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Network B. Welch
Internet-Draft B. Halevy
Expires: December 11, 2005 Panasas
G. Goodson
NetApp
D. Black
EMC
A. Adamson
CITI
June 9, 2005
pNFS Operations
draft-welch-pnfs-ops-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This Internet-Draft provides a description of the pNFS extension for
NFSv4.
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The key feature of the protocol extension is the ability for clients
to perform read and write operations that go directly from the client
to individual storage system elements without funneling all such
accesses through a single file server. Of course, the file server
must coordinate the client I/O so that the file system retains its
integrity.
The extension adds operations that query and manage layout
information that allows parallel I/O between clients and storage
system elements. The layouts are managed in a similar way as
delegations in that they have leases and can be recalled by the
server, but layout information is independent of delegations.
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 [1].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. General Definitions . . . . . . . . . . . . . . . . . . . . . 7
2.1 Metadata Server . . . . . . . . . . . . . . . . . . . . . 7
2.2 Client . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Storage Device . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Storage Protocol . . . . . . . . . . . . . . . . . . . . . 8
2.5 Management Protocol . . . . . . . . . . . . . . . . . . . 8
2.6 Metadata . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.7 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Layouts and Aggregation . . . . . . . . . . . . . . . . . . . 9
3.1 Layout Structure . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 Device IDs . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2 Aggregation Schemes . . . . . . . . . . . . . . . . . 10
3.2 Basic Layout Semantics . . . . . . . . . . . . . . . . . . 10
3.2.1 Layout Iomode . . . . . . . . . . . . . . . . . . . . 11
3.2.2 Operation Sequencing . . . . . . . . . . . . . . . . . 11
3.3 Obtaining a Layout . . . . . . . . . . . . . . . . . . . . 12
3.3.1 Identifying Layouts . . . . . . . . . . . . . . . . . 12
3.3.2 Overlapping Layouts . . . . . . . . . . . . . . . . . 12
3.3.3 Copy-on-write . . . . . . . . . . . . . . . . . . . . 13
3.4 Recalling a Layout . . . . . . . . . . . . . . . . . . . . 13
3.5 Committing a Layout . . . . . . . . . . . . . . . . . . . 13
3.5.1 LAYOUTCOMMIT and EOF . . . . . . . . . . . . . . . . . 14
3.6 Lease Renewals . . . . . . . . . . . . . . . . . . . . . . 15
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
4.1 File Layout Security . . . . . . . . . . . . . . . . . . . 17
4.2 Object Layout Security . . . . . . . . . . . . . . . . . . 17
4.3 Block Layout Security . . . . . . . . . . . . . . . . . . 18
5. NFSv4 File Layout Type . . . . . . . . . . . . . . . . . . . . 18
5.1 File Striping and Data Access . . . . . . . . . . . . . . 18
5.2 Global Stateid Requirements . . . . . . . . . . . . . . . 22
5.3 The Layout Iomode . . . . . . . . . . . . . . . . . . . . 22
5.4 Storage Device State Propagation . . . . . . . . . . . . . 22
5.4.1 Lock State Propagation . . . . . . . . . . . . . . . . 23
5.4.2 Open-mode Validation . . . . . . . . . . . . . . . . . 23
5.4.3 File Attributes . . . . . . . . . . . . . . . . . . . 24
5.4.4 Access State Propagation . . . . . . . . . . . . . . . 24
5.5 Extending EOF . . . . . . . . . . . . . . . . . . . . . . 24
5.5.1 READs and EOF . . . . . . . . . . . . . . . . . . . . 24
5.5.2 LAYOUTCOMMIT and EOF . . . . . . . . . . . . . . . . . 25
5.6 Security Considerations . . . . . . . . . . . . . . . . . 26
5.7 Alternate Approaches . . . . . . . . . . . . . . . . . . . 26
6. pNFS Typed Data Structures . . . . . . . . . . . . . . . . . . 27
6.1 pnfs_layouttype4 . . . . . . . . . . . . . . . . . . . . . 27
6.2 pnfs_deviceid4 . . . . . . . . . . . . . . . . . . . . . . 27
6.3 pnfs_devaddr4 . . . . . . . . . . . . . . . . . . . . . . 28
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6.4 pnfs_devlist_item4 . . . . . . . . . . . . . . . . . . . . 28
6.5 pnfs_layout4 . . . . . . . . . . . . . . . . . . . . . . . 29
7. pNFS File Attributes . . . . . . . . . . . . . . . . . . . . . 29
7.1 pnfs_layouttype4<> LAYOUT_TYPES . . . . . . . . . . . . . 29
7.2 pnfs_layouttype4 LAYOUT_TYPE . . . . . . . . . . . . . . . 29
7.3 pnfs_layouttypes4 LAYOUT_HINT . . . . . . . . . . . . . . 29
8. pNFS Error Definitions . . . . . . . . . . . . . . . . . . . . 30
9. pNFS Operations . . . . . . . . . . . . . . . . . . . . . . . 30
9.1 LAYOUTGET - Get Layout Information . . . . . . . . . . . . 30
9.2 LAYOUTCOMMIT - Commit writes made using a layout . . . . . 33
9.3 LAYOUTRETURN - Release Layout Information . . . . . . . . 35
9.4 GETDEVICEINFO - Get Device Information . . . . . . . . . . 36
9.5 GETDEVICELIST - Get List of Devices . . . . . . . . . . . 37
10. Callback Operations . . . . . . . . . . . . . . . . . . . . 37
10.1 CB_LAYOUTRECALL . . . . . . . . . . . . . . . . . . . . . 38
10.2 CB_EOFCHANGED . . . . . . . . . . . . . . . . . . . . . . 39
11. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . 40
11.1 Basic Read Scenario . . . . . . . . . . . . . . . . . . . 40
11.2 Multiple Reads to a File . . . . . . . . . . . . . . . . . 40
11.3 Multiple Reads to a File with Delegations . . . . . . . . 40
11.4 Read with existing writers . . . . . . . . . . . . . . . . 41
11.5 Read with later conflict . . . . . . . . . . . . . . . . . 41
11.6 Basic Write Case . . . . . . . . . . . . . . . . . . . . . 41
11.7 Large Write Case . . . . . . . . . . . . . . . . . . . . . 42
11.8 Create with special layout . . . . . . . . . . . . . . . . 42
12. Layouts and Aggregation . . . . . . . . . . . . . . . . . . 42
12.1 Simple Map . . . . . . . . . . . . . . . . . . . . . . . . 42
12.2 Block Map . . . . . . . . . . . . . . . . . . . . . . . . 43
12.3 Striped Map (RAID 0) . . . . . . . . . . . . . . . . . . . 43
12.4 Replicated Map . . . . . . . . . . . . . . . . . . . . . . 43
12.5 Concatenated Map . . . . . . . . . . . . . . . . . . . . . 43
12.6 Nested Map . . . . . . . . . . . . . . . . . . . . . . . . 44
13. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
13.1 Storage Protocol Negotiation . . . . . . . . . . . . . . . 44
13.2 Crash recovery . . . . . . . . . . . . . . . . . . . . . . 44
13.3 Storage Errors . . . . . . . . . . . . . . . . . . . . . . 44
14. Normative References . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 46
Intellectual Property and Copyright Statements . . . . . . . . 47
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1. Introduction
The NFSv4 protocol [2] specifies the interaction between a client
that accesses files and a server that provides access to files and is
responsible for coordinating access by multiple clients. As
described in the pNFS problem statement, this requires that all
access to a set of files exported by a single NFSv4 server be
performed by that server; at high data rates the server may become a
bottleneck.
The parallel NFS (pNFS) extensions to NFSv4 allow data accesses to
bypass this bottleneck by permitting direct client access to the
storage devices containing the file data. When file data for a
single NFSv4 server is stored on multiple and/or higher throughput
storage devices (by comparison to the server's throughput
capability), the result can be significantly better file access
performance. The relationship among multiple clients, a single
server, and multiple storage devices for pNFS (server and clients
have access to all storage devices) is shown in this diagram:
+-----------+
|+-----------+ +-----------+
||+-----------+ | |
||| | NFSv4 + pNFS | |
+|| Clients |<------------------------------>| Server |
+| | | |
+-----------+ | |
||| +-----------+
||| |
||| |
||| Storage +-----------+ |
||| Protocol |+-----------+ |
||+----------------||+-----------+ Management|
|+-----------------||| | Protocol|
+------------------+|| Storage |------------+
+| Devices |
+-----------+
Figure 1
In this structure, the responsibility for coordination of file access
by multiple clients is shared among the server, clients, and storage
devices, in contrast to NFSv4, by itself, where this is primarily the
server's responsibility, some of which can be delegated to clients
under strictly specified conditions.
The pNFS extension to NFSv4 takes the form of new operations that
manage data location information called a "layout". The layout is
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managed in a similar fashion as NFSv4 data delegations (e.g., they
are recallable and revocable). However, they are distinct
abstractions and are manipulated with new operations that are
described in Section 9. When a client holds a layout, it has rights
to access the data directly using the location information in the
layout.
There are new attributes that describe general layout
characteristics. However, attributes do not provide everything
needed to support layouts, hence the use of operations instead.
This document specifies both the NFSv4 extensions required to
distribute file access coordination between the server and its
clients and a NFSv4 file storage protocol that may be used to access
data stored on NFSv4 storage devices.
