Internet DRAFT - draft-welch-pnfs-ops
draft-welch-pnfs-ops
Network B. Welch
Internet-Draft B. Halevy
Expires: January 16, 2006 Panasas
G. Goodson
NetApp
D. Black
EMC
A. Adamson
CITI
July 15, 2005
pNFS Operations
draft-welch-pnfs-ops-03.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 provide sufficient coordination of the client I/O so that the
file system retains its integrity.
The extension adds operations that query and manage layout
information that allows parallel I/O between clients and storage
system elements. The layouts are managed in a similar way to
delegations in that they are associated with leases and can be
recalled by the server, but layout information is independent of
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 . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. General Definitions . . . . . . . . . . . . . . . . . . . . . 8
2.1 Metadata Server . . . . . . . . . . . . . . . . . . . . . 8
2.2 Client . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Storage Device . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Storage Protocol . . . . . . . . . . . . . . . . . . . . . 9
2.5 Management Protocol . . . . . . . . . . . . . . . . . . . 9
2.6 Metadata . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.7 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Layouts and Aggregation . . . . . . . . . . . . . . . . . . . 10
3.1 Layout Structure . . . . . . . . . . . . . . . . . . . . . 10
3.1.1 Device IDs . . . . . . . . . . . . . . . . . . . . . . 11
3.1.2 Aggregation Schemes . . . . . . . . . . . . . . . . . 11
3.2 Basic Layout Semantics . . . . . . . . . . . . . . . . . . 12
3.2.1 Layouts and access control . . . . . . . . . . . . . . 12
3.2.2 Layout Iomode . . . . . . . . . . . . . . . . . . . . 13
3.2.3 Operation Sequencing . . . . . . . . . . . . . . . . . 13
3.3 Obtaining a Layout . . . . . . . . . . . . . . . . . . . . 14
3.3.1 Identifying Layouts . . . . . . . . . . . . . . . . . 14
3.3.2 Overlapping Layouts . . . . . . . . . . . . . . . . . 15
3.3.3 Copy-on-write . . . . . . . . . . . . . . . . . . . . 15
3.4 Recalling a Layout . . . . . . . . . . . . . . . . . . . . 15
3.5 Committing a Layout . . . . . . . . . . . . . . . . . . . 16
3.5.1 LAYOUTCOMMIT and mtime/atime/change . . . . . . . . . 17
3.5.2 LAYOUTCOMMIT and size . . . . . . . . . . . . . . . . 18
3.5.3 LAYOUTCOMMIT and layoutupdate . . . . . . . . . . . . 19
3.6 Crash Recovery . . . . . . . . . . . . . . . . . . . . . . 19
3.6.1 Leases . . . . . . . . . . . . . . . . . . . . . . . . 19
3.6.2 Client Recovery . . . . . . . . . . . . . . . . . . . 21
3.6.3 Metadata Server Recovery . . . . . . . . . . . . . . . 21
4. Security Considerations . . . . . . . . . . . . . . . . . . . 23
4.1 File Layout Security . . . . . . . . . . . . . . . . . . . 24
4.2 Object Layout Security . . . . . . . . . . . . . . . . . . 24
4.3 Block Layout Security . . . . . . . . . . . . . . . . . . 25
5. NFSv4 File Layout Type . . . . . . . . . . . . . . . . . . . . 26
5.1 File Striping and Data Access . . . . . . . . . . . . . . 26
5.1.1 Sparse and Dense Storage Device Data Layouts . . . . . 28
5.1.2 Operations Issued to Storage Devices . . . . . . . . . 29
5.2 Global Stateid Requirements . . . . . . . . . . . . . . . 30
5.3 The Layout Iomode . . . . . . . . . . . . . . . . . . . . 30
5.4 Storage Device State Propagation . . . . . . . . . . . . . 31
5.4.1 Lock State Propagation . . . . . . . . . . . . . . . . 31
5.4.2 Open-mode Validation . . . . . . . . . . . . . . . . . 32
5.4.3 File Attributes . . . . . . . . . . . . . . . . . . . 32
5.5 Extending file size . . . . . . . . . . . . . . . . . . . 33
