there are multiple clients who wish
   to access the same data.  This policy SHOULD be implemented when
   storage devices do not provide atomicity for concurrent read/write
   and write/write operations to the same data.

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



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   in the rejected request queue can be removed when it reaches a
   certain age.

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

2.1.6. End-of-file Processing

   To avoid file-system corruption, close coordination between pNFS
   clients and the server is required when an NFSv4 client changes the
   end-of-file marker via the SETATTR call.  Whenever the end-of-file is
   set into the middle of a file-system block, the portion of the block
   which comes after the end-of-file must be zeroed on disk.  The pNFS
   clients and server share the responsibility for this zeroing as
   follows:

2.1.6.1. Server End-of-file Processing

   When an NFSv4 client changes the end-of-file marker via a SETATTR
   operation, the server MUST send a CB_SIZECHANGED notification to each
   pNFS client (with the exception of the client which sent the SETATTR)
   that holds a layout for the file.  Clients process this callback as
   described in Section 2.1.6.2.

   The CB_SIZECHANGED notification has the effect of invalidating all
   data beyond the new end-of-file.  Once the server receives a
   successful response to the CB_SIZECHANGED notification from a client,
   it will consider any portion of any layout held by the client beyond
   the new end-of-file to be invalid.  This requires that:







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   o  READ_WRITE_DATA extents are changed INVALID_DATA extents.

   o  READ_DATA extents are changed to NONE_DATA extents.  If this
      results in any portion of a NONE_DATA extent overlapping an
      INVALID_DATA extent, that portion of the NONE_DATA extent (which
      may be the entire extent) is immediately discarded as INVALID_DATA
      extents are not permitted to overlap NONE_DATA extents.

   If the new end-of-file is not set on a file-system block boundary,
   the server must ensure that zeroes are written to the partial block
   from the new end-of-file to the end of the block containing it.  If a
   pNFS client holds a writeable layout covering that block, that pNFS
   client is expected to perform this function, but the CB_SIZECHANGED
   callback response needs to have a way for the client to communicate
   that it has done so (see Section 3.2.2).  If no client performs this
   function, the server must do the zeroing, including recalling all
   layouts (readable and writable) for that block.

2.1.6.2. Client End-of-file Processing

   When a pNFS client receives the CB_SIZECHANGED notification from the
   server, or when a pNFS client issues a SETATTR operation to set the
   end-of-file marker for a file on which it holds a layout, it should
   proceed the same way.  First, it should consider any portions of any
   readable layout beyond the new end-of-file to be in the NONE_DATA
   state and it should immediately stop servicing read requests from
   such extents.  If the client holds a readable layout for the block
   containing a new end-of-file that is not at a block boundary, it
   SHOULD return at least that block before replying to the callback;
   this avoids a callback from the server in order to zero the partial
   block beyond the new end-of-file (how to do this is an open issue -
   see Section 3.2.3).  Next, the client should consider any portions of
   a writeable layout beyond the new end-of-file to be in the
   INVALID_DATA state, and should henceforth return zeroes in response
   to any read requests for these extents.  Data can, of course, be
   rewritten to these extents, turning them back into READ_WRITE_DATA
   extents, as desired.  Finally, if the new end-of-file marker is not
   on a file-system block boundary, and if the client has a writeable
   layout which covers the block containing the new end-of-file, then
   the client MUST zero-fill the portion of the block after the end-of-
   file marker and write this block to the storage before responding to
   the CB_SIZECHANGED notification.  In this case the new end-of-file
   block MUST be considered to be in the READ_WRITE_DATA state.






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2.2. Volume Identification

   Storage Systems such as storage arrays can have multiple physical
   network ports that need not be connected to a common network,
   resulting in a pNFS client having simultaneous multipath access to
   the same storage volumes via different ports on different networks.
   The networks may not even be the same technology - for example,
   access to the same volume via both iSCSI and Fibre Channel is
   possible, hence network address are difficult to use for volume
   identification.  For this reason, this pNFS block layout identifies
   storage volumes by content, for example providing the means to match
   (unique portions of) labels used by volume managers.  Any block pNFS
   system using this layout MUST support a means of content-based unique
   volume identification that can be employed via the data structure
   given here.

   struct sigComponent {         /*  disk signature component */

      offset4  sig_offset;       /* byte offset of component */

      length4  sig_length;       /* byte length of component */

      opaque contents<>;      /* contents of this component of the
                                 signature (this is opaque) */