Storage protocols used to access a variety of other storage devices
are deliberately not specified, these may include:
o Block protocols such as iSCSI, parallel SCSI, and FCP (SCSI over
Fibre Channel) [refs]. The block protocol support can be
independent of the addressing structure of the block protocol
used, allowing more than one protocol to access the same file data
and enabling extensibility to other block protocols.
o Object protocols such as OSD over iSCSI or Fibre Channel [3].
o Other storage protocols, including PVFS and other file systems
that are in use in HPC environments.
pNFS is designed to accommodate these protocols and be extensible to
new classes of storage protocols that may be of interest.
The distribution of file access coordination between the server and
its clients increases the level of responsibility placed on clients.
Clients are already responsible for ensuring that suitable access
checks are made to cached data and that attributes are suitably
propagated to the server. Generally, a misbehaving client that hosts
only a single-user can only impact files accessible to that single
user. Misbehavior by a client hosting multiple users may impact
files accessible to all of its users. NFSv4 delegations increase the
level of client responsibility as a client that carries out actions
requiring a delegation without obtaining that delegation will cause
its user(s) to see unexpected and/or incorrect behavior.
Some uses of pNFS extend the responsibility of clients beyond
delegations. In some configurations, the storage devices cannot
perform fine grain access checks to ensure that clients are only
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performing accesses within the bounds permitted to them by the pNFS
operations with the server (e.g., the checks may only be possible at
file system granularity rather than file granularity). In situations
where this added responsibility placed on clients creates
unacceptable security risks, pNFS configurations in which storage
devices cannot perform fine-grained access checks SHOULD NOT be used.
All pNFS server implementations MUST support NFSv4 access to any file
accessible via pNFS in order to provide an interoperable means of
file access in such situations. See Section 4 on Security for
further discussion.
Finally, there are issues about how layouts interact with the
existing NFSv4 abstractions of data delegations and byte range
locking. These issues (and more) are also discussed here.
2. General Definitions
This protocol extension partitions the NFSv4 file system protocol
into two parts, the control path and the data path. The control path
is implemented by the extended (p)NFSv4 server. When the file system
being exported by (p)NFSv4 uses storage devices that are visible to
clients over the network, the data path may be implemented by direct
communication between the extended (p)NFSv4 file system client and
the storage devices. This leads to a few new terms used to describe
the protocol extension and some clarifications of some existing
terms.
2.1 Metadata Server
A pNFS "server" or "metadata server" is a server as defined by
RFC3530 [2], with the addition of supporting the pNFS minor
extension. When using the pNFS NFSv4 minor extension, the metadata
server may hold only the metadata associated with a file, while the
data is stored on the storage devices. Note: directory data is
always accessed through the metadata server.
2.2 Client
A pNFS "client" is a client as defined by RFC3530 [2], with the
addition of supporting the pNFS minor extension server protocol and
with the addition of supporting at least one storage protocol (for
performing I/O directly to storage devices).
2.3 Storage Device
This is a device, or server, that controls the file's data, but
leaves other metadata management up to the metadata server. A
storage device could be another NFS server, or an Object Storage
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Device (OSD) or a block device accessed over a SAN (either
FiberChannel or iSCSI SAN). The goal of this extension is to allow
direct communication between clients and storage devices.
2.4 Storage Protocol
This is the protocol between the pNFS client and the storage device
used to access the file data. There are three primary types: file
protocols (such as NFSv4 or NFSv3), object protocols (OSD), and block
protocols (SCSI-block commands, or "SBC"). These protocols are in
turn layered over transport protocols such as RPC/TCP/IP or iSCSI/
TCP/IP or FC/SCSI. We anticipate there will be variations on these
storage protocols, including new protocols that are unknown at this
time or experimental in nature. The details of the storage protocols
will be described in other documents so that pNFS clients can be
written to use these storage protocols. A NFSv4 file protocol is
described in Section 5.
2.5 Management Protocol
This is the protocol used by the exported file system between the
server and storage devices. This protocol is outside the scope of
this draft, and is used for various management activities including
the allocation and deallocation of storage and the management of
state required by the storage devices to perform client access
control. The management protocol should not be confused with
protocols used to manage LUNs in a SAN and other sysadmin kinds of
tasks.
While the pNFS protocol allows for any management protocol, in
practice the mangement protocol is closely related to the storage
protocol. For example, if the storage devices are NFS servers, then
the protocol between the pNFS metadata server and the storage devices
is likely to involve NFS operations. Similarly, when object storage
devices are used, the pNFS metadata server will likely use iSCSI/OSD
commands to manipulate storage.
However, this document does not mandate any particular management
protocol. Instead, it just describes the requirements on the
managment protocol for maintaining attributes like modify time, the
change attribute, and the end-of-file position.
2.6 Metadata
This is information about a file, like its name, owner, where it
stored, and so forth. The information is managed by the exported
file system server (server). Metadata also includes lower-level
information like block addresses and indirect block pointers.
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Depending the storage protocol, block-level metadata may or may not
be managed by the metadata server, but is instead managed by Object
Storage Devices or other servers acting as a storage device.
2.7 Layout
A layout defines how a file's data is organized on one or more
storage devices. There are many possible layout types. They vary in
the storage protocol used to access the data, and in the aggregation
scheme that lays out the file data on the underlying storage devices.
Layouts are described in more detail below.
3. Layouts and Aggregation
3.1 Layout Structure
The layout is a typed data structure that has variants to handle
different storage protocols (block, object, and file). A layout
describes a range of a file's contents (e.g., the set of storage
devices on which a specific byte range of the file's data reside and
a method for identifying the data on those devices). A specific
layout structure belongs to a "layout type" (e.g., blocks, objects,
files). A metadata server, along with its management protocol, must
support at least one layout type. See Section 6.1 for the RPC
definition of a layout type. A private sub-range of the layout type
name space can be defined through the IANA (e.g., a type with the
high bit set to one). This private sub-range can be used for
internal testing or experimentation.
For example, a file layout type could be an array of tuples
(deviceID, file_handle), along with a definition (or aggregation
scheme) of how the data is stored across the devices (e.g.,
striping). A block layout might be an array of tuples that store
(deviceID, block_number, block count) along with information about
block size and the file offset of the first block. An object layout
is an array of tuples (deviceID, objectID) and an additional
structure (i.e., the aggregation map) that defines how the logical
byte sequence of the file data is serialized into the different
objects.
This document defines a NFSv4 file layout type using a stripe-based
aggregation scheme (see Section 5). Adjunct specifications must
exist that precisely define other layout formats (e.g., blocks,
objects, or other file-based layouts) to allow interoperability among
clients and metadata servers.
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3.1.1 Device IDs
The "deviceID" is a short name for a storage device. In practice, a
significant amount of information may be required to fully identify a
storage device. Instead of embedding all that information in a
layout, a level of indirection is used. Layouts embed device Ids,
and a new op (GETDEVICEINFO) is used to retrieve the complete
identity information about the storage device. For example, the
identity of a file server or object server could be an IP address and
port. The identity of a block device could be a volume label. Due
to multipath connectivity in a SAN environment, agreement on a volume
label is considered the reliable way to locate a particular storage
device. Another operation, GETDEVICELIST, has been added to allow
clients to fetch the mappings of multiple storage devices attached to
a metadata server. Clients SHOULD NOT expect the mapping between
deviceID and storage device address to exist across metadata server
reboots (i.e., clients should fetch new mappings upon startup or upon
detection of a metadata server reboot). If data are reorganized from
a storage device with a given deviceID to a different storage device,
the layout describing the data SHOULD be recalled rather than
assigning the new storage device to the old deviceID.
3.1.2 Aggregation Schemes
Aggregation schemes can describe layouts like simple one-to-one
mapping, concatenation, and striping. A general aggregation scheme
allows nested maps so that more complex layouts can be compactly
described. The canonical aggregation type for this extension is
striping, which allows a client to access storage devices in
parallel. Even a one-to-one mapping is useful for a file server that
wishes to distribute its load among a set of other file servers.
There are also experimental aggregation types such as writable
mirrors and RAID.
3.2 Basic Layout Semantics
Layouts delegate to the client the ability to access data out of
band. The layout guarantees the holder that the layout will be
recalled when the state encapsulated by the layout becomes invalid
(e.g., through some operation that directly or indirectly modifies
the layout) or, possibly, when a conflicting layout is requested, as
determined by the layout's iomode.
Holding a layout does not guarantee that a user of the layout has the
rights to access the data represented by the layout. All user access
rights MUST be obtained through the appropriate open and lock
commands. However, if a valid layout for a file is not held by the
client, the storage device may reject all I/Os to that file's byte
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range that originate from that client. In summary, layouts and
ordinary file access controls are independent. The act of modifying
a file for which a layout is held, does not necessarily conflict with
the holding of the layout (that describes the file being modified).
However, with certain layout types (e.g., block layouts), the
layout's iomode must agree with the type of I/O being performed.
3.2.1 Layout Iomode
When requesting a layout (through LAYOUTGET), the client MUST request
a layout pertaining to an "iomode" of either READ or READ/WRITE. The
iomode indicates to the metadata server the client's intent to
perform either READ-ONLY or READ/WRITE I/O to the storage devices
using the requested layout. For certain layout types, it is useful
for a client to specify this intent at LAYOUTGET time. E.g., for
block based protocols, block allocation could occur when a READ/WRITE
iomode is specified. A storage device may validate I/O with regards
to the iomode; this is dependent upon storage device implementation.
