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Keywords: [--------], OSD, storage device
Internet Engineering Task Force (IETF) B. Halevy
Request for Comments: 5664 B. Welch
Category: Standards Track J. Zelenka
ISSN: 2070-1721 Panasas
January 2010
Object-Based Parallel NFS (pNFS) Operations
Abstract
Parallel NFS (pNFS) extends Network File System version 4 (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, a.k.a. the Layout Type. The main pNFS
operations and data types in NFSv4 Minor version 1 specify a layout-
type-independent layer; layout-type-specific information is conveyed
using opaque data structures whose internal structure is further
defined by the particular layout type specification. This document
specifies the NFSv4.1 Object-Based pNFS Layout Type as a companion to
the main NFSv4 Minor version 1 specification.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5664.
Halevy, et al. Standards Track [Page 1]
RFC 5664 pNFS Objects January 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Language ......................................4
2. XDR Description of the Objects-Based Layout Protocol ............4
2.1. Code Components Licensing Notice ...........................4
3. Basic Data Type Definitions .....................................6
3.1. pnfs_osd_objid4 ............................................6
3.2. pnfs_osd_version4 ..........................................6
3.3. pnfs_osd_object_cred4 ......................................7
3.4. pnfs_osd_raid_algorithm4 ...................................8
4. Object Storage Device Addressing and Discovery ..................8
4.1. pnfs_osd_targetid_type4 ...................................10
4.2. pnfs_osd_deviceaddr4 ......................................10
4.2.1. SCSI Target Identifier .............................11
4.2.2. Device Network Address .............................11
5. Object-Based Layout ............................................12
5.1. pnfs_osd_data_map4 ........................................13
5.2. pnfs_osd_layout4 ..........................................14
5.3. Data Mapping Schemes ......................................14
5.3.1. Simple Striping ....................................15
5.3.2. Nested Striping ....................................16
5.3.3. Mirroring ..........................................17
5.4. RAID Algorithms ...........................................18
5.4.1. PNFS_OSD_RAID_0 ....................................18
5.4.2. PNFS_OSD_RAID_4 ....................................18
5.4.3. PNFS_OSD_RAID_5 ....................................18
5.4.4. PNFS_OSD_RAID_PQ ...................................19
5.4.5. RAID Usage and Implementation Notes ................19
6. Object-Based Layout Update .....................................20
6.1. pnfs_osd_deltaspaceused4 ..................................20
6.2. pnfs_osd_layoutupdate4 ....................................21
7. Recovering from Client I/O Errors ..............................21
Halevy, et al. Standards Track [Page 2]
RFC 5664 pNFS Objects January 2010
8. Object-Based Layout Return .....................................22
8.1. pnfs_osd_errno4 ...........................................23
8.2. pnfs_osd_ioerr4 ...........................................24
8.3. pnfs_osd_layoutreturn4 ....................................24
9. Object-Based Creation Layout Hint ..............................25
9.1. pnfs_osd_layouthint4 ......................................25
10. Layout Segments ...............................................26
10.1. CB_LAYOUTRECALL and LAYOUTRETURN .........................27
10.2. LAYOUTCOMMIT .............................................27
11. Recalling Layouts .............................................27
11.1. CB_RECALL_ANY ............................................28
12. Client Fencing ................................................29
13. Security Considerations .......................................29
13.1. OSD Security Data Types ..................................30
13.2. The OSD Security Protocol ................................30
13.3. Protocol Privacy Requirements ............................32
13.4. Revoking Capabilities ....................................32
14. IANA Considerations ...........................................33
15. References ....................................................33
15.1. Normative References .....................................33
15.2. Informative References ...................................34
Appendix A. Acknowledgments ......................................35
1. Introduction
In pNFS, the file server returns typed layout structures that
describe where file data is located. There are different layouts for
different storage systems and methods of arranging data on storage
devices. This document describes the layouts used with object-based
storage devices (OSDs) that are accessed according to the OSD storage
protocol standard (ANSI INCITS 400-2004 [1]).
An "object" is a container for data and attributes, and files are
stored in one or more objects. The OSD protocol specifies several
operations on objects, including READ, WRITE, FLUSH, GET ATTRIBUTES,
SET ATTRIBUTES, CREATE, and DELETE. However, using the object-based
layout the client only uses the READ, WRITE, GET ATTRIBUTES, and
FLUSH commands. The other commands are only used by the pNFS server.
An object-based layout for pNFS includes object identifiers,
capabilities that allow clients to READ or WRITE those objects, and
various parameters that control how file data is striped across their
component objects. The OSD protocol has a capability-based security
scheme that allows the pNFS server to control what operations and
what objects can be used by clients. This scheme is described in
more detail in the "Security Considerations" section (Section 13).
Halevy, et al. Standards Track [Page 3]
RFC 5664 pNFS Objects January 2010
1.1. Requirements Language
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 [2].
2. XDR Description of the Objects-Based Layout Protocol
This document contains the external data representation (XDR [3])
description of the NFSv4.1 objects layout protocol. The XDR
description is embedded in this document in a way that makes it
simple for the reader to extract into a ready-to-compile form. The
reader can feed this document into the following shell script to
produce the machine readable XDR description of the NFSv4.1 objects
layout protocol:
#!/bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'
That is, if the above script is stored in a file called "extract.sh",
and this document is in a file called "spec.txt", then the reader can
do:
sh extract.sh < spec.txt > pnfs_osd_prot.x
The effect of the script is to remove leading white space from each
line, plus a sentinel sequence of "///".
The embedded XDR file header follows. Subsequent XDR descriptions,
with the sentinel sequence are embedded throughout the document.
Note that the XDR code contained in this document depends on types
from the NFSv4.1 nfs4_prot.x file ([4]). This includes both nfs
types that end with a 4, such as offset4, length4, etc., as well as
more generic types such as uint32_t and uint64_t.
2.1. Code Components Licensing Notice
The XDR description, marked with lines beginning with the sequence
"///", as well as scripts for extracting the XDR description are Code
Components as described in Section 4 of "Legal Provisions Relating to
IETF Documents" [5]. These Code Components are licensed according to
the terms of Section 4 of "Legal Provisions Relating to IETF
Documents".
Halevy, et al. Standards Track [Page 4]
RFC 5664 pNFS Objects January 2010
/// /*
/// * Copyright (c) 2010 IETF Trust and the persons identified
/// * as authors of the code. All rights reserved.
/// *
/// * Redistribution and use in source and binary forms, with
/// * or without modification, are permitted provided that the
/// * following conditions are met:
/// *
/// * o Redistributions of source code must retain the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer.
/// *
/// * o Redistributions in binary form must reproduce the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer in the documentation and/or other
/// * materials provided with the distribution.
/// *
/// * o Neither the name of Internet Society, IETF or IETF
/// * Trust, nor the names of specific contributors, may be
/// * used to endorse or promote products derived from this
/// * software without specific prior written permission.
/// *
/// * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
/// * AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
/// * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
/// * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
/// * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
/// * EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
/// * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
/// * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
/// * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
/// * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
/// * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
/// * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
/// * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
/// * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
/// * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
/// *
/// * This code was derived from RFC 5664.
/// * Please reproduce this note if possible.
/// */
///
/// /*
/// * pnfs_osd_prot.x
/// */
///
/// %#include <nfs4_prot.x>
///
Halevy, et al. Standards Track [Page 5]
RFC 5664 pNFS Objects January 2010
3. Basic Data Type Definitions
The following sections define basic data types and constants used by
the Object-Based Layout protocol.
3.1. pnfs_osd_objid4
An object is identified by a number, somewhat like an inode number.