   };

   struct pnfs_block_deviceaddr4 {

      sigComponent ds<MAX_SIG_COMP>;   /* disk signature */

   };

   A volume is content-identified by a disk signature made up of extents
   within blocks and contents that must match.  The
   "pnfs_block_deviceaddr4" structure is returned by the server as the
   storage-protocol-specific opaque field in the "pnfs_deviceaddr4"
   structure, in response to the GETDEVICEINFO or GETDEVICELIST
   operations.  Note that the opaque "contents" field in the
   "sigComponent" structure MUST NOT be interpreted as a zero-terminated
   string, as it may contain embedded zero-valued octets.  It contains
   exactly sig_length octets.  There are no restrictions on alignment
   (e.g., neither sig_offset nor sig_length are required to be multiples
   of 4).





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3. New Operations Issues

   This section collects issues in the [NFVV4.1] draft encountered in
   writing this block/volume layout draft.

3.1. Server Controlling Client Access to Block Devices

   Typically, SAN disk arrays and SAN protocols provide access control
   mechanisms (access-logics, lun masking, etc.) which operate at the
   granularity of individual hosts.  The functionality provided by such
   mechanisms makes it possible for the server to "fence" individual
   client machines from certain physical disks---that is to say, to
   prevent individual client machines from reading or writing to certain
   physical disks.  Finer-grained access control methods are not
   generally available.  This affects the ability of the block/volume
   storage protocol to meet the requirements set out in the [NFSV4.1]
   draft in the following ways:

3.1.1. Guarantees Provided by Layouts

   See first paragraph of Security Considerations (Section 4).

3.1.2. I/O In-flight Issues

   See third paragraph of Security Considerations (Section 4).

3.1.3. Crash Recovery Issues

   The main [NFSV4.1] draft does not currently include a "reclaim bit"
   in the LAYOUTGET operation.  This means that after a server crash,
   during the server's recovery grace period, a client cannot be
   guaranteed that it will be able to reclaim a writable layout that it
   held before the crash (due to possibly conflicting requests from
   other clients), nor can the client be guaranteed that a writeable
   layout that it reclaims will have the same on-disk layout as the one
   it held prior to the crash.  There is one possible scenario where
   this lack of guarantees could lead to data corruption.

   The problematic case is when the server crashes while the client
   holds a writable layout, but has yet to commit some allocated block
   extents back to the server with a LAYOUTCOMMIT operation.  If the
   data that has been written to these extents is still cached by the
   client, the client can simply re-write the data via NFSv4, once the
   server has come back online.  However, the uncommitted extents may
   represent more data than can fit in the client's cache at one time.
   In this case, the data cannot be rewritten to the server---indeed, it
   cannot even be safely re-read from storage in the absence of a pNFS


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   layout which covers those disk blocks.  The ideal solution, from the
   client's perspective, would be a method that allows the client to
   reliably reclaim the previously held layout from the server.  For
   this, the client would need a "reclaim bit" in the LAYOUTGET
   operation to allow the server to recognize, and give priority to, a
   reclaim request during the server's recovery grace period, as well as
   a way of supplying an extent list indicating the blocks (including
   storage location) that were previously held in the writable layout.

   Without this support, the server would need to make sure that
   provisionally allocated extents that are supplied in response to
   writable LAYOUTGET commands are persistently stored and recoverable
   in the face of server crashes.  Otherwise, the client would have no
   guarantee that the new writable layout it receives in response to a
   reclaim request covers the same disk blocks as the old layout it held
   before the server crash.  The "reclaim bit" could be implemented as
   another iomode (e.g., LAYOUTIOMODE_RW_RECOVER).  If this is not done,
   the result may be that the amount of written-but-uncomitted data held
   by a client may need to be limited to the client's cache size,
   resulting in less effective cache usage and more commit traffic.

3.2. End-of-File (EOF) Handling issues

   The main pNFS draft [NFSV4.1] includes a callback (CB_SIZECHANGED)
   for a pNFS server to communicate a file size change (new EOF) to pNFS
   clients.  The callback documentation indicates that "the client
   should update its internal size for the file".  This is insufficient
   in a number of ways described below.  Some of these issues are unique
   to the block/ volume layout as this layout requires that either the
   client or server zero beyond the new EOF to the end of the block that
   contains the new EOF (both the file and object layouts can make this
   the responsibility of the storage server(s)).