Thus, if the client's layout iomode differs from the I/O being
performed the storage device may reject the client's I/O with an
error indicating a new layout with the correct I/O mode should be
fetched. E.g., if a client gets a layout with a READ iomode and
performs a WRITE to a storage device, the storage device is allowed
to reject that WRITE.
The iomode does not conflict with OPEN share modes or lock requests,
and these are the preferred method for restricting user access to
data files. E.g., an OPEN of read, deny-write does not conflict with
a LAYOUTGET containing an iomode of READ/WRITE performed by another
client. Applications that depend on writing into the same file
concurrently SHOULD use byte range locking to serialize their
accesses.
3.2.2 Operation Sequencing
As with other stateful operations, pNFS requires the correct
sequencing of layout operations. This proposal assumes that sessions
will precede pNFS into NFSv4.x and thus, pNFS will require the use of
sessions. If the sessions proposal does not precede pNFS, then this
proposal must be modified to provide for the correct sequencing of
pNFS layout operations.
One main issue with operation sequencing concerns callbacks. The
protocol must defend against races between the reply to a LAYOUTGET
operation and a subsequent CB_LAYOUTRECALL. It MUST not be possible
for a client to process the CB_LAYOUTRECALL for a layout that it has
not received in a reply message to a LAYOUTGET.
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3.3 Obtaining a Layout
The metadata server will give out layouts of a particular type
(block, object, or file) and aggregation. A client obtains a layout
through a new operation (LAYOUTGET). The layout returned to the
client may not line up exactly with the requested byte range.
However, at least a single byte overlap MUST exist between the
requested layout and the layout returned by the metadata server.
The storage protocol used by the client to access the data on the
storage device is determined by the layout's type. The client needs
to select a "layout driver" that understands how to interpret and use
that layout. The API used by the client to talk to its drivers is
outside the scope of the pNFS extension. The storage protocol
between the client's layout driver and the actual storage is covered
by other protocols specifications such as SBC (block storage), OSD
(object storage) or NFS (file storage).
Although, the metadata server is in control of the layout for a file,
the pNFS client can provide hints to the server when a file is opened
or created about preferred layout type. The pNFS extension
introduces a LAYOUT_HINT attribute that the client can query at
anytime, and can set with a compound SETATTR after OPEN to provide a
hint to the server for new files.
3.3.1 Identifying Layouts
A layout is identified by the following tuple: (ClientID, FH, offset,
length); the FH refers to the FH of the file on the metadata server,
the offset and length specify the byte range of the file the layout
covers. Since there is a desire to manage layouts as sub-dividable
entities, layouts are range-based and are identified in such a
manner. Sub-dividable layouts have the benefit of being returnable/
recallable or committable in smaller chunks without having to return,
recall, or commit the entire layout. E.g., this may be useful when
the layout is very large and a client is only actively using a small
range of the layout, thus the client may not want to commit the
entire layout, rather it could commit just the range of the layout it
is using.
3.3.2 Overlapping Layouts
A metadata server may hand-out layouts that overlap, as long as the
overlapping regions specify the same storage device/file mapping;
i.e., the records within the overlapping layouts should specify the
same storage device mapping for the same byte ranges they represent.
If two overlapping layouts differ, the old layout should be recalled.
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3.3.3 Copy-on-write
For block-based protocols it is useful to have the ability to direct
a client to read data from one group of blocks, but write to a
different group; e.g., to implement a snapshotting blocks system.
The client can not make the choice of where to place data, it
requires help by the metadata server, most probably communicated
through the layout. A single layout with the ability to mark (and
re-mark) portions read-only vs. read/write is sufficient for this to
work.
3.4 Recalling a Layout
Since a layout protects a client's access to a file via a direct
client-data-server path, a layout need only be recalled when it is
semantically unable to serve this function. Typically, this occurs
when the layout no longer encapsulates the true location of the file
over the byte range it represents. Any operation that changes the
layout will result in a recall of the layout. A REMOVE operation may
cause the metadata server to recall the layout to prevent the client
from accessing a non-existent file and to reclaim state stored on the
client. Since a REMOVE may be delayed until the last close of the
file has occurred, the recall may also be delayed until this time.
As well, once the file has been removed (after the last reference),
the client SHOULD no longer be able to perform I/O using the layout
(e.g., with file-based layouts an error such as ESTALE could be
returned).
Once a layout has been recalled, the client should no longer issue
I/Os to the storage devices for the file and byte range represented
by the recalled layout, even though the client may still have valid
stateids for that file (except to flush dirty data before returning
the layout). If a client does issue an I/O to a storage device for
which it does not hold a layout, the storage device SHOULD reject the
I/O. This can be verified by the storage device by mapping the
stateid used for I/O to the client instance and validating that the
client instance has a valid layout.
3.5 Committing a Layout
Due to the nature of the protocol, the file attributes that exist on
the metadata storage device may become inconsistent in relation to
the data stored on the storage devices; e.g., when WRITEs occur
before a layout has been committed (e.g., between a LAYOUTGET and a
LAYOUTCOMMIT). Thus, it is necessary to occasionally re-sync this
state and make it visible to other clients through the metadata
server.
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The LAYOUTCOMMIT operation is responsible for committing the modified
layout to the metadata server. Note: the data should be written (and
committed) to the appropriate storage devices before the LAYOUTCOMMIT
occurs. The scope of this operation depends on the storage protocol
in use. For block-based layouts, it may require updating the block
list that comprises the file and committing this layout to stable
storage. While, for file-layouts it requires the synchronization of
attributes between the metadata and storage devices (mainly the size/
EOF). The management protocol is free to sync this state before it
receives a LAYOUTCOMMIT, however upon successful completion of a
LAYOUTCOMMIT, state that exists on the metadata server that describes
the file MUST be in sync with the state existing on the storage
devices that comprises that file (assuming no intervening
operations). Thus, a client that queries the size of a file between
a WRITE to a storage device and the LAYOUTCOMMIT may not observe a
size that reflects the actual data written.
The change attribute and mtime may be updated, by the server, at
LAYOUTCOMMIT time; since for some layout protocols, the change
attribute can not be updated by a WRITE operation performed at a
storage device. However, for some layout protocols the change
attribute and mtime may be updated at or after the time of the WRITE
(e.g., if the storage device is able to communicate these attributes
to the metadata server). If, upon receiving a LAYOUTCOMMIT, the
server implementation is able to determine that the file did not
change since the last time the change attribute was updated (e.g., no
WRITEs or over-writes occurred), the implementation need not update
the change attribute (file-based protocols may have enough state to
make this determination or may update the change attribute upon each
file modification). This also applies for mtime; if the server
implementation is able to determine that the file has not been
modified since the last mtime update, the server need not update
mtime at LAYOUTCOMMIT time. Once LAYOUTCOMMIT completes, the new
change attribute and mtime should be visible if that file was
modified since the latest previous LAYOUTCOMMIT or LAYOUTGET. If a
client prefers to set a new mtime, it should do so through the
SETATTR operation.
3.5.1 LAYOUTCOMMIT and EOF
As well, the file's EOF may be updated at LAYOUTCOMMIT time. The
LAYOUTCOMMIT operation takes an EOF flag and length as arguments. If
the EOF flag is set, a new EOF SHOULD be specified by the client.
The EOF length may be used as a hint to the metadata server. The
metadata server may validate the EOF against state that exists on the
storage devices. The metadata server may either: update the file's
EOF based on the client specified length, it may ignore the EOF flag,
or it may use a value computed by querying the storage devices (e.g.,
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through the management protocol).
The method chosen to update EOF may depend on the storage device's
and/or the management protocol's implementation. For example, if the
storage devices are block devices with no knowledge of EOF, then the
metadata server must rely on the client to set the EOF appropriately.
An EOF flag and length are also returned in the results of a
LAYOUTCOMMIT. This union indicates whether a new EOF was set, and to
what length it was set. The server may return a new EOF regardless
of whether the client set the neweof field in the request, however if
the EOF flag was set in the request, the neweof MUST be returned.
The EOF flag SHOULD not be used to truncate or grow the file
sparsely; the SETATTR operation must be used to do so. The metadata
server in conjunction with the management protocol SHOULD ensure that
a new EOF is reflected by the storage device immediately upon return
of the LAYOUTCOMMIT operation; e.g., a READ up to the new EOF should
succeed on the storage devices (assuming no intervening truncations).
Since client layout holders may be unaware of changes made to EOF
(through LAYOUTCOMMIT or SETATTR) by other clients, an additional
callback/notification has been added for pNFS. CB_EOFCHANGED is a
notification that the metadata server sends to layout holders to
notify them of an EOF change to the file. This is preferred over
issuing CB_LAYOUTRECALL to each of the layout holders.
3.6 Lease Renewals
The current NFSv4 specification allows for implicit lease renewals to
occur upon receiving an I/O. However, due to the disjoint pNFS
architecture, implicit lease renewals are limited to operations
performed at the metadata server (including I/O performed through the
metadata server). So, READ and WRITE I/O to storage devices do not
implicitly renew lease state. It is suggested that explicit lease
renewal is used instead of relying on implicit renewals.