5.5.1 READs and EOF . . . . . . . . . . . . . . . . . . . . 33
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5.5.2 LAYOUTCOMMIT and EOF . . . . . . . . . . . . . . . . . 33
5.6 Crash Recovery Considerations . . . . . . . . . . . . . . 34
5.7 Security Considerations . . . . . . . . . . . . . . . . . 35
5.8 Alternate Approaches . . . . . . . . . . . . . . . . . . . 35
6. pNFS Typed Data Structures . . . . . . . . . . . . . . . . . . 36
6.1 pnfs_layouttype4 . . . . . . . . . . . . . . . . . . . . . 36
6.2 pnfs_deviceid4 . . . . . . . . . . . . . . . . . . . . . . 36
6.3 pnfs_devaddr4 . . . . . . . . . . . . . . . . . . . . . . 37
6.4 pnfs_devlist_item4 . . . . . . . . . . . . . . . . . . . . 37
6.5 pnfs_layout4 . . . . . . . . . . . . . . . . . . . . . . . 37
6.6 pnfs_layoutupdate4 . . . . . . . . . . . . . . . . . . . . 38
6.7 pnfs_layoutiomode4 . . . . . . . . . . . . . . . . . . . . 38
7. pNFS File Attributes . . . . . . . . . . . . . . . . . . . . . 38
7.1 pnfs_layouttype4<> FS_LAYOUT_TYPES . . . . . . . . . . . . 39
7.2 pnfs_layouttype4<> FILE_LAYOUT_TYPE . . . . . . . . . . . 39
7.3 pnfs_layouttypes4 FILE_LAYOUT_HINT . . . . . . . . . . . . 39
7.4 uint32_t FS_LAYOUT_PREFERRED_BLOCKSIZE . . . . . . . . . . 39
7.5 uint32_t FS_LAYOUT_PREFERRED_ALIGNMENT . . . . . . . . . . 39
8. pNFS Error Definitions . . . . . . . . . . . . . . . . . . . . 39
9. pNFS Operations . . . . . . . . . . . . . . . . . . . . . . . 40
9.1 LAYOUTGET - Get Layout Information . . . . . . . . . . . . 40
9.2 LAYOUTCOMMIT - Commit writes made using a layout . . . . . 42
9.3 LAYOUTRETURN - Release Layout Information . . . . . . . . 46
9.4 GETDEVICEINFO - Get Device Information . . . . . . . . . . 47
9.5 GETDEVICELIST - Get List of Devices . . . . . . . . . . . 49
10. Callback Operations . . . . . . . . . . . . . . . . . . . . 50
10.1 CB_LAYOUTRECALL . . . . . . . . . . . . . . . . . . . . . 50
10.2 CB_SIZECHANGED . . . . . . . . . . . . . . . . . . . . . . 52
11. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . 52
11.1 Basic Read Scenario . . . . . . . . . . . . . . . . . . . 53
11.2 Multiple Reads to a File . . . . . . . . . . . . . . . . . 53
11.3 Multiple Reads to a File with Delegations . . . . . . . . 53
11.4 Read with existing writers . . . . . . . . . . . . . . . . 53
11.5 Read with later conflict . . . . . . . . . . . . . . . . . 54
11.6 Basic Write Case . . . . . . . . . . . . . . . . . . . . . 54
11.7 Large Write Case . . . . . . . . . . . . . . . . . . . . . 55
11.8 Create with special layout . . . . . . . . . . . . . . . . 55
12. Layouts and Aggregation . . . . . . . . . . . . . . . . . . 55
12.1 Simple Map . . . . . . . . . . . . . . . . . . . . . . . . 55
12.2 Block Map . . . . . . . . . . . . . . . . . . . . . . . . 55
12.3 Striped Map (RAID 0) . . . . . . . . . . . . . . . . . . . 56
12.4 Replicated Map . . . . . . . . . . . . . . . . . . . . . . 56
12.5 Concatenated Map . . . . . . . . . . . . . . . . . . . . . 56
12.6 Nested Map . . . . . . . . . . . . . . . . . . . . . . . . 56
13. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
13.1 Storage Protocol Negotiation . . . . . . . . . . . . . . . 57
13.2 Storage Errors . . . . . . . . . . . . . . . . . . . . . . 57
14. Normative References . . . . . . . . . . . . . . . . . . . . 57
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 57
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . . 59
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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. This is in contrast to NFSv4 without pNFS extensions, in
which this is primarily the server's responsibility, some of which
can be delegated to clients under strictly specified conditions.