3.2.1. Truncation

   There is a race between a pNFS client extending a file via direct
   pNFS writes and an ordinary NFS client truncating the file via a
   SETATTR - the order of these operations may be visible to the
   application(s) using NFS, and hence it is necessary that they be done
   in the right order.  List discussion suggests that the absence of
   operation sequencing in pNFS (in contrast to HighRoad) results in the
   best approach being to require a recall of the portions of the layout
   that are invalidated by a truncation so that the server can perform
   the truncation (this MUST include the entire layout beyond the new
   EOF).  For the block/volume layout, the server is responsible for
   zeroing the partial block beyond the new EOF in this case.



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3.2.2. Extension

   When extending a file via SETATTR, a similar race can arise between
   that SETATTR and direct pNFS writes.  When CB_SIZECHANGED is used to
   pass the new end-of-file to a client that holds a writable layout for
   the block containing the new end-of-file, and that new end-of-file is
   not at a filesystem block boundary, it is desirable for the client
   holding the writable layout to take responsibility for zeroing the
   partial block beyond end-of file.  Note that the client may not be
   able to do so in all cases, as the client may have been in the
   process of returning that portion of the layout when the callback
   arrives, or the server and client may not agree on whether the client
   holds a writable layout containing that block.  Hence the
   CB_SIZECHANGED reply needs to carry an indication from the client as
   to whether the client has zeroed that partial block.

   Open Issue: How to convey that indication.  The least intrusive way
   to do this may be to define an NFS Error that indicates that the
   callback was successful and the client zeroed the resulting partial
   block.

   Open Issue: Can callback reply be delayed until this write is done?
   Immediate callback reply with promise to zero the partial block
   followed by client crash results in potential data corruption due to
   failure to zero the partial block (junk appears if the file is
   extended by moving end-of-file forward).

3.2.3. Extension vs. Readable Layouts

   There is an analogous issue for a client holding a readable layout
   (READ_DATA) including the block that contains a new non-block-aligned
   end-of-file.  In this case, the client cannot zero the partial block
   itself, as it does not hold a writable layout, but if it retains the
   readable layout, the server is likely to immediately issue a recall
   callback to revoke the readable layout before the server writes the
   zeroes.  This second callback should be avoided.

   Issue: How?  Allowing an arbitrary return in the response to
   CB_SIZECHANGED would get the job done, but is only needed by the
   block/volume layout, and complicates the end-of-file change code on
   the server by potentially returning unrelated areas of the layout.
   Delaying the callback reply to allow the client to do a RETURN helps
   some (2 round-trips instead of 3 if the recall were used) at the cost
   of delaying the recall reply.  Best bet may be to enlarge the
   CB_SIZECHANGED callback so the server can tell the client what needs
   to be returned (in the readable layout case, the server asks for one
   block) and for the client to accept/reject that (client accepts if it


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   can do so immediately - it would reject if it needed to do a commit);
   that should yield one round-trip with the server getting exactly what
   it needs back (and server MUST NOT use this to recall a writable
   layout).

4. Security Considerations

   Certain security responsibilities are delegated to pNFS clients.
   Block/volume storage systems generally control access at a volume
   granularity, and hence pNFS clients have to be trusted to only
   perform accesses allowed by the layout extents they currently hold
   (e.g., and not access storage for files on which a layout extent is
   not held).  In general, the server will not be able to prevent a
   client which holds a layout for a file from accessing parts of the
   physical disk not covered by the layout.  Similarly, the server will
   not be able to prevent a client from accessing blocks covered by a
   layout that it has already returned.  This block-based level of
   protection must be provided by the client software.  In environments
   where the security requirements are such that client-side protection
   from access to storage outside of the layout is not sufficient, pNFS
   block/volume storage layouts for pNFS SHOULD NOT be used.

   This also has implications for some NFSv4 functionality outside pNFS.
   For instance, if a file is covered by a mandatory read-only lock, the
   server can ensure that only readable layouts for the file are granted
   to pNFS clients.  However, it is up to each pNFS client to ensure
   that the readable layout is used only to service read requests, and
   not to allow writes to the existing parts of the file.  Since
   block/volume storage systems are generally not capable of enforcing
   such file-based security, in environments where pNFS clients cannot
   be trusted to enforce such policies, pNFS block/volume storage
   layouts SHOULD NOT be used.