The impact on lease renewals and storage devices needs to be better
defined. For now it is left up to the management protocol to manage
leases that may exist on storage devices.
4. 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
(see Figure 1) is required to preserve the security properties of
NFSv4 with respect to an entity accessing data via a client,
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including security countermeasures to defend against threats that
NFSv4 provides defenses for in environments where these threats are
considered significant.
In some cases, the security countermeasures for connections to
storage devices may take the form of physical isolation or a
recommendation not to use pNFS in an environment. For example, it is
currently infeasible to provide confidentiality protection for some
storage device access protocols to protect against eavesdropping; in
environments where eavesdropping on such protocols is of sufficient
concern to require countermeasures, physical isolation of the
communication channel (e.g., via direct connection from client(s) to
storage device(s)) and/or a decision to forego use of pNFS (e.g., and
fall back to NFSv4) may be appropriate courses of action.
In full generality where communication with storage devices is
subject to the same threats as client-server communication, the
protocols used for that communication need to provide security
mechanisms comparable to those available via RPSEC_GSS for NFSv4.
Many situations in which pNFS is likely to be used will not be
subject to the overall threat profile for which NFSv4 is required to
provide countermeasures.
pNFS implementations MUST NOT remove NFSv4's access controls. The
combination of clients, storage devices, and the server are
responsible for ensuring that all client to storage device file data
access respects NFSv4 ACLs and file open modes. This entails
performing both of these checks on every access in the client, the
storage device, or both. If a pNFS configuration performs these
checks only in the client, the risk of a misbehaving client obtaining
unauthorized access is an important consideration in determining when
it is appropriate to use such a pNFS configuration. Such
configurations SHOULD NOT be used when client- only access checks do
not provide sufficient assurance that NFSv4 access control is being
applied correctly.
The following subsections describe security considerations
specifically applicable to each of the three major storage device
protocol types supported for pNFS.
[Additional security info - the object protocol needs this, but it
may be out-of-band; the OSD experts will know for sure. For Block
and File an approach of the client being expected to know what it
needs when it sees what it's being asked to access probably suffices,
although we might be able to help (e.g., pass iSCSI CHAP
authentication identities, but NOT secrets, via pNFS). For File in
particular, defaulting to the NFSv4 principal is probably a good
idea, although it's not strictly necessary.]
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[Requiring strict equivalence to NFSv4 security mechanisms is the
wrong approach. Will need to lay down a set of statements that each
protocol has to make starting with access check location/properties.]
4.1 File Layout Security
A NFSv4 file layout type is defined in Section 5; see Section 5.6 for
additional security considerations and details. In summary, the
NFSv4 file layout type requires that all I/O access checks MUST be
performed by the storage devices, as defined by the NFSv4
specification. If another file layout type is being used, additional
access checks may be required. But in all cases, the access control
performed by the storage devices must be at least as strict as that
specified by the NFSv4 protocol.
4.2 Object Layout Security
The object storage protocol relies on a cryptographically secure
capability to control accesses at the object storage devices.
Capabilities are generated by the metadata server, returned to the
client, and used by the client as described below to authenticate
their requests to the Object Storage Device (OSD). Capabilities
therefore achieve the required access and open mode checking. They
allow the file server to define and check a policy (e.g., open mode)
and the OSD to check and enforce that policy without knowing the
details (e.g., user IDs and ACLs).
Each capability is specific to a particular object, an operation on
that object, a byte range w/in the object, and has an explicit
expiration time. The capabilities are signed with a secret key that
is shared by the object storage devices (OSD) and the metadata
managers. clients do not have device keys so they are unable to forge
capabilities.
The details of the security and privacy model for Object Storage are
out of scope of this document and will be specified in the Object
Storage version of the storage protocol definition. However, the
following sketch of the algorithm should help the reader understand
the basic model.
LAYOUTGET returns
{CapKey = MAC<SecretKey>(CapArgs), CapArgs}
The client uses CapKey to sign all the requests it issues for that
object using the respective CapArgs. In other words, the CapArgs
appears in the request to the storage device, and that request is
signed with the CapKey as follows:
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ReqMAC = MAC<CapKey>(Req, Nonceln)
The following is sent to the OSD: {CapArgs, Req, Nonceln, ReqMAC}.
The OSD uses the SecretKey it shares with the metadata server to
compare the ReqMAC the client sent with a locally computed
MAC<MAC<SecretKey>(CapArgs)>(Req, Nonceln)
and if they match the OSD assumes that the capabilities came from an
authentic metadata server and allows access to the object, as allowed
by the CapArgs. Therefore, if the server LAYOUTGET reply, holding
CapKey and CapArgs, is snooped by another client, it can be used to
generate valid OSD requests (within the CapArgs access restriction).
To provide the required privacy requirements for the capabilities
returned by LAYOUTGET, the GSS-API can be used, e.g. by using a
session key known to the file server and to the client to encrypt the
whole layout or parts of it. Two general ways to provide privacy in
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.
4.3 Block Layout Security
Block protocols rely on clients to enforce file access checks, as the
storage devices are generally unaware of the files they are storing
(and in particular are unaware of which block belongs to which file).
In environments where access control is important and client-only
access checks provide insufficient assurance of access control
enforcement (e.g., there is concern about a malicious client skipping
the access check), the storage devices will generally be unable to
compensate for these client deficiencies. In such threat
environments, Block protocols SHOULD NOT be used with pNFS; NFSv4
without pNFS may be a more suitable means of accessing files in the
presence of such threats. Storage-device/protocol-specific methods
(e.g., LUN masking/mapping) may be available to prevent malicious or
high-risk clients from directly accessing storage devices.
5. NFSv4 File Layout Type
This section describes the semantics and format of NFSv4 file-based
layouts.
5.1 File Striping and Data Access
The file layout specifies an ordered array of (deviceID,
start_offset, filehandle) tuples, as well as the stripe_size, skip
and the file's current EOF (current as of LAYOUTGET time). Devices
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and filehandles may be repeated multiple times within the device list
(dev_list). Data MUST be distributed in a round-robin fashion across
the list of devices in increments of stripe_size bytes. A data file
stored on a storage device MUST map to a single file as defined by
the metadata server; i.e., data from two files as viewed by the
metadata server MUST NOT be stored within the same data file on any
storage device.
struct pnfs_nfsv4_file_layout {
pnfs_deviceid4 dev_id;
offset4 start_offset;
nfs_fh4 fh;
};
struct pnfs_nfsv4_file_layouttype4 {
uint64_t skip;
uint64_t stripe_size;
length4 eof;
pnfs_nfsv4_file_layout dev_list<>;
};
The "start_offset" field indicates the initial byte offset in the
file represented by the filehandle "fh" on the device indicated by
"dev_id". The "skip" field indicates the number of bytes to skip
between the stripes on a particular storage device. This
representation allows for a variety of storage device file layouts.
The two data file layouts anticipated to be most common are "sparse
file-layouts" and "dense file-layouts".
For example:
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Sparse file-layout
------------------
The following layout: stripe_size=4KB, skip=8KB and dev_list:
[{dev_id: 0, start_offset: 0},
{dev_id: 1, start_offset: 4KB},
{dev_id: 2, start_offset: 8KB}]
Is represented by the following file layout on the storage devices:
Offset ID:0 ID:1 ID:2
0 +--+ +--+ +--+ +--+ indicates a
|//| | | | | |//| stripe that
4KB +--+ +--+ +--+ +--+ contains data
| | |//| | |
8KB +--+ +--+ +--+
| | | | |//|
12KB +--+ +--+ +--+
|//| | | | |
16KB +--+ +--+ +--+
| | |//| | |
+--+ +--+ +--+
The sparse file-layout has holes for the byte ranges not exported by
that storage device. This allows clients to access data using the
real offset into the file, regardless of the storage device's
position within the stripe. However, if a client writes to one of
these holes (e.g., offset 4-12KB on device 1), then an error MUST be
returned by the storage device. This requires that the storage
device have knowledge of the layout for each file.
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Dense/packed file-layout
------------------------
The following layout: stripe_size=4KB, skip=0 and dev_list:
[{dev_id: 0, start_offset: 0},
{dev_id: 1, start_offset: 0},
{dev_id: 2, start_offset: 0}]
Is represented by the following file layout on the storage devices:
Offset ID:0 ID:1 ID:2
0 +--+ +--+ +--+
|//| |//| |//|
4KB +--+ +--+ +--+
|//| |//| |//|
8KB +--+ +--+ +--+
|//| |//| |//|
12KB +--+ +--+ +--+
|//| |//| |//|
16KB +--+ +--+ +--+
|//| |//| |//|
+--+ +--+ +--+
The dense or packed file-layout does not leave holes on the storage
devices. Each stripe unit is spread in a round-robin fashion across
the storage devices. As such, the storage devices need not know the
file's layout since the client is allowed to write to any offset.
Regardless of the layout, the calculation to determine the index into
the device array is the same:
dev_idx = floor(file_offset / stripe_size) mod dev_list_num
The calculation to determine the byte offset within the data file (on
the storage device referenced by the device index) is:
dev_offset = (floor(floor(file_offset / stripe_size) /
dev_list_num) * (stripe_size + skip)) +
(file_offset mod stripe_size) +
dev_list[dev_idx].start_offset
[NOTE: I have no problem simplifying this layout scheme. It seems
there is a desire for sparse and dense layouts. We can always
simplify this by adding a flag which indicates the type of layout
(sparse or dense).]