The object storage model has a two-level scheme, where the objects
within an object storage device are grouped into partitions.
/// struct pnfs_osd_objid4 {
/// deviceid4 oid_device_id;
/// uint64_t oid_partition_id;
/// uint64_t oid_object_id;
/// };
///
The pnfs_osd_objid4 type is used to identify an object within a
partition on a specified object storage device. "oid_device_id"
selects the object storage device from the set of available storage
devices. The device is identified with the deviceid4 type, which is
an index into addressing information about that device returned by
the GETDEVICELIST and GETDEVICEINFO operations. The deviceid4 data
type is defined in NFSv4.1 [6]. Within an OSD, a partition is
identified with a 64-bit number, "oid_partition_id". Within a
partition, an object is identified with a 64-bit number,
"oid_object_id". Creation and management of partitions is outside
the scope of this document, and is a facility provided by the object-
based storage file system.
3.2. pnfs_osd_version4
/// enum pnfs_osd_version4 {
/// PNFS_OSD_MISSING = 0,
/// PNFS_OSD_VERSION_1 = 1,
/// PNFS_OSD_VERSION_2 = 2
/// };
///
pnfs_osd_version4 is used to indicate the OSD protocol version or
whether an object is missing (i.e., unavailable). Some of the
object-based layout-supported RAID algorithms encode redundant
information and can compensate for missing components, but the data
placement algorithm needs to know what parts are missing.
Halevy, et al. Standards Track [Page 6]
RFC 5664 pNFS Objects January 2010
At this time, the OSD standard is at version 1.0, and we anticipate a
version 2.0 of the standard (SNIA T10/1729-D [14]). The second
generation OSD protocol has additional proposed features to support
more robust error recovery, snapshots, and byte-range capabilities.
Therefore, the OSD version is explicitly called out in the
information returned in the layout. (This information can also be
deduced by looking inside the capability type at the format field,
which is the first byte. The format value is 0x1 for an OSD v1
capability. However, it seems most robust to call out the version
explicitly.)
3.3. pnfs_osd_object_cred4
/// enum pnfs_osd_cap_key_sec4 {
/// PNFS_OSD_CAP_KEY_SEC_NONE = 0,
/// PNFS_OSD_CAP_KEY_SEC_SSV = 1
/// };
///
/// struct pnfs_osd_object_cred4 {
/// pnfs_osd_objid4 oc_object_id;
/// pnfs_osd_version4 oc_osd_version;
/// pnfs_osd_cap_key_sec4 oc_cap_key_sec;
/// opaque oc_capability_key<>;
/// opaque oc_capability<>;
/// };
///
The pnfs_osd_object_cred4 structure is used to identify each
component comprising the file. The "oc_object_id" identifies the
component object, the "oc_osd_version" represents the osd protocol
version, or whether that component is unavailable, and the
"oc_capability" and "oc_capability_key", along with the
"oda_systemid" from the pnfs_osd_deviceaddr4, provide the OSD
security credentials needed to access that object. The
"oc_cap_key_sec" value denotes the method used to secure the
oc_capability_key (see Section 13.1 for more details).
To comply with the OSD security requirements, the capability key
SHOULD be transferred securely to prevent eavesdropping (see
Section 13). Therefore, a client SHOULD either issue the LAYOUTGET
or GETDEVICEINFO operations via RPCSEC_GSS with the privacy service
or previously establish a secret state verifier (SSV) for the
sessions via the NFSv4.1 SET_SSV operation. The
pnfs_osd_cap_key_sec4 type is used to identify the method used by the
server to secure the capability key.
Halevy, et al. Standards Track [Page 7]
RFC 5664 pNFS Objects January 2010
o PNFS_OSD_CAP_KEY_SEC_NONE denotes that the oc_capability_key is
not encrypted, in which case the client SHOULD issue the LAYOUTGET
or GETDEVICEINFO operations with RPCSEC_GSS with the privacy
service or the NFSv4.1 transport should be secured by using
methods that are external to NFSv4.1 like the use of IPsec [15]
for transporting the NFSV4.1 protocol.
o PNFS_OSD_CAP_KEY_SEC_SSV denotes that the oc_capability_key
contents are encrypted using the SSV GSS context and the
capability key as inputs to the GSS_Wrap() function (see GSS-API
[7]) with the conf_req_flag set to TRUE. The client MUST use the
secret SSV key as part of the client's GSS context to decrypt the
capability key using the value of the oc_capability_key field as
the input_message to the GSS_unwrap() function. Note that to
prevent eavesdropping of the SSV key, the client SHOULD issue
SET_SSV via RPCSEC_GSS with the privacy service.
The actual method chosen depends on whether the client established a
SSV key with the server and whether it issued the operation with the
RPCSEC_GSS privacy method. Naturally, if the client did not
establish an SSV key via SET_SSV, the server MUST use the
PNFS_OSD_CAP_KEY_SEC_NONE method. Otherwise, if the operation was
not issued with the RPCSEC_GSS privacy method, the server SHOULD
secure the oc_capability_key with the PNFS_OSD_CAP_KEY_SEC_SSV
method. The server MAY use the PNFS_OSD_CAP_KEY_SEC_SSV method also
when the operation was issued with the RPCSEC_GSS privacy method.
3.4. pnfs_osd_raid_algorithm4
/// enum pnfs_osd_raid_algorithm4 {
/// PNFS_OSD_RAID_0 = 1,
/// PNFS_OSD_RAID_4 = 2,
/// PNFS_OSD_RAID_5 = 3,
/// PNFS_OSD_RAID_PQ = 4 /* Reed-Solomon P+Q */
/// };
///
pnfs_osd_raid_algorithm4 represents the data redundancy algorithm
used to protect the file's contents. See Section 5.4 for more
details.
4. Object Storage Device Addressing and Discovery
Data operations to an OSD require the client to know the "address" of
each OSD's root object. The root object is synonymous with the Small
Computer System Interface (SCSI) logical unit. The client specifies
SCSI logical units to its SCSI protocol stack using a representation
Halevy, et al. Standards Track [Page 8]
RFC 5664 pNFS Objects January 2010
local to the client. Because these representations are local,
GETDEVICEINFO must return information that can be used by the client
to select the correct local representation.
In the block world, a set offset (logical block number or track/
sector) contains a disk label. This label identifies the disk
uniquely. In contrast, an OSD has a standard set of attributes on
its root object. For device identification purposes, the OSD System
ID (root information attribute number 3) and the OSD Name (root
information attribute number 9) are used as the label. These appear
in the pnfs_osd_deviceaddr4 type below under the "oda_systemid" and
"oda_osdname" fields.
In some situations, SCSI target discovery may need to be driven based
on information contained in the GETDEVICEINFO response. One example
of this is Internet SCSI (iSCSI) targets that are not known to the
client until a layout has been requested. The information provided
as the "oda_targetid", "oda_targetaddr", and "oda_lun" fields in the
pnfs_osd_deviceaddr4 type described below (see Section 4.2) allows
the client to probe a specific device given its network address and
optionally its iSCSI Name (see iSCSI [8]), or when the device network
address is omitted, allows it to discover the object storage device
using the provided device name or SCSI Device Identifier (see SPC-3
[9].)
The oda_systemid is implicitly used by the client, by using the
object credential signing key to sign each request with the request
integrity check value. This method protects the client from
unintentionally accessing a device if the device address mapping was
changed (or revoked). The server computes the capability key using
its own view of the systemid associated with the respective deviceid
present in the credential. If the client's view of the deviceid
mapping is stale, the client will use the wrong systemid (which must
be system-wide unique) and the I/O request to the OSD will fail to
pass the integrity check verification.
