NFSv4 Working Group                                          S. Faibish
Internet-Draft                                          EMC Corporation
Intended status: Proposed Standard                             D. Black
Expires: April 14, 2011                                 EMC Corporation
Updates: 5661, 5662                                           M. Eisler
                                                             J. Glasgow
                                                       October 14, 2010

                       pNFS Access Permissions Check

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   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.


   This document extends the pNFS protocol to communicate errors caused
   by inability to access data servers referenced by layouts, including
   checks performed by both clients and the MDS. The extension provides
   means for clients to communicate client-detected access denial errors
   to the MDS, including the case in which a client requests direct NFS
   access via the MDS that the MDS cannot perform.

Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................5
   3. Changes to Operation 51: LAYOUTRETURN (RFC 5661)...............5
      3.1. ARGUMENT (18.44.1)........................................5
      3.2. RESULT (18.44.2)..........................................7
      3.3. DESCRIPTION (18.44.3).....................................7
      3.4. IMPLEMENTATION (18.44.4)..................................7
         3.4.1. Storage Device Error Mapping (, new)........9
   4. Change to NFS4ERR_NXIO Usage..................................10
   5. Security Considerations.......................................10
   6. IANA Considerations...........................................10
   7. Conclusions...................................................10
   8. References....................................................10
      8.1. Normative References.....................................10

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1. Introduction

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

          |+-----------+                                 +-----------+
          ||+-----------+                                |           |
          |||           |       NFSv4.1 + pNFS           |           |
          +||  Clients  |<------------------------------>|    MDS    |
           +|           |                                |           |
            +-----------+                                |           |
                 |||                                     +-----------+
                 |||                                           |
                 |||                                           |
                 ||| Storage        +-----------+              |
                 ||| Protocol       |+-----------+             |
                 ||+----------------||+-----------+  Control   |
                 |+-----------------|||           |  Protocol  |
                 +------------------+||  Storage  |------------+
                                     +|  Devices  |

                           Figure 1 pNFS Architecture

   In this document, "storage device" is used as a general term for a
   data server and/or storage server for the file, block or object pNFS

   The current pNFS protocol [RFC5661] assumes that a client can access
   every storage device (SD) included in a valid layout sent by the MDS
   server, and provides no means to communicate client access failures
   to the MDS. Access failures can impair pNFS performance scaling and
   allow significant errors to go unreported. If the MDS can access all
   the storage devices involved, but the client doesn't have sufficient
   access rights to some storage devices, the client may choose to fall
   back to accessing the file system using NFSV4.1 without pNFS support;
   there are environments in which this behavior is undesirable,
   especially if it occurs silently. An important example is addition of
   a new storage device to which a large population of pNFS clients
   (e.g., 1000s) lacks access permission. Layouts granted that use this
   new device, result in client errors, requiring that all I/Os to that
   new storage device be served by the MDS server. This creates a
   performance and scalability bottleneck that may be difficult to
   detect based on I/O behavior because the other storage devices are
   functioning correctly.

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   The preferable approach to this scenario is to report the access
   failures before any client attempts to issue any I/Os that can only
   be serviced by the MDS server.  This makes the problem explicit,
   rather than forcing the MDS, or a system administrator, to diagnose
   the performance problem caused by client I/O using NFS instead of
   pNFS. There are limits to this approach because complex mount
   structures may prevent a client from detecting this situation at
   mount time, but at a minimum, access problems involving the root of
   the mount structure can be detected.

   The most suitable time for the client to report inability to access a
   storage device is at mount time, but this is not always possible.
   If the application uses a special tag or a switch to the mount
   command (e.g., -pnfs) and syscall to declare its intention to use
   pNFS, at the client, the client can check for both pNFS support and
   device accessibility.

   This document introduces an error reporting mechanism that is an
   extension to the return of a pNFS layout; a pNFS client MAY use this
   mechanism to inform the MDS that the layout is being returned because
   one or more data servers are not accessible to the client. Error
   reporting at I/O time is not affected because the result of an
   inaccessible data server may not be an I/O error if a subsequent
   retry of the operation via the MDS is successful.