Clients MUST use the filehandle described within the layout when
accessing data on the storage devices. The client MUST only issue
READ, WRITE, and COMMIT operations to the storage devices. In
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response to a WRITE or COMMIT, a storage device may return a
writeverf unique to that storage device. GETATTR and SETATTR MUST be
directed to the metadata server. In the case of a SETATTR of the
size attribute, the management protocol is responsible for
propagating size updates/truncations to the storage devices. In the
case of extending WRITEs to the storage devices, the new size must be
visible on the metadata server once a LAYOUTCOMMIT has completed (see
Section 3.5.1, Section 5.5.2). All size attribute updates MUST be
effective on the storage devices immediately (by the time the
metadata operation returns), so that READs past EOF can be
recognized.
As described in Section 3.2, a client MUST NOT issue I/Os to storage
devices for which it does not hold a valid layout. The storage
devices SHOULD reject such requests.
5.2 Global Stateid Requirements
Note, there are no stateids returned embedded within the layout. The
client MUST use the stateid representing open or lock state as
returned by an earlier metadata operation (e.g., OPEN, LOCK) to
perform I/O on the data-servers (as would be used in regular NFSv4).
Special stateids may be used when accessing data files on the storage
devices. Special stateid usage for I/O is subject to the NFSv4
protocol specification. The stateid used for I/O MUST have the same
affect and be subject to the same validation on storage device as it
would if the I/O was being performed on the metadata server itself
(in the absence of pNFS). This has the implication that stateids are
globally valid on both the metadata and storage devices. This
requires the metadata server to propagate changes in lock and open
state to the data-servers, so that the data-servers may validate I/O
accesses. This is discussed further in Section 5.4.
5.3 The Layout Iomode
The layout iomode is not used by the metadata server when servicing
NFSv4 file-based layouts. As such, the client SHOULD set the iomode
to READ/WRITE at LAYOUTGET time. If an iomode of READ/WRITE is not
specified, the metadata server may return an error. The iomode need
not be checked by the storage devices when clients perform I/O.
However, the storage devices SHOULD still validate that the client
holds a valid layout and return an error if the client does not.
5.4 Storage Device State Propagation
Since the metadata server, which handles lock and open-mode state
changes, as well as ACLs, may not be collocated with the storage
devices (where I/O access is validated), the server implementation
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MUST take care of propagating changes of this state to the storage
devices. Once the propagation to the storage devices is complete,
the full effect of those changes must be in effect at the storage
devices. However, some state changes need not be propagated
immediately, although all changes SHOULD be propagated promptly.
These state propagations have an impact on the design of the
management protocol, even though the management protocol is outside
of the scope of this specification.
5.4.1 Lock State Propagation
Mandatory locks MUST be made effective at the storage devices before
the request that establishes them returns to the caller. Thus,
mandatory lock state MUST be synchronously propagated to the storage
devices. On the other hand, since advisory lock state is not used
for checking I/O accesses at the storage devices, there is no
semantic reason for propagating advisory lock state to the storage
devices. However, since all lock, unlock, open downgrades and
upgrades affect the sequence ID stored within the stateid, the
stateid changes which may cause difficulty if this state is not
propagated.
Thus, when a client uses a stateid on a storage device for I/O with a
newer sequence number than the one the storage device has, the
storage device should query the metadata server and get any pending
updates to that stateid. Since updates to advisory locks neither
confer nor remove privileges, these changes need not be propagated
immediately, and may not need to be propagated promptly. The updates
to advisory locks need only be propagated when the storage device
needs to resolve a question about a stateid. In fact, if byte-range
locking is not mandatory (is advisory) the clients are advised not to
use the lock-based stateids for I/O at all. The ones returned by
open are sufficient and eliminate overhead for this kind of state
propagation.
5.4.2 Open-mode Validation
Open-mode validation MUST be performed against the open mode(s) held
by the storage devices. However, the server implementation may not
always require the immediate propagation of changes. Reduction in
access because of CLOSEs or DOWNGRADEs do not have to be propagated
immediately, but SHOULD be propagated promptly (whereas changes due
to revocation MUST be propagated immediately). On the other hand,
changes that expand access (e.g., new OPEN's and upgrades) don't have
to be propagated immediately but the storage device SHOULD NOT reject
a request because of mode issues without making sure that the upgrade
is not in flight.
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5.4.3 File Attributes
Since the SETATTR operation has the ability to modify state that is
visible on the both the metadata and storage devices (e.g., the
size), care must be taken to ensure that the resultant state (across
the set of storage devices) is consistent; especially when truncating
or growing the file.
As described earlier, the LAYOUTCOMMIT operation is used to ensure
that the metadata is synced with changes made to the storage devices.
For the file-based protocol, it is necessary to re-sync state such as
the size attribute, and the setting of mtime/atime. The management
protocol is free to sync this state before it receives a
LAYOUTCOMMIT, however upon successful completion of a LAYOUTCOMMIT
the size attribute MUST be in sync (across the metadata server and
storage devices, baring any intervening operations).
5.4.4 Access State Propagation
Any changes to the state of a file that controls access as reflected
by ACCESS calls or READs and WRITEs on the metadata server, MUST be
propagated to the storage devices for enforcement on READ and WRITE
I/O calls. If the changes made on the metadata server result in more
restrictive access permissions for any user, those changes MUST be
propagated to the storage devices synchronously.
Recall, the NFSv4 protocol [2] specifies that:
...since the NFS version 4 protocol does not impose any
requirement that READs and WRITEs issued for an open file have the
same credentials as the OPEN itself, the server still must do
appropriate access checking on the READs and WRITEs themselves.
This also includes changes to ACLs. The propagation of ACLs may be
asynchronous only if the server implementation is able to determine
that the updated ACL is not more restrictive for any user specified
in the old ACL. Due to the relative infrequency of ACL updates, it
is suggested that they are propagated synchronously.
5.5 Extending EOF
5.5.1 READs and EOF
A potential problem exists when a data file on a particular storage
device is grown past EOF; it exists for both dense and sparse
layouts. Imagine the following scenario: a client creates a new file
(EOF == 0) and writes to byte 128KB; the client then seeks to the
beginning of the file and reads byte 100. The client should receive
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0s back as a result of the read. However, if the read falls on a
different storage device to the client's original write, the storage
device servicing the READ may still believe that the EOF is at 0 and
return no data (with the EOF flag set). The storage device can only
return 0s if it knows that the EOF has been extended. This would
require the immediate propagation of EOF to all storage devices,
which is potentially very costly, instead another approach is
outlined below.
First, the EOF is returned within the layout by LAYOUTGET. This EOF
must reflect the latest EOF at the metadata server as set by the most
recent of either the last LAYOUTCOMMIT or SETATTR; however, it may be
more recent. Second, if a client performs a read that is returned
short (i.e., is fully within EOF, but the storage device indicates
EOF and returns partial or no data), the client must assume that it
is a hole and substitute 0s for the data not read (up until its known
local EOF). If a client extends the file, it must update its local
EOF. Third, if the metadata server receives a SETATTR of the size or
a LAYOUTCOMMIT that alters the EOF, the metadata server MUST send out
CB_EOFCHANGED messages with the new EOF to clients holding layouts
(it need not send a notification to the client that performed the
operation that resulted in EOF changing). Upon reception of the
CB_EOFCHANGED notification, clients must update their local EOF. As
well, if a new EOF is returned as a result to LAYOUTCOMMIT, the
client must update their local EOF.
5.5.2 LAYOUTCOMMIT and EOF
Another complication can arise due to EOF. If a file has been grown
by a set of WRITEs prior to a LAYOUTCOMMIT, the management protocol
must ensure that the corresponding file on each storage device is
grown (possibly sparsely) up until the offset represented by the EOF
length before LAYOUTCOMMIT returns.
For example: Imagine a file is striped across four storage devices,
using a sparse file layout, with 64KB on each storage device. A
WRITE of 64KB occurs starting at offset 192KB (the first stripe on
the 4th storage device) followed by a LAYOUTCOMMIT. The new EOF
offset is now at 256KB, however the corresponding file size on the
first three storage devices is 0, since they did not service any
WRITE operations. Immediately upon completion of LAYOUTCOMMIT, the
server implementation MUST ensure that READs to any of the storage
devices, at an offset below EOF, succeed; indeed, in this example, a
read to any of the first three storage devices (below EOF) must
return all 0s. The easiest way to accomplish this is to set the file
size on each of the storage devices to EOF. Note, this only need
occur at LAYOUTCOMMIT time or upon the reception of a SETATTR that
modifies the size.
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5.6 Security Considerations
The NFSv4 file layout type MUST adhere to the security considerations
outlined in Section 4. More specifically, storage devices must make
all of the required access checks on each READ or WRITE I/O as
determined by the NFSv4 protocol [2]. This impacts the management
protocol and the propagation of state from the metadata server to the
storage devices; see Section 5.4 for more details.
5.7 Alternate Approaches
Two alternate approaches exist for file-based layouts and the method
used by clients to obtain stateids used for I/O. Both approaches
embed stateids within the layout.