To recover from this condition the client should report the error and
return the layout using LAYOUTRETURN, and invalidate all the device
address mappings associated with this layout. The client can then
ask for a new layout if it wishes using LAYOUTGET and resolve the
referenced deviceids using GETDEVICEINFO or GETDEVICELIST.
The server MUST provide the oda_systemid and SHOULD also provide the
oda_osdname. When the OSD name is present, the client SHOULD get the
root information attributes whenever it establishes communication
with the OSD and verify that the OSD name it got from the OSD matches
the one sent by the metadata server. To do so, the client uses the
root_obj_cred credentials.
Halevy, et al. Standards Track [Page 9]
RFC 5664 pNFS Objects January 2010
4.1. pnfs_osd_targetid_type4
The following enum specifies the manner in which a SCSI target can be
specified. The target can be specified as a SCSI Name, or as an SCSI
Device Identifier.
/// enum pnfs_osd_targetid_type4 {
/// OBJ_TARGET_ANON = 1,
/// OBJ_TARGET_SCSI_NAME = 2,
/// OBJ_TARGET_SCSI_DEVICE_ID = 3
/// };
///
4.2. pnfs_osd_deviceaddr4
The specification for an object device address is as follows:
/// union pnfs_osd_targetid4 switch (pnfs_osd_targetid_type4 oti_type) {
/// case OBJ_TARGET_SCSI_NAME:
/// string oti_scsi_name<>;
///
/// case OBJ_TARGET_SCSI_DEVICE_ID:
/// opaque oti_scsi_device_id<>;
///
/// default:
/// void;
/// };
///
/// union pnfs_osd_targetaddr4 switch (bool ota_available) {
/// case TRUE:
/// netaddr4 ota_netaddr;
/// case FALSE:
/// void;
/// };
///
/// struct pnfs_osd_deviceaddr4 {
/// pnfs_osd_targetid4 oda_targetid;
/// pnfs_osd_targetaddr4 oda_targetaddr;
/// opaque oda_lun[8];
/// opaque oda_systemid<>;
/// pnfs_osd_object_cred4 oda_root_obj_cred;
/// opaque oda_osdname<>;
/// };
///
Halevy, et al. Standards Track [Page 10]
RFC 5664 pNFS Objects January 2010
4.2.1. SCSI Target Identifier
When "oda_targetid" is specified as an OBJ_TARGET_SCSI_NAME, the
"oti_scsi_name" string MUST be formatted as an "iSCSI Name" as
specified in iSCSI [8] and [10]. Note that the specification of the
oti_scsi_name string format is outside the scope of this document.
Parsing the string is based on the string prefix, e.g., "iqn.",
"eui.", or "naa." and more formats MAY be specified in the future in
accordance with iSCSI Names properties.
Currently, the iSCSI Name provides for naming the target device using
a string formatted as an iSCSI Qualified Name (IQN) or as an Extended
Unique Identifier (EUI) [11] string. Those are typically used to
identify iSCSI or Secure Routing Protocol (SRP) [16] devices. The
Network Address Authority (NAA) string format (see [10]) provides for
naming the device using globally unique identifiers, as defined in
Fibre Channel Framing and Signaling (FC-FS) [17]. These are
typically used to identify Fibre Channel or SAS [18] (Serial Attached
SCSI) devices. In particular, such devices that are dual-attached
both over Fibre Channel or SAS and over iSCSI.
When "oda_targetid" is specified as an OBJ_TARGET_SCSI_DEVICE_ID, the
"oti_scsi_device_id" opaque field MUST be formatted as a SCSI Device
Identifier as defined in SPC-3 [9] VPD Page 83h (Section 7.6.3.
"Device Identification VPD Page"). If the Device Identifier is
identical to the OSD System ID, as given by oda_systemid, the server
SHOULD provide a zero-length oti_scsi_device_id opaque value. Note
that similarly to the "oti_scsi_name", the specification of the
oti_scsi_device_id opaque contents is outside the scope of this
document and more formats MAY be specified in the future in
accordance with SPC-3.
The OBJ_TARGET_ANON pnfs_osd_targetid_type4 MAY be used for providing
no target identification. In this case, only the OSD System ID, and
optionally the provided network address, are used to locate the
device.
4.2.2. Device Network Address
The optional "oda_targetaddr" field MAY be provided by the server as
a hint to accelerate device discovery over, e.g., the iSCSI transport
protocol. The network address is given with the netaddr4 type, which
specifies a TCP/IP based endpoint (as specified in NFSv4.1 [6]).
When given, the client SHOULD use it to probe for the SCSI device at
the given network address. The client MAY still use other discovery
mechanisms such as Internet Storage Name Service (iSNS) [12] to
locate the device using the oda_targetid. In particular, such an
Halevy, et al. Standards Track [Page 11]
RFC 5664 pNFS Objects January 2010
external name service SHOULD be used when the devices may be attached
to the network using multiple connections, and/or multiple storage
fabrics (e.g., Fibre-Channel and iSCSI).
The "oda_lun" field identifies the OSD 64-bit Logical Unit Number,
formatted in accordance with SAM-3 [13]. The client uses the Logical
Unit Number to communicate with the specific OSD Logical Unit. Its
use is defined in detail by the SCSI transport protocol, e.g., iSCSI
[8].
5. Object-Based Layout
The layout4 type is defined in the NFSv4.1 [6] as follows:
enum layouttype4 {
LAYOUT4_NFSV4_1_FILES = 1,
LAYOUT4_OSD2_OBJECTS = 2,
LAYOUT4_BLOCK_VOLUME = 3
};
struct layout_content4 {
layouttype4 loc_type;
opaque loc_body<>;
};
struct layout4 {
offset4 lo_offset;
length4 lo_length;
layoutiomode4 lo_iomode;
layout_content4 lo_content;
};
This document defines structure associated with the layouttype4
value, LAYOUT4_OSD2_OBJECTS. The NFSv4.1 [6] specifies the loc_body
structure as an XDR type "opaque". The opaque layout is
uninterpreted by the generic pNFS client layers, but obviously must
be interpreted by the object storage layout driver. This section
defines the structure of this opaque value, pnfs_osd_layout4.
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RFC 5664 pNFS Objects January 2010
5.1. pnfs_osd_data_map4
/// struct pnfs_osd_data_map4 {
/// uint32_t odm_num_comps;
/// length4 odm_stripe_unit;
/// uint32_t odm_group_width;
/// uint32_t odm_group_depth;
/// uint32_t odm_mirror_cnt;
/// pnfs_osd_raid_algorithm4 odm_raid_algorithm;
/// };
///
The pnfs_osd_data_map4 structure parameterizes the algorithm that
maps a file's contents over the component objects. Instead of
limiting the system to simple striping scheme where loss of a single
component object results in data loss, the map parameters support
mirroring and more complicated schemes that protect against loss of a
component object.
"odm_num_comps" is the number of component objects the file is
striped over. The server MAY grow the file by adding more components
to the stripe while clients hold valid layouts until the file has
reached its final stripe width. The file length in this case MUST be
limited to the number of bytes in a full stripe.
The "odm_stripe_unit" is the number of bytes placed on one component
before advancing to the next one in the list of components. The
number of bytes in a full stripe is odm_stripe_unit times the number
of components. In some RAID schemes, a stripe includes redundant
information (i.e., parity) that lets the system recover from loss or
damage to a component object.
The "odm_group_width" and "odm_group_depth" parameters allow a nested
striping pattern (see Section 5.3.2 for details). If there is no
nesting, then odm_group_width and odm_group_depth MUST be zero. The
size of the components array MUST be a multiple of odm_group_width.