   When a layout lease timer expires, it is important for the sake of
   correctness that any in-flight I/Os that the client issued before the
   expiration of the timer are rejected at the storage.  For the
   block/volume protocol, this is possible by fencing a client with an
   expired layout timer from the physical storage.  Note, however, that
   the granularity of this operation can only be at the host/logical-
   unit level.  Thus, if a client's lease timer expires for a single
   layout, it will effectively render useless *all* of the clients
   layouts for files in the containing filesystem.

   Access to block/volume storage is logically at a lower layer of the
   I/O stack than NFSv4, and hence NFSv4 security is not directly
   applicable to protocols that access such storage directly.  Depending
   on the protocol, some of the security mechanisms provided by NFSv4


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   (e.g., encryption, cryptographic integrity) may not be available, or
   may be provided via different means.  At one extreme, pNFS with
   block/volume storage can be used with storage access protocols (e.g.,
   parallel SCSI) that provide essentially no security functionality.
   At the other extreme, pNFS may be used with storage protocols such as
   iSCSI that provide significant functionality.  It is the
   responsibility of those administering and deploying pNFS with a
   block/volume storage access protocol to ensure that appropriate
   protection is provided to that protocol (physical security is a
   common means for protocols not based on IP).  In environments where
   the security requirements for the storage protocol cannot be met,
   pNFS block/volume storage layouts SHOULD NOT be used.

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

5. Conclusions

   This draft specifies the block/volume layout type for pNFS and
   associated functionality.

6. IANA Considerations

   There are no IANA considerations in this document.  All pNFS IANA
   Considerations are covered in [NFSV4.1].

7. Revision History

   -00: Initial Version as draft-black-pnfs-block-00

   -01: Rework discussion of extents as locks to talk about extents
   granting access permissions.  Rewrite operation ordering section to
   discuss deadlocks and races that can cause problems.  Add new section
   on recall completion.  Add client copy-on-write based on text from
   Craig Everhart.

   -02: Fix glitches in extent state descriptions.  Describe most issues
   as RESOLVED.  Most of Section 3 has been incorporated into the the
   main PNFD draft, add NOTE to that effect and say that it will be


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   deleted in the next version of this draft (which should be a draft-
   ietf-nfsv4 draft).  Cleaning up a number of things have been left to
   that draft revision, including the interlocks with the types in the
   main pNFS draft, layout striping support, and finishing the Security
   Considerations section.

   -00: New version as draft-ietf-nfsv4-pnfs-block.  Removed resolved
   operations issues (Section 3).  Align types with main pNFS draft
   (which is now part of the NFSv4.1 minor version draft), add volume
   striping and slicing support.  New operations issues are in Section 3
   - the need for a "reclaim bit" and EOF concerns are the two major
   issues.  Extended and improved the Security Considerations section,
   but it still needs work.  Added 1-sentence conclusion that also still
   needs work.

8. Acknowledgments

   This draft draws extensively on the authors' familiarity with the
   mapping functionality and protocol in EMC's HighRoad system
   [HighRoad].  The protocol used by HighRoad is called FMP (File
   Mapping Protocol); it is an add-on protocol that runs in parallel
   with filesystem protocols such as NFSv3 to provide pNFS-like
   functionality for block/volume storage.  While drawing on HighRoad
   FMP, the data structures and functional considerations in this draft
   differ in significant ways, based on lessons learned and the
   opportunity to take advantage of NFSv4 features such as COMPOUND
   operations.  The design to support pNFS client participation in copy-
   on-write is based on text and ideas contributed by Craig Everhart
   (formerly with IBM).

9. References

9.1. Normative References

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

   [NFSV4.1] Shepler, S., ed., "NFSv4 Minor Version 1", draft-ietf-
             nfsv4-minorversion1-01.txt, Work in Progress, December
             2005.

9.2. Informative References

   [HighRoad] EMC Corporation, "EMC Celerra HighRoad", EMC C819.1 white
              paper, available at:
   http://www.emc.com/pdf/products/celerra_file_server/HighRoad_wp.pdf
              link checked 30 December 2005.


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Author's Addresses

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

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


   Stephen Fridella
   EMC Corporation
   32 Coslin Drive
   Southboro, MA  01772

   Phone: +1 (508) 305-8512
   Email: fridella_stephen@emc.com


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Disclaimer of Validity

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