However, before examining these approaches it is important to
understand the distinction between clients and owners. Delegations
belong to clients, while locks (record and share reservations) are
held by owners (who belong to a specific client). As such,
delegations can only protect against inter-client conflicts, not
intra-client conflicts. Layouts are held by clients and SHOULD NOT
be associated with state held by owners. Therefore, if stateids used
for data access are embedded within a layout, these stateids can only
act as delegation stateids, protecting against inter-client
conflicts; stateids pertaining to an owner can not be embedded within
the layout. This has the implication that the client MUST arbitrate
among all intra-client conflicts (such as arbitrating among lock
requests by different processes) before issuing pNFS operations.
Using the stateids stored within the layout, storage devices can only
arbitrate between clients (not owners).
The first alternate approach is to do away with global stateids
(stateids returned by OPEN/LOCK that are valid on the metadata server
and storage devices) and use only stateids embedded within the
layout. This approach has the drawback that the stateids used for
I/O access can not be validated against per owner state (rather they
are validated against per client state), since they are only
associated with the client holding the layout. It breaks the
semantics of tieing a stateid used for I/O to an open instance. This
has the implication that clients must delegate per owner lock and
open requests internally, rather than push the work onto the storage
devices. The storage devices can still arbitrate and enforce inter-
client lock and open state.
[Comment: What goes wrong when the stateids are not used as expected?
Is it that byte range locks are not honored? Does it matter in the
world of advisory locks?]
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The second approach is a hybrid approach. This approach allows for
stateids to be embedded with the layout, but also allows for the
possibility of global stateids. If the stateid embedded within the
layout is a special stateid of all zeros, then the stateid referring
to the last successful OPEN/LOCK should be used (as a global stateid
presented earlier in the proposal). This approach is recommended if
it is decided that using NFSv4 as a management protocol is required.
This proposal suggests the global stateid approach due to the cleaner
semantics it provides regarding the relationship between stateids
used for I/O and their corresponding open instance (or lock state).
However, it does have a profound impact on the management protocol's
implementation and the state propagation that is required (as
described in Section 5.4).
6. pNFS Typed Data Structures
6.1 pnfs_layouttype4
enum pnfs_layouttype4 {
LAYOUT_NFSV4_FILES = 1
};
A layout type specifies the layout being used. The implication is
that clients have "layout drivers" that support one or more layout
types. The file server advertises the layout types it supports
through the LAYOUT_TYPES file system attribute. A client asks for
layouts of a particular type in LAYOUTGET, and passes those layouts
to its layout driver. The set of well known layout types must be
defined through IANA. A private range of layout types should exist
as defined through IANA. This would allow custom installations to
introduce new layout types.
The LAYOUT_NFSV4_FILES enumeration specifies that the NFSv4 file
layout type is to be used.
6.2 pnfs_deviceid4
uint32_t pnfs_deviceid4; /* 32-bit device ID */
Layout information includes device IDs that specify a storage device
through a compact handle. Addressing and type information is
obtained with the GETDEVICEINFO operation. Device IDs may not be
valid across metadata server reboots. See Section 3.1.1 for more
details.
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6.3 pnfs_devaddr4
enum pnfs_devaddrtypes4 {
DEVADDR_SERVER_PORT = 1
};
union pnfs_devaddr4 switch (pnfs_devaddrtypes4 type) {
case DEVADDR_SERVER_PORT:
string r_netid<>; /* network ID */
string r_addr<>; /* universal address */
default:
opaque devaddr<>; /* For other layouts */
};
This structure is used to set up a communication channel with the
storage device. Different layout types will require different types
of structures to define how they communicate with storage devices.
The pnfs_devaddrtypes4 enumeration should list the types of
structures required by the different layout types. The pnfs_devaddr4
union switches of this enumeration. Currently, DEVADDR_SERVER_PORT
has been defined to identify a storage device by network IP address
and port number. This is sufficient for the NFSv4 file layout
storage driver to communicate with the NFSv4 storage devices, and for
object-based storage drivers to communicate with iSCSI/OSD devices.
Again, the pnfs_devaddrtypes4 should be defined through IANA.
6.4 pnfs_devlist_item4
struct pnfs_devlist_item4 {
pnfs_deviceid4 id;
pnfs_deviceaddr4 addr;
};
An array of these values is returned by the GETDEVICELIST operation.
They define the set of devices associated with a file system.
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6.5 pnfs_layout4
union pnfs_layouttypes4 switch (pnfs_layouttype4 layout_type) {
case LAYOUT_NFSV4_FILES:
pnfs_nfsv4_layouttype4 file_layout;
default:
opaque layout_data<>;
};
struct pnfs_layout4 {
offset4 offset;
length4 length;
pnfs_layouttypes4 layout;
};
The pnfs_layout4 structure defines a layout for a file. The
pnfs_layouttypes4 union contains the portion of the layout specific
to the layout type. Currently, only the NFSv4 file layout type is
defined; see Section 5.1 for its definition. Since layouts are sub-
dividable, the offset and length (together with the file's filehandle
and the clientid), identifies the layout.
7. pNFS File Attributes
7.1 pnfs_layouttype4<> LAYOUT_TYPES
This attribute applies to a file system and indicates what layout
types are supported by the file system. We expect this attribute to
be queried when a client encounters a new fsid. This attribute is
used by the client to determine if it has applicable layout drivers.
7.2 pnfs_layouttype4 LAYOUT_TYPE
This attribute indicates the particular layout type used for a file.
This is for informational purposes only. The client needs to use the
LAYOUTGET operation in order to get enough information (e.g.,
specific device information) in order to perform I/O.
7.3 pnfs_layouttypes4 LAYOUT_HINT
This attribute may be set on newly created files to influence the
metadata server's choice for the file's layout. The metadata server
may ignore this attribute. This attribute is a sub-set of the layout
returned by LAYOUTGET. For example, instead of specifying particular
devices, this would be used to suggest the stripe width of a file.
It is up to the server implementation to determine which fields
within the layout it uses.
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8. pNFS Error Definitions
NFS4ERR_BADLAYOUT Layout specified is invalid.
NFS4ERR_LAYOUTUNAVAILABLE Layouts are not available for the file or
its containing file system.
NFS4ERR_LAYOUTTRYLATER Layouts are temporarily unavailable for the
file, client should retry later.
9. pNFS Operations
9.1 LAYOUTGET - Get Layout Information
SYNOPSIS
(cfh), clientid, layout_type, iomode, offset, length -> layout
ARGUMENT
enum layoutget_iomode4 {
LAYOUTGET_READ = 1,
LAYOUTGET_RW = 2
};
struct LAYOUTGET4args {
/* CURRENT_FH: file */
clientid4 clientid;
pnfs_layouttype4 layout_type;
layoutget_iomode4 iomode;
offset4 offset;
length4 length;
};
RESULT
struct LAYOUTGET4resok {
pnfs_layout4 layout;
};
union LAYOUTGET4res switch (nfsstat4 status) {
case NFS4_OK:
LAYOUTGET4resok resok4;
default:
void;
};
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DESCRIPTION
Requests a layout for reading or writing the file given by the
filehandle at the byte range specified by offset and length. Layouts
are identified through the clientid, filehandle, and byte range
(offset, length pair). The iomode specifies whether the client
intends to read or read/write the data pertaining to the layout. The
metadata server's use of the iomode may depend on the layout type
being used. The storage devices may validate I/O accesses against
the iomode (and reject invalid accesses).
As well, the metadata server may adjust the range of the returned
layout based on striping patterns and usage implied by the iomode.
The client must be prepared to get a layout that does not line up
exactly with their request; there MUST be at least one byte of
overlap between the layout returned by the server and the client's
request, or the server SHOULD reject the request. See Section 3.3
for more details.
The LAYOUTGET operation returns layout information for the specified
byte range. To get a layout from a specific offset through the end-
of-file (no matter how long the file actually is) use a length field
with all bits set to 1 (one). If the length is zero, or if a length
which is not all bits set to one is specified, and length when added
to the offset exceeds the maximum 64-bit unsigned integer value, the
error NFS4ERR_INVAL will result.
The format of the returned layout is specific to the underlying file
system. Layout types other than the NFSv4 file layout type should be
specified outside of this document.
If layouts are not supported for the requested file or its containing
file system the server SHOULD return NFS4ERR_LAYOUTUNAVAILABLE.
If layout for the file is unavailable due to transient conditions,
e.g. file sharing prohibits layouts, the server SHOULD return
NFS4ERR_LAYOUTTRYLATER.
On success, the current filehandle retains its value.
IMPLEMENTATION
Typically, LAYOUTGET will be called as part of a compound RPC after
an OPEN operation and results in the client having location
information for the file; a client may also hold a layout across
multiple OPENs. The client specifies a layout type that limits what
kind of layout the server will return. This prevents servers from
issuing layouts that are unusable by the client.
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[Comment: The notion of the layout class indicating a sub-set of
possible layout types is gone. Now that the class is a flat number
space, there is no official way to reference a "class" of layouts
(e.g., files, blocks, or objects). This means that the type in the
LAYOUTGET may be too restrictive, or that it is up to the server to
decide if it can give out a "closely associated" layout that the
client may be able to use.]