The pNFS extension to NFSv4 takes the form of new operations that
manage data location information called a "layout". The layout is
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managed in a similar fashion as NFSv4 data delegations (e.g., they
are recallable and revocable). However, they are distinct
abstractions and are manipulated with new operations that are
described in Section 9. When a client holds a layout, it has rights
to access the data directly using the location information in the
layout.
There are new attributes that describe general layout
characteristics. However, much of the required information cannot be
managed solely within the attribute framework, because it will need
to have a strictly limited term of validity, subject to invalidation
by the server. This requires the use of new operations to obtain,
return, recall, and modify layouts, in addition to new attributes.
This document specifies both the NFSv4 extensions required to
distribute file access coordination between the server and its
clients and a NFSv4 file storage protocol that may be used to access
data stored on NFSv4 storage devices.
Storage protocols used to access a variety of other storage devices
are deliberately not specified here. These might include:
o Block 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.
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Some uses of pNFS extend the responsibility of clients beyond
delegations. In some configurations, the storage devices cannot
perform fine-grained access checks to ensure that clients are only
performing accesses within the bounds permitted to them by the pNFS
operations with the server (e.g., the checks may only be possible at
file system granularity rather than file granularity). In situations
where this added responsibility placed on clients creates
unacceptable security risks, pNFS configurations in which storage
devices cannot perform fine-grained access checks SHOULD NOT be used.
All pNFS server implementations MUST support NFSv4 access to any file
accessible via pNFS in order to provide an interoperable means of
file access in such situations. See Section 4 on Security for
further discussion.
Finally, there are issues about how layouts interact with the
existing NFSv4 abstractions of data delegations and byte range
locking. These issues (and 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 existing terms.
2.1 Metadata Server
A pNFS "server" or "metadata server" is a server as defined by
RFC3530 [2], which additionally provides support of the pNFS minor
extension. When using the pNFS NFSv4 minor extension, the metadata
server may hold only the metadata associated with a file, while the
data 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
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leaves other metadata management up to the metadata server. A
storage device could be another NFS server, or an Object Storage
Device (OSD) or a block device accessed over a SAN (either
FiberChannel or iSCSI SAN). The goal of this extension is to allow
direct communication between clients and storage devices.
2.4 Storage Protocol
This is the protocol between the pNFS client and the storage device
used to access the file data. Three following types have been
described: file protocols (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. Use of
NFSv4 itself as a file-based storage protocol is described in
Section 5.
2.5 Management Protocol
This is a protocol used by the exported file system between the
server and storage devices. Specification of such protocols is
outside the scope of this draft. Such management protocols would be
used to control such activities as the allocation and deallocation of
storage and the management of state required by the storage devices
to perform client access control. The 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 management 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
management 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
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stored, and so forth. The information is managed by the exported
file system server (metadata server). Metadata also includes lower-
level information like block addresses and indirect block pointers.
Depending the storage protocol, block-level metadata may or may not
be managed by the metadata server, but is instead managed by Object
Storage Devices or other servers acting as a storage device.
2.7 Layout
A layout defines how a file's data is organized on one or more
storage devices. There are many possible layout types. They vary in
the storage protocol used to access the data, and in the aggregation
scheme that lays out the file data on the underlying storage devices.