The "odm_mirror_cnt" is used to replicate a file by replicating its
component objects. If there is no mirroring, then odm_mirror_cnt
MUST be 0. If odm_mirror_cnt is greater than zero, then the size of
the component array MUST be a multiple of (odm_mirror_cnt+1).
See Section 5.3 for more details.
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RFC 5664 pNFS Objects January 2010
5.2. pnfs_osd_layout4
/// struct pnfs_osd_layout4 {
/// pnfs_osd_data_map4 olo_map;
/// uint32_t olo_comps_index;
/// pnfs_osd_object_cred4 olo_components<>;
/// };
///
The pnfs_osd_layout4 structure specifies a layout over a set of
component objects. The "olo_components" field is an array of object
identifiers and security credentials that grant access to each
object. The organization of the data is defined by the
pnfs_osd_data_map4 type that specifies how the file's data is mapped
onto the component objects (i.e., the striping pattern). The data
placement algorithm that maps file data onto component objects
assumes that each component object occurs exactly once in the array
of components. Therefore, component objects MUST appear in the
olo_components array only once. The components array may represent
all objects comprising the file, in which case "olo_comps_index" is
set to zero and the number of entries in the olo_components array is
equal to olo_map.odm_num_comps. The server MAY return fewer
components than odm_num_comps, provided that the returned components
are sufficient to access any byte in the layout's data range (e.g., a
sub-stripe of "odm_group_width" components). In this case,
olo_comps_index represents the position of the returned components
array within the full array of components that comprise the file.
Note that the layout depends on the file size, which the client
learns from the generic return parameters of LAYOUTGET, by doing
GETATTR commands to the metadata server. The client uses the file
size to decide if it should fill holes with zeros or return a short
read. Striping patterns can cause cases where component objects are
shorter than other components because a hole happens to correspond to
the last part of the component object.
5.3. Data Mapping Schemes
This section describes the different data mapping schemes in detail.
The object layout always uses a "dense" layout as described in
NFSv4.1 [6]. This means that the second stripe unit of the file
starts at offset 0 of the second component, rather than at offset
stripe_unit bytes. After a full stripe has been written, the next
stripe unit is appended to the first component object in the list
without any holes in the component objects.
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5.3.1. Simple Striping
The mapping from the logical offset within a file (L) to the
component object C and object-specific offset O is defined by the
following equations:
L = logical offset into the file
W = total number of components
S = W * stripe_unit
N = L / S
C = (L-(N*S)) / stripe_unit
O = (N*stripe_unit)+(L%stripe_unit)
In these equations, S is the number of bytes in a full stripe, and N
is the stripe number. C is an index into the array of components, so
it selects a particular object storage device. Both N and C count
from zero. O is the offset within the object that corresponds to the
file offset. Note that this computation does not accommodate the
same object appearing in the olo_components array multiple times.
For example, consider an object striped over four devices, <D0 D1 D2
D3>. The stripe_unit is 4096 bytes. The stripe width S is thus 4 *
4096 = 16384.
Offset 0:
N = 0 / 16384 = 0
C = 0-0/4096 = 0 (D0)
O = 0*4096 + (0%4096) = 0
Offset 4096:
N = 4096 / 16384 = 0
C = (4096-(0*16384)) / 4096 = 1 (D1)
O = (0*4096)+(4096%4096) = 0
Offset 9000:
N = 9000 / 16384 = 0
C = (9000-(0*16384)) / 4096 = 2 (D2)
O = (0*4096)+(9000%4096) = 808
Offset 132000:
N = 132000 / 16384 = 8
C = (132000-(8*16384)) / 4096 = 0 (D0)
O = (8*4096) + (132000%4096) = 33696
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5.3.2. Nested Striping
The odm_group_width and odm_group_depth parameters allow a nested
striping pattern. odm_group_width defines the width of a data stripe
and odm_group_depth defines how many stripes are written before
advancing to the next group of components in the list of component
objects for the file. The math used to map from a file offset to a
component object and offset within that object is shown below. The
computations map from the logical offset L to the component index C
and offset relative O within that component object.
L = logical offset into the file
W = total number of components
S = stripe_unit * group_depth * W
T = stripe_unit * group_depth * group_width
U = stripe_unit * group_width
M = L / S
G = (L - (M * S)) / T
H = (L - (M * S)) % T
N = H / U
C = (H - (N * U)) / stripe_unit + G * group_width
O = L % stripe_unit + N * stripe_unit + M * group_depth * stripe_unit
In these equations, S is the number of bytes striped across all
component objects before the pattern repeats. T is the number of
bytes striped within a group of component objects before advancing to
the next group. U is the number of bytes in a stripe within a group.
M is the "major" (i.e., across all components) stripe number, and N
is the "minor" (i.e., across the group) stripe number. G counts the
groups from the beginning of the major stripe, and H is the byte
offset within the group.
For example, consider an object striped over 100 devices with a
group_width of 10, a group_depth of 50, and a stripe_unit of 1 MB.
In this scheme, 500 MB are written to the first 10 components, and
5000 MB are written before the pattern wraps back around to the first
component in the array.
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Offset 0:
W = 100
S = 1 MB * 50 * 100 = 5000 MB
T = 1 MB * 50 * 10 = 500 MB
U = 1 MB * 10 = 10 MB
M = 0 / 5000 MB = 0
G = (0 - (0 * 5000 MB)) / 500 MB = 0
H = (0 - (0 * 5000 MB)) % 500 MB = 0
N = 0 / 10 MB = 0
C = (0 - (0 * 10 MB)) / 1 MB + 0 * 10 = 0
O = 0 % 1 MB + 0 * 1 MB + 0 * 50 * 1 MB = 0
Offset 27 MB:
M = 27 MB / 5000 MB = 0
G = (27 MB - (0 * 5000 MB)) / 500 MB = 0
H = (27 MB - (0 * 5000 MB)) % 500 MB = 27 MB
N = 27 MB / 10 MB = 2
C = (27 MB - (2 * 10 MB)) / 1 MB + 0 * 10 = 7
O = 27 MB % 1 MB + 2 * 1 MB + 0 * 50 * 1 MB = 2 MB
Offset 7232 MB:
M = 7232 MB / 5000 MB = 1
G = (7232 MB - (1 * 5000 MB)) / 500 MB = 4
H = (7232 MB - (1 * 5000 MB)) % 500 MB = 232 MB
N = 232 MB / 10 MB = 23
C = (232 MB - (23 * 10 MB)) / 1 MB + 4 * 10 = 42
O = 7232 MB % 1 MB + 23 * 1 MB + 1 * 50 * 1 MB = 73 MB
5.3.3. Mirroring
The odm_mirror_cnt is used to replicate a file by replicating its
component objects. If there is no mirroring, then odm_mirror_cnt
MUST be 0. If odm_mirror_cnt is greater than zero, then the size of
the olo_components array MUST be a multiple of (odm_mirror_cnt+1).
Thus, for a classic mirror on two objects, odm_mirror_cnt is one.
Note that mirroring can be defined over any RAID algorithm and
striping pattern (either simple or nested). If odm_group_width is
also non-zero, then the size of the olo_components array MUST be a
multiple of odm_group_width * (odm_mirror_cnt+1). Replicas are
adjacent in the olo_components array, and the value C produced by the
above equations is not a direct index into the olo_components array.
Instead, the following equations determine the replica component
index RCi, where i ranges from 0 to odm_mirror_cnt.
C = component index for striping or two-level striping
i ranges from 0 to odm_mirror_cnt, inclusive
RCi = C * (odm_mirror_cnt+1) + i
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5.4. RAID Algorithms
pnfs_osd_raid_algorithm4 determines the algorithm and placement of
redundant data. This section defines the different redundancy
algorithms. Note: The term "RAID" (Redundant Array of Independent
Disks) is used in this document to represent an array of component
objects that store data for an individual file. The objects are
stored on independent object-based storage devices. File data is
encoded and striped across the array of component objects using
algorithms developed for block-based RAID systems.