ERRORS
NFS4ERR_INVAL
NFS4ERR_NOTSUPP
NFS4ERR_LAYOUTUNAVAILABLE
NFS4ERR_LAYOUTTRYLATER
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9.2 LAYOUTCOMMIT - Commit writes made using a layout
SYNOPSIS
(cfh), layout_stateid, offset, length, neweof, newlayout -> neweof
ARGUMENT
union neweof4 switch (bool eofchanged) {
case TRUE:
length4 eof;
case FALSE:
void;
};
union newlayout4 switch (bool layoutchanged) {
case TRUE:
pnfs_layouttypes4 layout;
case FALSE:
void;
};
struct LAYOUTCOMMIT4args {
/* CURRENT_FH: file */
clientid4 clientid;
neweof4 neweof;
offset4 offset;
length4 length;
newlayout4 newlayout;
};
RESULT
struct LAYOUTCOMMIT4resok {
neweof4 neweof;
};
union LAYOUTCOMMIT4res switch (nfsstat4 status) {
case NFS4_OK:
LAYOUTCOMMIT4resok resok4;
default:
void;
};
DESCRIPTION
Commits changes in the layout portion represented by the current
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filehandle, clientid, and byte range. Since layouts are sub-
dividable, a smaller portion of a layout, retrieved via LAYOUTGET,
may be committed. The region being committed is specified through
the byte range (length and offset). Note: the "newlayout" field does
not include the length and offset, as they are already specified in
the arguments.
The LAYOUTCOMMIT operation indicates that the client has completed
writes using a layout obtained by a previous LAYOUTGET. The client
may have only written a subset of the data range it previously
requested. LAYOUTCOMMIT allows it to commit or discard provisionally
allocated space and to update the server with a new end of file. The
metadata server may use the included new EOF as a hint. If the
metadata server changes the EOF of the file, it MUST return the new
EOF as part of the results.
The layout argument to LAYOUTCOMMIT describes what regions have been
used and what regions can be deallocated. NFSv4 file layout type
implementations should ignore this field. The resulting layout is
still valid after LAYOUTCOMMIT and can be continued to be referenced
by the clientid, filehandle, and byte range.
The layout information is more verbose for block devices than for
objects and files because the latter hide the details of block
allocation behind their storage protocols. At the minimum, the
client needs to communicate changes to the end of file location back
to the server, and its view of the file modify and access times
(unless it wants the server to set those times to the time of
LAYOUTCOMMIT). For blocks, it needs to specify precisely which
blocks have been used.
The metadata server should use the time of the LAYOUTCOMMIT operation
as the file modify time, unless it is able to determine that the file
has not been updated since the last mtime update. The client may use
a SETATTR operation in a compound right after LAYOUTCOMMIT in order
to override the access and modify times of the file. See Section 3.5
for more details.
On success, the current filehandle retains its value.
ERRORS
NFS4ERR_INVAL
NFS4ERR_BADLAYOUT
TBD
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9.3 LAYOUTRETURN - Release Layout Information
SYNOPSIS
(cfh), clientid, offset, length -> -
ARGUMENT
struct LAYOUTRETURN4args {
/* CURRENT_FH: file */
clientid4 clientid;
offset4 offset;
length4 length;
};
RESULT
struct LAYOUTRETURN4res {
nfsstat4 status;
};
DESCRIPTION
Returns the layout represented by the current filehandle, clientid,
and byte range. After this call, the client MUST NOT use the layout
and the associated storage protocol to access the file data. The
layout being returned may be a subdivision of a layout previously
fetched through LAYOUTGET. If the length is all 1s, the layout
covers the range from offset to EOF.
Layouts may be returned when recalled or voluntarily (i.e., before
the server has recalled them). In either case the client must
properly propagate state changed under the context of the layout to
storage or to the server before returning the layout.
If a client fails to return a layout in a timely manner, then the
metadata server should use its management protocol with the storage
devices to fence the client from accessing the data referenced by the
layout. See Section 3.4 for more details.
On success, the current filehandle retains its value.
[TODO: We need to work out how clients return error information if
they encounter problems with storage (if they should). We could
return a single OK bit, or we could return more extensive information
from the layout driver that describes the error condition in more
detail. We could use an opaque "layout_error" type that is defined
by the storage protocol along with its layout types.]
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[There is a proposal for communicating extended error information in
the "newlayout" argument to LAYOUTCOMMIT. This could provide an
additional status code for each device in the layout, for example.]
ERRORS
NFS4ERR_INVAL
NFS4ERR_BADLAYOUT
TBD
9.4 GETDEVICEINFO - Get Device Information
SYNOPSIS
(cfh), device_id -> device_addr
ARGUMENT
struct GETDEVICEINFO4args {
pnfs_deviceid4 device_id;
};
RESULT
struct GETDEVICEINFO4resok {
pnfs_devaddr4 device_addr;
};
union GETDEVICEINFO4res switch (nfsstat4 status) {
case NFS4_OK:
GETDEVICEINFO4resok resok4;
default:
void;
};
DESCRIPTION
Returns device type and device address information for a specified
device. The returned device_addr includes a type that indicates how
to interpret the addressing information for that device. At this
time we expect two main kinds of device addresses, either IP address
and port numbers, or SCSI volume identifiers. The final protocol
specification will detail the allowed values for device_type and the
format of their associated location information.
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See Section 3.1.1 for more details on device ID assignment.
9.5 GETDEVICELIST - Get List of Devices
SYNOPSIS
(cfh), max_bytes, cookie, cookie_verf -> device_addr<>
ARGUMENT
struct GETDEVICELIST4args {
/* Current file handle */
uint32_t max_bytes;
nfs_cookie4 cookie;
verifier4 cookie_verf;
};
RESULT
struct GETDEVICELIST4resok {
pnfs_devlist_item4 device_addr_list<>;
};
union GETDEVICEINFO4res switch (nfsstat4 status) {
case NFS4_OK:
GETDEVICEINFO4resok resok4;
default:
void;
};
DESCRIPTION
In some applications, especially SAN environments, it is convenient
to find out about all the devices associated with a file system.
This lets a client determine if it has access to these devices, e.g.,
at mount time.
This operation returns a list of items that establish the association
between the short pnfs_deviceid4 and the addressing information for
that device. This operation may not be able to fetch all device
information at once, thus it uses a cookie based approach, similar to
READDIR, to fetch additional device information.
10. Callback Operations
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10.1 CB_LAYOUTRECALL
SYNOPSIS
fh, offset, length -> -
ARGUMENT
struct CB_LAYOUTRECALLargs {
nfs_fh4 fh;
offset4 offset;
length4 length;
};
RESULT
struct CB_LAYOUTRECALLres {
nfsstat4 status;
};
DESCRIPTION
The CB_LAYOUTRECALL operation is used to begin the process of
recalling a layout, or a portion thereof, and returning it to the
server. The offset and length fields specify the portion of the
layout to be returned. A length of all 1s specifies that the layout
to EOF MUST be returned.
If the handle specified is not one for which the client holds a
layout, an NFS4ERR_BADHANDLE error is returned.
If the layout byte range specified does not correspond to a valid
layout for the file specified by the filehandle, an NFS4ERR_BADLAYOUT
is returned. If the byte range overlaps with a layout being held,
the portion of the layout represented by the overlap MUST be
returned.
IMPLEMENTATION
The client should reply to the callback immediately. Replying does
not complete the recall except when an error was returned. The
recall is not complete until the layout is returned using a
LAYOUTRETURN.
The client should complete any in-flight I/O operations using the
recalled layout before returning it via LAYOUTRETURN. If the client
has buffered dirty data, it may chose to write it directly to storage
before calling LAYOUTRETURN, or to write it later using normal NFSv4
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WRITE operations to the metadata server.
ERRORS
NFS4ERR_BADHANDLE
NFS4ERR_BADLAYOUT
10.2 CB_EOFCHANGED
SYNOPSIS
fh, eof -> -
ARGUMENT
struct CB_EOFCHANGEDargs {
nfs_fh4 fh;
length4 eof;
};
RESULT
struct CB_EOFCHANGEDres {
nfsstat4 status;
};
DESCRIPTION
The CB_EOFCHANGED operation is used to notify the client that the EOF
pertaining to the filehandle associated with "fh", has changed. The
new EOF is specified in the "eof" field. Upon reception of this
notification callback, the client should update its internal EOF for
the file. If the layout being held for the file is of the NFSv4 file
layout type, then the EOF field within that layout should be updated
(see Section 5.5.1). For other layout types see Section 3.5.1 for
more details.
If the handle specified is not one for which the client holds a
layout, an NFS4ERR_BADHANDLE error is returned.
ERRORS
NFS4ERR_BADHANDLE
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11. Usage Scenarios
This section has a description of common open, close, read, write
interactions and how those work with layout delegations. [TODO: this
section feels rough and I'm not sure it adds value in its present
form.]
11.1 Basic Read Scenario
Client does an OPEN to get a file handle. Client does a LAYOUTGET
for a range of the file, gets back a layout. Client uses the storage
protocol and the layout to access the file. Client returns the
layout with LAYOUTRETURN. Client closes stateID and open delegation
with CLOSE.
This is rather boring as the client is careful to clean up all server
state after only a single use of the file.