   There is a related problem scenario involving an MDS that cannot
   access some storage devices and hence cannot perform I/Os on behalf
   of a client. In the case of the block layout [RFC5663] if the MDS
   lacks access to a storage device (e.g., LUN), MDS implementations
   generally do not export any filesystem using that storage device.  In
   contrast to the block layout, MDSs for the file [RFC5661] and object
   [RFC5664] layouts may be unable to access the storage devices that
   store data for an exported filesystem.  This enables a file or object
   layout MDS to provide layouts that contain client-inaccessible
   devices. For the specific case of adding a new storage device to a
   filesystem, MDS issuance of test I/Os to the newly added device
   before using it in layouts avoids this problem scenario, but does not
   cover loss of access to existing storage devices at a later time.

   In addition, [RFC5661] states that a client can write through or read
   from the MDS, even if it has a layout; this assumes that the MDS can
   access all the storage devices. This document makes that assumed
   access an explicit requirement.

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2. Conventions used in this document

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

3. Changes to Operation 51: LAYOUTRETURN (RFC 5661)

   The existing LAYOUTRETURN operation is extended by introducing three
   new layout return types that correspond to the existing types:

   o  LAYOUT4_RET_REC_FILE_NO_ACCESS at file scope;

   o  LAYOUT4_RET_REC_FSID_NO_ACCESS at fsid scope; and

   o  LAYOUT4_RET_REC_ALL_NO_ACCESS at client scope.

   The first return type returns the layout for an individual file and
   informs the server that the reason for the return is a storage device
   connectivity problem. The second return type performs that function
   for all layouts held by the client for the filesystem that
   corresponds to the current filehandle used for the LAYOUTRETURN
   operation.  The third return type performs that function for all
   layouts held by the client; it is intended for situations in which a
   device is shared across all or most of the filesystems from a server
   for which the client has layouts.

3.1. ARGUMENT (18.44.1)

   The ARGUMENT specification of the LAYOUTRETURN operation in section
   18.44.1 of [RFC5661] is replaced by the following XDR code [XDR]:

     /* Constants used for new LAYOUTRETURN and CB_LAYOUTRECALL */
     const LAYOUT4_RET_REC_FILE      = 1;
     const LAYOUT4_RET_REC_FSID      = 2;
     const LAYOUT4_RET_REC_ALL       = 3;
     const LAYOUT4_RET_REC_FILE_NO_ACCESS    = 4;
     const LAYOUT4_RET_REC_FSID_NO_ACESSS    = 5;
     const LAYOUT4_RET_REC_ALL_NO_ACCESS     = 6;

     enum layoutreturn_type4 {

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     struct layoutreturn_file4 {
           offset4         lrf_offset;
           length4         lrf_length;
           stateid4        lrf_stateid;
           /* layouttype4 specific data */
           opaque          lrf_body<>;

     struct layoutreturn_device_no_access4 {
           deviceid4     lrdna_deviceid;
           nfsstat4      lrdna_status;

     struct layoutreturn_file_no_access4 {
           offset4         lrfna_offset;
           length4         lrfna_length;
           stateid4        lrfna_stateid;
           deviceid4       lrfna_deviceid;
           nfsstat4        lrfna_status;
           /* layouttype4 specific data */
           opaque          lrfna_body<>;

     union layoutreturn4 switch(layoutreturn_type4 lr_returntype) {
           case LAYOUTRETURN4_FILE:
                   layoutreturn_file4             lr_layout;
                   layoutreturn_file_no_access4   lr_layout_na;
                   layoutreturn_device_no_access4      lr_device<>;

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3.2. RESULT (18.44.2)

   The RESULT of the LAYOUTRETURN operation is unchanged; see section
   18.44.2 of [RFC5661].

3.3. DESCRIPTION (18.44.3)

   The following text is added to the end of the LAYOUTRETURN operation
   DESCRIPTION in section 18.44.3 of [RFC5661]:

   There are three NO_ACCESS layoutreturn_type4 values that indicate a
   persistent lack of client ability to access storage device(s),
   LAYOUT4_RET_REC_ALL_NO_ACCESS. A client uses these return types to
   return a layout (or portion thereof) for a file, return all layouts
   for an FSID or all layouts from that server held by the client, and
   in all cases to inform the server that the reason for the return is
   the client's inability to access one or more storage devices. The
   same stateid may be used or the client MAY force use of a new stateid
   in order to report a new error.