5.4.1. PNFS_OSD_RAID_0
PNFS_OSD_RAID_0 means there is no parity data, so all bytes in the
component objects are data bytes located by the above equations for C
and O. If a component object is marked as PNFS_OSD_MISSING, the pNFS
client MUST either return an I/O error if this component is attempted
to be read or, alternatively, it can retry the READ against the pNFS
server.
5.4.2. PNFS_OSD_RAID_4
PNFS_OSD_RAID_4 means that the last component object, or the last in
each group (if odm_group_width is greater than zero), contains parity
information computed over the rest of the stripe with an XOR
operation. If a component object is unavailable, the client can read
the rest of the stripe units in the damaged stripe and recompute the
missing stripe unit by XORing the other stripe units in the stripe.
Or the client can replay the READ against the pNFS server that will
presumably perform the reconstructed read on the client's behalf.
When parity is present in the file, then there is an additional
computation to map from the file offset L to the offset that accounts
for embedded parity, L'. First compute L', and then use L' in the
above equations for C and O.
L = file offset, not accounting for parity
P = number of parity devices in each stripe
W = group_width, if not zero, else size of olo_components array
N = L / (W-P * stripe_unit)
L' = N * (W * stripe_unit) +
(L % (W-P * stripe_unit))
5.4.3. PNFS_OSD_RAID_5
PNFS_OSD_RAID_5 means that the position of the parity data is rotated
on each stripe or each group (if odm_group_width is greater than
zero). In the first stripe, the last component holds the parity. In
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the second stripe, the next-to-last component holds the parity, and
so on. In this scheme, all stripe units are rotated so that I/O is
evenly spread across objects as the file is read sequentially. The
rotated parity layout is illustrated here, with numbers indicating
the stripe unit.
0 1 2 P
4 5 P 3
8 P 6 7
P 9 a b
To compute the component object C, first compute the offset that
accounts for parity L' and use that to compute C. Then rotate C to
get C'. Finally, increase C' by one if the parity information comes
at or before C' within that stripe. The following equations
illustrate this by computing I, which is the index of the component
that contains parity for a given stripe.
L = file offset, not accounting for parity
W = odm_group_width, if not zero, else size of olo_components array
N = L / (W-1 * stripe_unit)
(Compute L' as describe above)
(Compute C based on L' as described above)
C' = (C - (N%W)) % W
I = W - (N%W) - 1
if (C' <= I) {
C'++
}
5.4.4. PNFS_OSD_RAID_PQ
PNFS_OSD_RAID_PQ is a double-parity scheme that uses the Reed-Solomon
P+Q encoding scheme [19]. In this layout, the last two component
objects hold the P and Q data, respectively. P is parity computed
with XOR, and Q is a more complex equation that is not described
here. The equations given above for embedded parity can be used to
map a file offset to the correct component object by setting the
number of parity components to 2 instead of 1 for RAID4 or RAID5.
Clients may simply choose to read data through the metadata server if
two components are missing or damaged.
5.4.5. RAID Usage and Implementation Notes
RAID layouts with redundant data in their stripes require additional
serialization of updates to ensure correct operation. Otherwise, if
two clients simultaneously write to the same logical range of an
object, the result could include different data in the same ranges of
mirrored tuples, or corrupt parity information. It is the
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responsibility of the metadata server to enforce serialization
requirements such as this. For example, the metadata server may do
so by not granting overlapping write layouts within mirrored objects.
6. Object-Based Layout Update
layoutupdate4 is used in the LAYOUTCOMMIT operation to convey updates
to the layout and additional information to the metadata server. It
is defined in the NFSv4.1 [6] as follows:
struct layoutupdate4 {
layouttype4 lou_type;
opaque lou_body<>;
};
The layoutupdate4 type is an opaque value at the generic pNFS client
level. If the lou_type layout type is LAYOUT4_OSD2_OBJECTS, then the
lou_body opaque value is defined by the pnfs_osd_layoutupdate4 type.
Object-Based pNFS clients are not allowed to modify the layout.
Therefore, the information passed in pnfs_osd_layoutupdate4 is used
only to update the file's attributes. In addition to the generic
information the client can pass to the metadata server in
LAYOUTCOMMIT such as the highest offset the client wrote to and the
last time it modified the file, the client MAY use
pnfs_osd_layoutupdate4 to convey the capacity consumed (or released)
by writes using the layout, and to indicate that I/O errors were
encountered by such writes.
6.1. pnfs_osd_deltaspaceused4
/// union pnfs_osd_deltaspaceused4 switch (bool dsu_valid) {
/// case TRUE:
/// int64_t dsu_delta;
/// case FALSE:
/// void;
/// };
///
pnfs_osd_deltaspaceused4 is used to convey space utilization
information at the time of LAYOUTCOMMIT. For the file system to
properly maintain capacity-used information, it needs to track how
much capacity was consumed by WRITE operations performed by the
client. In this protocol, the OSD returns the capacity consumed by a
write (*), which can be different than the number of bytes written
because of internal overhead like block-level allocation and indirect
blocks, and the client reflects this back to the pNFS server so it
can accurately track quota. The pNFS server can choose to trust this
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information coming from the clients and therefore avoid querying the
OSDs at the time of LAYOUTCOMMIT. If the client is unable to obtain
this information from the OSD, it simply returns invalid
olu_delta_space_used.
6.2. pnfs_osd_layoutupdate4
/// struct pnfs_osd_layoutupdate4 {
/// pnfs_osd_deltaspaceused4 olu_delta_space_used;
/// bool olu_ioerr_flag;
/// };
///
"olu_delta_space_used" is used to convey capacity usage information
back to the metadata server.
The "olu_ioerr_flag" is used when I/O errors were encountered while
writing the file. The client MUST report the errors using the
pnfs_osd_ioerr4 structure (see Section 8.1) at LAYOUTRETURN time.
If the client updated the file successfully before hitting the I/O
errors, it MAY use LAYOUTCOMMIT to update the metadata server as
described above. Typically, in the error-free case, the server MAY
turn around and update the file's attributes on the storage devices.
However, if I/O errors were encountered, the server better not
attempt to write the new attributes on the storage devices until it
receives the I/O error report; therefore, the client MUST set the
olu_ioerr_flag to true. Note that in this case, the client SHOULD
send both the LAYOUTCOMMIT and LAYOUTRETURN operations in the same
COMPOUND RPC.
7. Recovering from Client I/O Errors
The pNFS client may encounter errors when directly accessing the
object storage devices. However, it is the responsibility of the
metadata server to handle the I/O errors. When the
LAYOUT4_OSD2_OBJECTS layout type is used, the client MUST report the
I/O errors to the server at LAYOUTRETURN time using the
pnfs_osd_ioerr4 structure (see Section 8.1).
The metadata server analyzes the error and determines the required
recovery operations such as repairing any parity inconsistencies,
recovering media failures, or reconstructing missing objects.
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The metadata server SHOULD recall any outstanding layouts to allow it
exclusive write access to the stripes being recovered and to prevent
other clients from hitting the same error condition. In these cases,
the server MUST complete recovery before handing out any new layouts
to the affected byte ranges.