11.2 Multiple Reads to a File
Client does an OPEN to get a file handle. Client does a LAYOUTGET
for a range of the file, gets back a layout. Client uses the storage
protocol and the layout to access the file. Client closes stateID
and with CLOSE.
Client does an OPEN to get a file handle. Client finds cached layout
associated with file handle. Client uses the storage protocol and
the layout to access the file. Client closes stateID and with CLOSE.
A bit more interesting as we've saved the LAYOUTGET operation, but we
are still doing server round-trips.
11.3 Multiple Reads to a File with Delegations
Client does an OPEN to get a file handle and an open delegation.
Client does a LAYOUTGET for a range of the file, gets back a layout.
Client uses the storage protocol and the layout to access the file.
Application does a close(), but client keeps state under the
delegation. (time passes) Application does another open(), which
client handles under the delegation. Client finds cached layout
associated with file handle. Client uses the storage protocol and
the layout to access the file. (pattern continues until open
delegation and/or layout is recalled)
This illustrates the efficiency of combining open delegations and
layouts to eliminate interactions with the file server altogether.
Of course, we assume the client's operating system is only allowing
the local open() to succeed based on the file permissions. The use
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of layouts does not change anything about the semantics of open
delegations.
11.4 Read with existing writers
NOTE: This scenario was under some debate, but we have resolved that
the server is able to give out overlapping/conflicting layout
information to different clients. In these cases we assume that
clients are using an external mechanism such as MPI-IO to synchronize
and serialize access to shared data. One can argue that even
unsynchronized clients get the same open-to-close consistency
semantics as NFS already provides, even when going direct to storage.
Client does an OPEN to get an open stateID and open delegation. The
file is open for writing elsewhere by different clients and so no
open delegation is returned. Client does a LAYOUT get and gets a
layout from the server. Client either synchronizes with the writers,
or not, and accesses data via the layout and storage protocol. There
are no guarantees about when data that is written by the writer is
visible to the reader. Once the writer has closed the file and
flushed updates to storage, then they are visible to the client.
[We should state explicitly that COMMIT and LAYOUTCOMMIT represent
explicit points where changes should be visible to other clients.]
11.5 Read with later conflict
ClientA does an OPEN to get an open stateID and open delegation.
ClientA does a LAYOUTGET for a range of the file, gets back a map and
layout stateid. ClientA uses the storage protocol to access the file
data. ClientB opens the file for WRITE. File server issues
CB_RECALL to ClientA. ClientA issues DELEGRETURN.
ClientA continues to use the storage protocol to access file data.
If it is accessing data from its cache, it will periodically check
that its data is still up-to-date because it has no open delegation.
[This is an odd scenario that mixes in open delegations for no real
value. Basically this is a "regular writer" being mixed with a pNFS
reader. I guess this example shows that no particular semantics are
provided during the simultaneous access. If the server so chose, it
could also recall the layout with CB_LAYOUTRECALL to force the
different clients to serialize at the file server.]
11.6 Basic Write Case
Client does an OPEN to get a file handle. Client does a LAYOUTGET
for a range of the file, gets back a layout and layout stateid.
Client writes to the file using the storage protocol. Client uses
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LAYOUTCOMMIT to communicate new EOF position. Client does SETATTR to
update timestamps. Client does a LAYOUTRETURN. Client does a CLOSE.
Again, the boring case where the client cleans up all of its server
state by returning the layout.
11.7 Large Write Case
Client does an OPEN to get a file handle. (loop.) Client does a
LAYOUTGET for a range of the file, gets back a layout and layout
stateid. Client writes to the file using the storage protocol.
Client fills up the range covered by the layout. Client updates the
server with LAYOUTCOMMIT, communicating about new EOF position.
Client does SETATTR to update timestamps. Client releases the layout
with LAYOUTRELEASE. (end loop.) Client does a CLOSE.
11.8 Create with special layout
Client does an OPEN and a SETATTR that specifies a particular layout
type using the LAYOUT_HINT attribute. Client gets back an open
stateID and file handle. (etc)
12. Layouts and Aggregation
This section describes several layout formats in a semi-formal way to
provide context for the layout delegations. These definitions will
be formalized in other protocols. However, the set of understood
types is part of this protocol in order to provide for basic
interoperability.
The layout descriptions include (deviceID, objectID) tuples that
identify some storage object on some storage device. The addressing
formation associated with the deviceID is obtained with
GETDEVICEINFO. The interpretation of the objectID depends on the
storage protocol. The objectID could be a filehandle for an NFSv4
storage device. It could be a OSD object ID for an object server.
The layout for a block device generally includes additional block map
information to enumerate blocks or extents that are part of the
layout.
12.1 Simple Map
The data is located on a single storage device. In this case the
file server can act as the front end for several storage devices and
distribute files among them. Each file is limited in its size and
performance characteristics by a single storage device. The simple
map consists of (deviceID, objectID).
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12.2 Block Map
The data is located on a LUN in the SAN. The layout consists of an
array of (deviceID, blockID, blocksize) tuples. Alternatively, the
blocksize could be specified once to apply to all entries in the
layout.
12.3 Striped Map (RAID 0)
The data is striped across storage devices. The parameters of the
stripe include the number of storage devices (N) and the size of each
stripe unit (U). A full stripe of data is N * U bytes. The stripe
map consists of an ordered list of (deviceID, objectID) tuples and
the parameter value for U. The first stripe unit (the first U bytes)
are stored on the first (deviceID, objectID), the second stripe unit
on the second (deviceID, objectID) and so forth until the first
complete stripe. The data layout then wraps around so that byte
(N*U) of the file is stored on the first (deviceID, objectID) in the
list, but starting at offset U within that object. The striped
layout allows a client to read or write to the component objects in
parallel to achieve high bandwidth.
The striped map for a block device would be slightly different. The
map is an ordered list of (deviceID, blockID, blocksize), where the
deviceID is rotated among a set of devices to achieve striping.
12.4 Replicated Map
The file data is replicated on N storage devices. The map consists
of N (deviceID, objectID) tuples. When data is written using this
map, it should be written to N objects in parallel. When data is
read, any component object can be used.
This map type is controversial because it highlights the issues with
error recovery. Those issues get interesting with any scheme that
employs redundancy. The handling of errors (e.g., only a subset of
replicas get updated) is outside the scope of this protocol
extension. Instead, it is a function of the storage protocol and the
metadata management protocol.
12.5 Concatenated Map
The map consists of an ordered set of N (deviceID, objectID, size)
tuples. Each successive tuple describes the next segment of the
file.
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12.6 Nested Map
The nested map is used to compose more complex maps out of simpler
ones. The map format is an ordered set of M sub-maps, each submap
applies to a byte range within the file and has its own type such as
the ones introduced above. Any level of nesting is allowed in order
to build up complex aggregation schemes.
13. Issues
13.1 Storage Protocol Negotiation
Clients may want to negotiate with the metadata server about their
preferred storage protocol, and to find out what storage protocols
the server offers. Client can do this by querying the LAYOUT_TYPES
file system attribute. They respond by specifying a particular
layout type in their LAYOUTGET operation.
13.2 Crash recovery
We use the existing client crash recovery and server state recovery
mechanisms in NFSv4. The main new issue introduced by pNFS is that
the client may have to do a lot of I/O in response to a layout
recall. The client may need to remember to send RENEW ops to the
server during this period if it were to risk not doing anything
within the lease time. Of course, the client should only reply with
its LAYOUTRETURN after it knows its I/O has completed.
13.3 Storage Errors
As noted under LAYOUTRETURN, there may be a need for the client to
communicate about errors it has when accessing storage directly.
14. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[2] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
C., Eisler, M., and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 3530, April 2003.
[3] Weber, R., "Object-Based Storage Device Commands (OSD)",
INCITS 400-2004, July 2004,
<http://www.t10.org/ftp/t10/drafts/osd/osd-r10.pdf>.
[4] Gibson, G., "pNFS Problem Statement", July 2004, <ftp://
www.ietf.org/internet-drafts/
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draft-gibson-pnfs-problem-statement-01.txt>.
Authors' Addresses
Brent Welch
Panasas, Inc.
6520 Kaiser Drive
Fremont, CA 95444
USA
Phone: +1-650-608-7770
Email: welch@panasas.com
URI: http://www.panasas.com/
Benny Halevy
Panasas, Inc.
1501 Reedsdale St., #400
Pittsburgh, PA 15233
USA
Phone: +1-412-323-3500
Email: bhalevy@panasas.com
URI: http://www.panasas.com/
Garth Goodson
Network Appliance
495 E. Java Dr
Sunnyvale, CA 94089
USA
Phone: +1-408-822-6847
Email: goodson@netapp.com
David L. Black
EMC Corporation
176 South Street
Hopkinton, MA 01748
USA
Phone: +1-508-293-7953
Email: black_david@emc.com
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Andy Adamson
CITI University of Michigan
519 W. William
Ann Arbor, MI 48103-4943
USA
Phone: +1-734-764-9465
Email: andros@umich.edu
Appendix A. Acknowledgments
Many members of the pNFS informal working group have helped
considerably. The authors would like to thank Gary Grider, Peter
Corbett, Dave Noveck, and Peter Honeyman. This work is inspired by
the NASD and OSD work done by Garth Gibson. Gary Grider of the
national labs (LANL) has been a champion of high-performance parallel
I/O.
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