   An NFS error value (nfsstat4) is included for each device for these
   three NO_ACCESS return types to provide additional information on the
   cause. The allowed NFS errors are those that are valid for an NFS
   READ or WRITE operation, and NFS4ERR_NXIO is also allowed to report
   an inaccessible device. The server SHOULD log the received NFS error
   value, but that error value does not affect server processing of the
   LAYOUTRETURN operation. All uses of the NO_ACCESS layout return types
   that report NFS errors SHOULD be logged by the client.

   The client MAY use the new LAYOUT4_RET_REC_FILE_NO_ACCESS when only
   one file, or a small number of files are affected. If the access
   problem affects multiple devices, the client may use multiple file
   layout return operations; each return operation SHOULD return a
   layout extent obtained from the device for which an error is being
   reported. In contrast, both LAYOUT4_RET_REC_FSID_NO_ACCESS and
   LAYOUT4_RET_REC_ALL_NO_ACCESS include an array of <device, status>
   pairs to enable a single operation to report errors for multiple
   devices in a single operation.

3.4. IMPLEMENTATION (18.44.4)

   The following text is added to the end of the LAYOUTRETURN operation
   IMPLEMENTATION in section 18.4.4 of [RFC5661]:

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   A client that expects to use pNFS for a mounted filesystem SHOULD
   check for pNFS support at mount time. This check SHOULD be performed
   by sending a GETDEVICELIST operation, followed by layout-type-
   specific checks for accessibility of each storage device returned by
   GETDEVICELIST.  If the NFS server does not support pNFS, the
   GETDEVICELIST operation will be rejected with an NFS4ERR_NOTSUPP
   error; in this situation it is up to the client to determine whether
   it is acceptable to proceed with NFS-only access.

   Clients are expected to tolerate transient storage device errors, and
   hence clients SHOULD NOT use the NO_ACCESS layout return types for
   device access problems that may be transient. The methods by which a
   client decides whether an access problem is transient vs. persistent
   are implementation-specific, but may include retrying I/Os to a data
   server under appropriate conditions.

   When an I/O fails because a storage device is inaccessible, the
   client SHOULD retry the failed I/O via the MDS. In this situation,
   before retrying the I/O, the client SHOULD return the layout, or
   inaccessible portion thereof, and SHOULD indicate which storage
   device or devices was or were inaccessible. If the client does not do
   this, the MDS may issue a layout recall callback in order to perform
   the retried I/O.

   Backwards compatibility may require a client to perform two layout
   return operations to deal with servers that don't implement the
   NO_ACCESS layoutreturn_type4 values and hence respond to them with
   NFS4ERR_INVAL. In this situation, the client SHOULD perform an
   ordinary layout return operation and remember that the new layout
   NO_ACCESS return types are not to be used with that server.

   The metadata server (MDS) SHOULD NOT use storage devices in pNFS
   layouts that are not accessible to the MDS. At a minimum, the server
   SHOULD check its own storage device accessibility before exporting a
   filesystem that supports pNFS and when the device configuration for
   such an exported filesystem is changed (e.g., to add a storage

   If an MDS is aware that a storage device is inaccessible to a client,
   the MDS SHOULD NOT include that storage device in any pNFS layouts
   sent to that client. An MDS SHOULD react to a client return of
   inaccessible layouts by not using the inaccessible storage devices in
   layouts for that client, but the MDS is not required to indefinitely
   retain per-client storage device inaccessibility information. An MDS
   is also not required to automatically reinstate use of a previously
   inaccessible storage device; administrative intervention may be
   required instead.

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   A client MAY perform I/O via the MDS even when the client holds a
   layout that covers the I/O; servers MUST support this client
   behavior, and MAY recall layouts as needed to complete I/Os.

3.4.1. Storage Device Error Mapping (, new)

   The following text is added as new subsection of [RFC5661]:

   An NFS error value is sent for each device that the client reports as
   inaccessible via a NO_ACCESS layout return type. In general:

   o  If the client is unable to access the storage device, NFS4ERR_NXIO
      SHOULD be used.

   o  If the client is able to access the storage device, but permission
      is denied, NFS4ERR_ACCESS SHOULD be used.