Although it MAY be acceptable for the client to propagate a
corresponding error to the application that initiated the I/O
operation and drop any unwritten data, the client SHOULD attempt to
retry the original I/O operation by requesting a new layout using
LAYOUTGET and retry the I/O operation(s) using the new layout, or the
client MAY just retry the I/O operation(s) using regular NFS READ or
WRITE operations via the metadata server. The client SHOULD attempt
to retrieve a new layout and retry the I/O operation using OSD
commands first and only if the error persists, retry the I/O
operation via the metadata server.
8. Object-Based Layout Return
layoutreturn_file4 is used in the LAYOUTRETURN operation to convey
layout-type specific information to the server. It is defined in the
NFSv4.1 [6] as follows:
struct layoutreturn_file4 {
offset4 lrf_offset;
length4 lrf_length;
stateid4 lrf_stateid;
/* layouttype4 specific data */
opaque lrf_body<>;
};
union layoutreturn4 switch(layoutreturn_type4 lr_returntype) {
case LAYOUTRETURN4_FILE:
layoutreturn_file4 lr_layout;
default:
void;
};
struct LAYOUTRETURN4args {
/* CURRENT_FH: file */
bool lora_reclaim;
layoutreturn_stateid lora_recallstateid;
layouttype4 lora_layout_type;
layoutiomode4 lora_iomode;
layoutreturn4 lora_layoutreturn;
};
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If the lora_layout_type layout type is LAYOUT4_OSD2_OBJECTS, then the
lrf_body opaque value is defined by the pnfs_osd_layoutreturn4 type.
The pnfs_osd_layoutreturn4 type allows the client to report I/O error
information back to the metadata server as defined below.
8.1. pnfs_osd_errno4
/// enum pnfs_osd_errno4 {
/// PNFS_OSD_ERR_EIO = 1,
/// PNFS_OSD_ERR_NOT_FOUND = 2,
/// PNFS_OSD_ERR_NO_SPACE = 3,
/// PNFS_OSD_ERR_BAD_CRED = 4,
/// PNFS_OSD_ERR_NO_ACCESS = 5,
/// PNFS_OSD_ERR_UNREACHABLE = 6,
/// PNFS_OSD_ERR_RESOURCE = 7
/// };
///
pnfs_osd_errno4 is used to represent error types when read/write
errors are reported to the metadata server. The error codes serve as
hints to the metadata server that may help it in diagnosing the exact
reason for the error and in repairing it.
o PNFS_OSD_ERR_EIO indicates the operation failed because the object
storage device experienced a failure trying to access the object.
The most common source of these errors is media errors, but other
internal errors might cause this as well. In this case, the
metadata server should go examine the broken object more closely;
hence, it should be used as the default error code.
o PNFS_OSD_ERR_NOT_FOUND indicates the object ID specifies an object
that does not exist on the object storage device.
o PNFS_OSD_ERR_NO_SPACE indicates the operation failed because the
object storage device ran out of free capacity during the
operation.
o PNFS_OSD_ERR_BAD_CRED indicates the security parameters are not
valid. The primary cause of this is that the capability has
expired, or the access policy tag (a.k.a., capability version
number) has been changed to revoke capabilities. The client will
need to return the layout and get a new one with fresh
capabilities.
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o PNFS_OSD_ERR_NO_ACCESS indicates the capability does not allow the
requested operation. This should not occur in normal operation
because the metadata server should give out correct capabilities,
or none at all.
o PNFS_OSD_ERR_UNREACHABLE indicates the client did not complete the
I/O operation at the object storage device due to a communication
failure. Whether or not the I/O operation was executed by the OSD
is undetermined.
o PNFS_OSD_ERR_RESOURCE indicates the client did not issue the I/O
operation due to a local problem on the initiator (i.e., client)
side, e.g., when running out of memory. The client MUST guarantee
that the OSD command was never dispatched to the OSD.
8.2. pnfs_osd_ioerr4
/// struct pnfs_osd_ioerr4 {
/// pnfs_osd_objid4 oer_component;
/// length4 oer_comp_offset;
/// length4 oer_comp_length;
/// bool oer_iswrite;
/// pnfs_osd_errno4 oer_errno;
/// };
///
The pnfs_osd_ioerr4 structure is used to return error indications for
objects that generated errors during data transfers. These are hints
to the metadata server that there are problems with that object. For
each error, "oer_component", "oer_comp_offset", and "oer_comp_length"
represent the object and byte range within the component object in
which the error occurred; "oer_iswrite" is set to "true" if the
failed OSD operation was data modifying, and "oer_errno" represents
the type of error.
Component byte ranges in the optional pnfs_osd_ioerr4 structure are
used for recovering the object and MUST be set by the client to cover
all failed I/O operations to the component.
8.3. pnfs_osd_layoutreturn4
/// struct pnfs_osd_layoutreturn4 {
/// pnfs_osd_ioerr4 olr_ioerr_report<>;
/// };
///
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When OSD I/O operations failed, "olr_ioerr_report<>" is used to
report these errors to the metadata server as an array of elements of
type pnfs_osd_ioerr4. Each element in the array represents an error
that occurred on the object specified by oer_component. If no errors
are to be reported, the size of the olr_ioerr_report<> array is set
to zero.
9. Object-Based Creation Layout Hint
The layouthint4 type is defined in the NFSv4.1 [6] as follows:
struct layouthint4 {
layouttype4 loh_type;
opaque loh_body<>;
};
The layouthint4 structure is used by the client to pass a hint about
the type of layout it would like created for a particular file. If
the loh_type layout type is LAYOUT4_OSD2_OBJECTS, then the loh_body
opaque value is defined by the pnfs_osd_layouthint4 type.
9.1. pnfs_osd_layouthint4
/// union pnfs_osd_max_comps_hint4 switch (bool omx_valid) {
/// case TRUE:
/// uint32_t omx_max_comps;
/// case FALSE:
/// void;
/// };
///
/// union pnfs_osd_stripe_unit_hint4 switch (bool osu_valid) {
/// case TRUE:
/// length4 osu_stripe_unit;
/// case FALSE:
/// void;
/// };
///
/// union pnfs_osd_group_width_hint4 switch (bool ogw_valid) {
/// case TRUE:
/// uint32_t ogw_group_width;
/// case FALSE:
/// void;
/// };
///
/// union pnfs_osd_group_depth_hint4 switch (bool ogd_valid) {
/// case TRUE:
/// uint32_t ogd_group_depth;
/// case FALSE:
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/// void;
/// };
///
/// union pnfs_osd_mirror_cnt_hint4 switch (bool omc_valid) {
/// case TRUE:
/// uint32_t omc_mirror_cnt;
/// case FALSE:
/// void;
/// };
///
/// union pnfs_osd_raid_algorithm_hint4 switch (bool ora_valid) {
/// case TRUE:
/// pnfs_osd_raid_algorithm4 ora_raid_algorithm;
/// case FALSE:
/// void;
/// };
///
/// struct pnfs_osd_layouthint4 {
/// pnfs_osd_max_comps_hint4 olh_max_comps_hint;
/// pnfs_osd_stripe_unit_hint4 olh_stripe_unit_hint;
/// pnfs_osd_group_width_hint4 olh_group_width_hint;
/// pnfs_osd_group_depth_hint4 olh_group_depth_hint;
/// pnfs_osd_mirror_cnt_hint4 olh_mirror_cnt_hint;
/// pnfs_osd_raid_algorithm_hint4 olh_raid_algorithm_hint;
/// };
///
This type conveys hints for the desired data map. All parameters are
optional so the client can give values for only the parameters it
cares about, e.g. it can provide a hint for the desired number of
mirrored components, regardless of the RAID algorithm selected for
the file. The server should make an attempt to honor the hints, but
it can ignore any or all of them at its own discretion and without
failing the respective CREATE operation.
The "olh_max_comps_hint" can be used to limit the total number of
component objects comprising the file. All other hints correspond
directly to the different fields of pnfs_osd_data_map4.