Layouts are described in more detail below.
3. 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 (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 (according to its
layout type). For example, the identity of a file server or object
server could be an IP address and port. The identity of a block
device could be a volume label. Due to multipath connectivity in a
SAN environment, agreement on a volume label is considered the
reliable way to locate a particular storage device.
The device ID is qualified by the layout type. This allows different
layout drivers to generate device IDs without the need for co-
ordination. In addition to GETDEVICEINFO, another operation,
GETDEVICELIST, has been added to allow clients to fetch the mappings
of multiple storage devices attached to a metadata server.
Clients SHOULD NOT expect the mapping between device ID 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 device ID to a different storage device (i.e., if
the mapping between storage device and data changes), the layout
describing the data MUST be recalled rather than assigning the new
storage device to the old device ID.
[OPEN ISSUE: we could associate leases with device IDs; this seems
overly complex. As well, we could introduce an invalidation process,
instead of recalling layouts if mapping changes; again seems overly
complex.]
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 client-implemented RAID that could be defined.
[OPEN ISSUE: should aggregation schemes (striping) be moved up a
level and made not opaque. I.e., there would exist a generic
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striping mechanisms that could be shared by file/block/object
specifications.]
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. When a layout is recalled, and
then returned by the client, the client retains the ability to access
file data with normal NFSv4 I/O operations through the metadata
server. Only the right to do I/O out-of-band is affected.
3.2.1 Layouts and access control
Holding a layout does not guarantee that a user of the layout has the
rights to access the data represented by the layout. All user access
rights MUST be obtained through the appropriate open, lock, and
access operations. However, if a valid layout for a file is not held
by the client, the storage device will reject all I/Os to that file's
byte range that originate from that client. In summary, layouts and
ordinary file access controls are independent. The act of modifying
a file for which a layout is held, does not necessarily conflict with
the holding of the layout (that describes the file being modified).
However, with certain layout types (e.g., block layouts), the
layout's iomode must agree with the type of I/O being performed.
Depending upon the layout type and storage protocol in use, storage
device access permissions may be granted by LAYOUTGET and may be
encoded within the type specific layout. If access permissions are
encoded within the layout, the metadata server should recall the
layout when the file's ACL or mode changes. For example, the object
layout protocol encodes access permissions within the capability
embedded within the layout; if these permissions change, the layout
is recalled and the capability revoked. Note, clients are still
required to perform the appropriate access operations as described
above (e.g., open and lock ops). The degree to which it is possible
for the client to circumvent these access operations must be clearly
addressed by the individual layout type documents, as well as the
consequences of doing so. In addition, these documents must be clear
about the requirements and non-requirements for the checking
performed by the server.
If the pNFS metadata server supports mandatory byte range locks then
byte range locks must behave as specified by the NFSv4 protocol (as
seen by users of files). If a storage device is unable to restrict
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access by a pNFS client who does not hold a required mandatory byte
range lock then the metadata server must not grant layouts to a
client, for that storage device, that permits any access that
conflicts with a mandatory byte range lock held by another client.
In this scenario, it is also necessary for the metadata server to
ensure that byte range locks are not granted to a client if any other
client holds a conflicting layout; in this case all conflicting
layouts must be recalled and returned before the lock request can be
granted. This requires the pNFS server to understand the
capabilities of its storage devices.
3.2.2 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 READs (only) or a mixture of I/O possibly containing
WRITEs as well (READ/WRITE). 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 may use byte range locking to serialize their accesses.
3.2.3 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 needs to be modified to provide for the correct sequencing
of pNFS layout operations. It is the server's responsibility to
avoid inconsistencies regarding the layouts it hands out.
One critical issue with operation sequencing concerns callbacks. The
protocol must defend against races between the reply to a LAYOUTGET
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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.
[OPEN ISSUE: the mechanism for doing this is still an open issue. It
may be sufficient to add a seqid to LAYOUTGET/LAYOUTRETURN.]