   Beyond these two rules, error code usage is layout-type specific:

   o  For the pNFS file layout, an indicative NFS error from a failed
      read or write operation on the inaccessible device SHOULD be used.

   o  For the pNFS block layout, other errors from the Storage Protocol
      SHOULD be mapped to NFS4ERR_IO. In addition, the client SHOULD log
      information about the actual storage protocol error (e.g., SCSI
      status and sense data), but that information is not sent to the
      pNFS server.

   o  For the pNFS object layout, occurrences of the object error types
      specified in [RFC5664] SHOULD be mapped to the following NFS
      errors for use in LAYOUTRETURN:

       o    PNFS_OSD_ERR_EIO            -> NFS4ERR_IO


       o    PNFS_OSD_ERR_NO_SPACE       -> NFS4ERR_NOSPC

       o    PNFS_OSD_ERR_BAD_CRED       -> NFS4ERR_INVAL




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   The LAYOUTRETURN NO_ACCESS return types are used for persistent
   device errors; they do not replace other error reporting mechanisms
   that also apply to transient errors (e.g., as specified for the
   object layout in [RFC5664]).

4. Change to NFS4ERR_NXIO Usage

   This document specifies that the NFS4ERR_NXIO error SHOULD be used to
   report an inaccessible storage device. To enable that usage, this
   document updates [RFC5661] to allow use of the currently obsolete
   remains obsolete for all other uses of NFS errors.

5. Security Considerations

   This document adds a small extension to the NFSv4 LAYOUTRETURN
   operation.  The NFS and pNFS security considerations in [RFC5661],
   [RFC5663] and [RFC5664] apply to the extended LAYOUTRETURN operation.

6. IANA Considerations

   There are no additional IANA considerations in this document beyond
   the IANA Considerations covered in [RFC5661].

7. Conclusions

   This draft specifies additions to the pNFS protocol addressing
   inability to access storage devices used in pNFS layouts.

8. References

8.1. Normative References

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

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

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

   [RFC5664] Halevy, B., Welch, B., Zelenka, J., "Object-Based Parallel
             NFS (pNFS) Operations",,
             January 2010

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


   This draft includes ideas from discussions with the primary author of
   the pNFS object layout, Benny Halevy, and the Linux kernel pNFS
   maintainers, including Bruce Fields.  In addition, we thank the IETF
   nfsv4 WG and the following individuals for their comments on prior
   versions of this draft: Tom Haynes.

   This document was prepared using

Changes from draft-faibish-nfsv4-pnfs-access-permissions-check-03

   - First nfsv4 WG draft version.
   - Add ALL NO_ACCESS return type, so that there's a NO_ACCESS
      return type for every current return type.
   - Use NFS4ERR_ACCESS instead of NFS4ERR_PERM, and allow use of
      NFS4ERR_NXIO for an unreachable data server.
   - Simplify recommendation for initial access checks to only discuss
   - Add client guidance on riding through transient errors.
   - State that server does not need to indefinitely retain device
      inaccessibility information, and administrative intervention
      may be required to restore use of a previously inaccessible
      storage device.
   - Remove "MUST" requirement for layout return if retry via MDS
      fails.  Add warning that if the layout isn't returned in advance
      of MDS I/O retry, the MDS may issue a callback to get it.
   - Specify allowed errors (in payload) via reference to errors
      allowed for READ and WRITE, plus allow NFS4ERR_NXIO.
   - Provide information about how to map device errors (especially
      from non-file layout types) to NFS errors.

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   Authors' Addresses

   Sorin Faibish (editor)
   EMC Corporation
   228 South Street
   Hopkinton, MA 01748

   Phone: +1 (508) 249-5745

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

   Phone: +1 (508) 293-7953

   Michael Eisler
   5765 Chase Point Circle
   Colorado Springs, CO  80919

   Phone: +1 (719) 599-9026

   Jason Glasgow
   5 Cambridge Center, Floors 3-6
   Cambridge, MA  02142

   Phone: +1 (617) 575-1599

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