10. Layout Segments
The pnfs layout operations operate on logical byte ranges. There is
no requirement in the protocol for any relationship between byte
ranges used in LAYOUTGET to acquire layouts and byte ranges used in
CB_LAYOUTRECALL, LAYOUTCOMMIT, or LAYOUTRETURN. However, using OSD
byte-range capabilities poses limitations on these operations since
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the capabilities associated with layout segments cannot be merged or
split. The following guidelines should be followed for proper
operation of object-based layouts.
10.1. CB_LAYOUTRECALL and LAYOUTRETURN
In general, the object-based layout driver should keep track of each
layout segment it got, keeping record of the segment's iomode,
offset, and length. The server should allow the client to get
multiple overlapping layout segments but is free to recall the layout
to prevent overlap.
In response to CB_LAYOUTRECALL, the client should return all layout
segments matching the given iomode and overlapping with the recalled
range. When returning the layouts for this byte range with
LAYOUTRETURN, the client MUST NOT return a sub-range of a layout
segment it has; each LAYOUTRETURN sent MUST completely cover at least
one outstanding layout segment.
The server, in turn, should release any segment that exactly matches
the clientid, iomode, and byte range given in LAYOUTRETURN. If no
exact match is found, then the server should release all layout
segments matching the clientid and iomode and that are fully
contained in the returned byte range. If none are found and the byte
range is a subset of an outstanding layout segment with for the same
clientid and iomode, then the client can be considered malfunctioning
and the server SHOULD recall all layouts from this client to reset
its state. If this behavior repeats, the server SHOULD deny all
LAYOUTGETs from this client.
10.2. LAYOUTCOMMIT
LAYOUTCOMMIT is only used by object-based pNFS to convey modified
attributes hints and/or to report the presence of I/O errors to the
metadata server (MDS). Therefore, the offset and length in
LAYOUTCOMMIT4args are reserved for future use and should be set to 0.
11. Recalling Layouts
The object-based metadata server should recall outstanding layouts in
the following cases:
o When the file's security policy changes, i.e., Access Control
Lists (ACLs) or permission mode bits are set.
o When the file's aggregation map changes, rendering outstanding
layouts invalid.
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o When there are sharing conflicts. For example, the server will
issue stripe-aligned layout segments for RAID-5 objects. To
prevent corruption of the file's parity, multiple clients must not
hold valid write layouts for the same stripes. An outstanding
READ/WRITE (RW) layout should be recalled when a conflicting
LAYOUTGET is received from a different client for LAYOUTIOMODE4_RW
and for a byte range overlapping with the outstanding layout
segment.
11.1. CB_RECALL_ANY
The metadata server can use the CB_RECALL_ANY callback operation to
notify the client to return some or all of its layouts. The NFSv4.1
[6] defines the following types:
const RCA4_TYPE_MASK_OBJ_LAYOUT_MIN = 8;
const RCA4_TYPE_MASK_OBJ_LAYOUT_MAX = 9;
struct CB_RECALL_ANY4args {
uint32_t craa_objects_to_keep;
bitmap4 craa_type_mask;
};
Typically, CB_RECALL_ANY will be used to recall client state when the
server needs to reclaim resources. The craa_type_mask bitmap
specifies the type of resources that are recalled and the
craa_objects_to_keep value specifies how many of the recalled objects
the client is allowed to keep. The object-based layout type mask
flags are defined as follows. They represent the iomode of the
recalled layouts. In response, the client SHOULD return layouts of
the recalled iomode that it needs the least, keeping at most
craa_objects_to_keep object-based layouts.
/// enum pnfs_osd_cb_recall_any_mask {
/// PNFS_OSD_RCA4_TYPE_MASK_READ = 8,
/// PNFS_OSD_RCA4_TYPE_MASK_RW = 9
/// };
///
The PNFS_OSD_RCA4_TYPE_MASK_READ flag notifies the client to return
layouts of iomode LAYOUTIOMODE4_READ. Similarly, the
PNFS_OSD_RCA4_TYPE_MASK_RW flag notifies the client to return layouts
of iomode LAYOUTIOMODE4_RW. When both mask flags are set, the client
is notified to return layouts of either iomode.
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12. Client Fencing
In cases where clients are uncommunicative and their lease has
expired or when clients fail to return recalled layouts within a
lease period at the least (see "Recalling a Layout"[6]), the server
MAY revoke client layouts and/or device address mappings and reassign
these resources to other clients. To avoid data corruption, the
metadata server MUST fence off the revoked clients from the
respective objects as described in Section 13.4.
13. Security Considerations
The pNFS extension partitions the NFSv4 file system protocol into two
parts, the control path and the data path (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
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.
The metadata server enforces the file access-control policy at
LAYOUTGET time. The client should use suitable authorization
credentials for getting the layout for the requested iomode (READ or
RW) and the server verifies the permissions and ACL for these
credentials, possibly returning NFS4ERR_ACCESS if the client is not
allowed the requested iomode. If the LAYOUTGET operation succeeds
the client receives, as part of the layout, a set of object
capabilities allowing it I/O access to the specified objects
corresponding to the requested iomode. When the client acts on I/O
operations on behalf of its local users, it MUST authenticate and
authorize the user by issuing respective OPEN and ACCESS calls to the
metadata server, similar to having NFSv4 data delegations. If access
is allowed, the client uses the corresponding (READ or RW)
capabilities to perform the I/O operations at the object storage
devices. When the metadata server receives a request to change a
file's permissions or ACL, it SHOULD recall all layouts for that file
and it MUST change the capability version attribute on all objects
comprising the file to implicitly invalidate any outstanding
capabilities before committing to the new permissions and ACL. Doing
this will ensure that clients re-authorize their layouts according to
the modified permissions and ACL by requesting new layouts.
Recalling the layouts in this case is courtesy of the server intended
to prevent clients from getting an error on I/Os done after the
capability version changed.
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The object storage protocol MUST implement the security aspects
described in version 1 of the T10 OSD protocol definition [1]. The
standard defines four security methods: NOSEC, CAPKEY, CMDRSP, and
ALLDATA. To provide minimum level of security allowing verification
and enforcement of the server access control policy using the layout
security credentials, the NOSEC security method MUST NOT be used for
any I/O operation. 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.
13.1. OSD Security Data Types
There are three main data types associated with object security: a
capability, a credential, and security parameters. The capability is
a set of fields that specifies an object and what operations can be
performed on it. A credential is a signed capability. Only a
security manager that knows the secret device keys can correctly sign
a capability to form a valid credential. In pNFS, the file server
acts as the security manager and returns signed capabilities (i.e.,
credentials) to the pNFS client. The security parameters are values
computed by the issuer of OSD commands (i.e., the client) that prove
they hold valid credentials. The client uses the credential as a
signing key to sign the requests it makes to OSD, and puts the
resulting signatures into the security_parameters field of the OSD
command. The object storage device uses the secret keys it shares
with the security manager to validate the signature values in the
security parameters.
The security types are opaque to the generic layers of the pNFS
client. The credential contents are defined as opaque within the
pnfs_osd_object_cred4 type. Instead of repeating the definitions
here, the reader is referred to Section 4.9.2.2 of the OSD standard.
13.2. The OSD Security Protocol
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-based storage device. 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 enforce that policy without knowing the details (e.g.,
user IDs and ACLs).
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Since capabilities are tied to layouts, and since they are used to
enforce access control, when the file ACL or mode changes the
outstanding capabilities MUST be revoked to enforce the new access
permissions. The server SHOULD recall layouts to allow clients to
gracefully return their capabilities before the access permissions
change.
Each capability is specific to a particular object, an operation on
that object, a byte range within the object (in OSDv2), and has an
explicit expiration time. The capabilities are signed with a secret
key that is shared by the object storage devices and the metadata
managers. Clients do not have device keys so they are unable to
forge the signatures in the security parameters. The combination of
a capability, the OSD System ID, and a signature is called a
"credential" in the OSD specification.
The details of the security and privacy model for object storage are
defined in the T10 OSD standard. The following sketch of the
algorithm should help the reader understand the basic model.
LAYOUTGET returns a CapKey and a Cap, which, together with the OSD
System ID, are also called a credential. It is a capability and a
signature over that capability and the SystemID. The OSD Standard
refers to the CapKey as the "Credential integrity check value" and to
the ReqMAC as the "Request integrity check value".
CapKey = MAC<SecretKey>(Cap, SystemID)
Credential = {Cap, SystemID, CapKey}
The client uses CapKey to sign all the requests it issues for that
object using the respective Cap. In other words, the Cap appears in
the request to the storage device, and that request is signed with
the CapKey as follows:
ReqMAC = MAC<CapKey>(Req, ReqNonce)
Request = {Cap, Req, ReqNonce, ReqMAC}
The following is sent to the OSD: {Cap, Req, ReqNonce, ReqMAC}. The
OSD uses the SecretKey it shares with the metadata server to compare
the ReqMAC the client sent with a locally computed value:
LocalCapKey = MAC<SecretKey>(Cap, SystemID)
LocalReqMAC = MAC<LocalCapKey>(Req, ReqNonce)
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 Cap.
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RFC 5664 pNFS Objects January 2010
13.3. Protocol Privacy Requirements
Note that if the server LAYOUTGET reply, holding CapKey and Cap, is
snooped by another client, it can be used to generate valid OSD
requests (within the Cap access restrictions).
To provide the required privacy requirements for the capability key
returned by LAYOUTGET, the GSS-API [7] framework can be used, e.g.,
by using the RPCSEC_GSS privacy method to send the LAYOUTGET
operation or by using the SSV key to encrypt the oc_capability_key
using the GSS_Wrap() function. Two general ways to provide privacy
in 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 IPsec
[15].
13.4. Revoking Capabilities
At any time, the metadata server may invalidate all outstanding
capabilities on an object by changing its POLICY ACCESS TAG
attribute. The value of the POLICY ACCESS TAG is part of a
capability, and it must match the state of the object attribute. If
they do not match, the OSD rejects accesses to the object with the
sense key set to ILLEGAL REQUEST and an additional sense code set to
INVALID FIELD IN CDB. When a client attempts to use a capability and
is rejected this way, it should issue a LAYOUTCOMMIT for the object
and specify PNFS_OSD_BAD_CRED in the olr_ioerr_report parameter. The
client may elect to issue a compound LAYOUTRETURN/LAYOUTGET (or
LAYOUTCOMMIT/LAYOUTRETURN/LAYOUTGET) to attempt to fetch a refreshed
set of capabilities.
The metadata server may elect to change the access policy tag on an
object at any time, for any reason (with the understanding that there
is likely an associated performance penalty, especially if there are
outstanding layouts for this object). The metadata server MUST
revoke outstanding capabilities when any one of the following occurs:
o the permissions on the object change,
o a conflicting mandatory byte-range lock is granted, or
o a layout is revoked and reassigned to another client.
A pNFS client will typically hold one layout for each byte range for
either READ or READ/WRITE. The client's credentials are checked by
the metadata server at LAYOUTGET time and it is the client's
responsibility to enforce access control among multiple users
accessing the same file. It is neither required nor expected that
the pNFS client will obtain a separate layout for each user accessing
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RFC 5664 pNFS Objects January 2010
a shared object. The client SHOULD use OPEN and ACCESS calls to
check user permissions when performing I/O so that the server's
access control policies are correctly enforced. The result of the
ACCESS operation may be cached while the client holds a valid layout
as the server is expected to recall layouts when the file's access
permissions or ACL change.
14. IANA Considerations
As described in NFSv4.1 [6], new layout type numbers have been
assigned by IANA. This document defines the protocol associated with
the existing layout type number, LAYOUT4_OSD2_OBJECTS, and it
requires no further actions for IANA.
15. References
15.1. Normative References
[1] Weber, R., "Information Technology - SCSI Object-Based Storage
Device Commands (OSD)", ANSI INCITS 400-2004, December 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Eisler, M., "XDR: External Data Representation Standard",
STD 67, RFC 4506, May 2006.
[4] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed., "Network
File System (NFS) Version 4 Minor Version 1 External Data
Representation Standard (XDR) Description", RFC 5662,
January 2010.
[5] IETF Trust, "Legal Provisions Relating to IETF Documents",
November 2008,
<http://trustee.ietf.org/docs/IETF-Trust-License-Policy.pdf>.
[6] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed., "Network
File System (NFS) Version 4 Minor Version 1 Protocol",
RFC 5661, January 2010.
[7] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[8] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC 3720, April 2004.
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RFC 5664 pNFS Objects January 2010
[9] Weber, R., "SCSI Primary Commands - 3 (SPC-3)", ANSI
INCITS 408-2005, October 2005.
[10] Krueger, M., Chadalapaka, M., and R. Elliott, "T11 Network
Address Authority (NAA) Naming Format for iSCSI Node Names",
RFC 3980, February 2005.
[11] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
Registration Authority",
<http://standards.ieee.org/regauth/oui/tutorials/EUI64.html>.
[12] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C., and J.
Souza, "Internet Storage Name Service (iSNS)", RFC 4171,
September 2005.
[13] Weber, R., "SCSI Architecture Model - 3 (SAM-3)", ANSI
INCITS 402-2005, February 2005.
15.2. Informative References
[14] Weber, R., "SCSI Object-Based Storage Device Commands -2
(OSD-2)", January 2009,
<http://www.t10.org/cgi-bin/ac.pl?t=f&f=osd2r05a.pdf>.
[15] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[16] T10 1415-D, "SCSI RDMA Protocol (SRP)", ANSI INCITS 365-2002,
December 2002.
[17] T11 1619-D, "Fibre Channel Framing and Signaling - 2
(FC-FS-2)", ANSI INCITS 424-2007, February 2007.
[18] T10 1601-D, "Serial Attached SCSI - 1.1 (SAS-1.1)", ANSI
INCITS 417-2006, June 2006.
[19] MacWilliams, F. and N. Sloane, "The Theory of Error-Correcting
Codes, Part I", 1977.
Halevy, et al. Standards Track [Page 34]
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Appendix A. Acknowledgments
Todd Pisek was a co-editor of the initial versions of this document.
Daniel E. Messinger, Pete Wyckoff, Mike Eisler, Sean P. Turner, Brian
E. Carpenter, Jari Arkko, David Black, and Jason Glasgow reviewed and
commented on this document.
Authors' Addresses
Benny Halevy
Panasas, Inc.
1501 Reedsdale St. Suite 400
Pittsburgh, PA 15233
USA
Phone: +1-412-323-3500
EMail: bhalevy@panasas.com
URI: http://www.panasas.com/
Brent Welch
Panasas, Inc.
6520 Kaiser Drive
Fremont, CA 95444
USA
Phone: +1-510-608-7770
EMail: welch@panasas.com
URI: http://www.panasas.com/
Jim Zelenka
Panasas, Inc.
1501 Reedsdale St. Suite 400
Pittsburgh, PA 15233
USA
Phone: +1-412-323-3500
EMail: jimz@panasas.com
URI: http://www.panasas.com/
Halevy, et al. Standards Track [Page 35]