3.3 Obtaining a Layout
The metadata server will give out layouts of a particular type
(block, object, or file) and aggregation as requested by the client.
A client obtains a layout through a new operation (LAYOUTGET). The
client selects an appropriate layout type which the server supports
and the client is prepared to use. The layout returned to the client
may not line up exactly with the requested byte range. However, at
least a single byte overlap MUST exist between the requested layout
and the layout returned by the metadata server.
There is no implied ordering between getting a layout and performing
a file OPEN. For example, a layout may first be retrieved by placing
a LAYOUTGET operation in the same compound as the initial file OPEN.
Once the layout has been retrieved, it can be held across multiple
OPEN and CLOSE sequences.
The storage protocol used by the client to access the data on the
storage device is determined by the layout's type. The client needs
to select a "layout driver" that understands how to interpret and use
that layout. The API used by the client to talk to its drivers is
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 and aggregation scheme. The
pNFS extension introduces a LAYOUT_HINT attribute that the client can
set at creation time to provide a hint to the server for new files.
It is suggested that this attribute be set as one of the initial
attributes to OPEN when creating a new file. Setting this attribute
separately, after the file has been created could make it difficult
(or impossible) for the server implementation to comply.
3.3.1 Identifying Layouts
A layout is identified by the following tuple: (ClientID, FH, offset,
length, layout type, iomode); the FH refers to the FH of the file on
the metadata server, the offset and length specify the byte range of
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the file covered by the layout. The iomode specifies the client's
intent for data access (as described previously). Including the
iomode in the layout allows for distinct read-only and writable (and
readable) layouts to be held, possibly simultaneously (depending on
the layout type specific protocol).
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, to the same client, 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, of the same type
and iomode, differ, the old layout should be recalled and returned,
before giving out the new layout.
3.3.3 Copy-on-write
[OPEN ISSUE]
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 cannot make the choice of where to place data, it requires
help from 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.
Alternatively, separate read-only vs. read/write layouts, as
identified by the iomode, may be given out.
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 or action (e.g.,
server driven restriping or load balancing) that changes the layout
will result in a recall of the layout. A layout is recalled by the
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CB_LAYOUTRECALL callback operation (see Section 10.1). This callback
can either recall a layout identified by a byte range, or all the
layouts associated with a file system (FSID). The iomode must also
be specified when recalling layouts. A special LAYOUTIOMODE_ANY
enumeration is defined to enable recalling a layout of any type
(i.e., the client must return both read-only and read/write layouts).
A REMOVE operation may cause the metadata server to recall the layout
to prevent the client from accessing a non-existent file and to
reclaim state stored on the client. Since a REMOVE may be delayed
until the last close of the file has occurred, the recall may also be
delayed until this time. As well, once the file has been removed
(after the last reference), the client SHOULD no longer be able to
perform I/O using the layout (e.g., with file-based layouts an error
such as ESTALE could be returned).
Although, the pNFS extension does not alter the caching capabilities
of clients (or their semantics) it recognizes that some clients may
perform more aggressive write-behind caching to optimize the benefits
provided by pNFS. However, write-behind caching may impact the
latency in returning a layout in response to a CB_LAYOUTRECALL; just
as caching impacts DELEGRETURN with regards to data delegations.
Client implementations should limit the amount of dirty data they
have outstanding at any one time. Server implementations may fence
clients from performing direct I/O to the storage devices if they
perceive that the client is taking too long to return a layout once
recalled. A server may be able to monitor client progress by
watching client I/Os or by observing LAYOUTRETURNs of sub-portions of
the recalled layout. The server can also limit the amount of dirty
data to be flushed to storage devices by limiting the byte ranges
covered in the layouts it gives out.
Once a layout has been returned, the client should no longer issue
I/Os to the storage devices for the file, byte range, and iomode
represented by the returned layout. If a client does issue an I/O to
a storage device for which it does not hold a layout, the storage
device may reject the I/O.
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 some synchronization of
attributes between the metadata and storage devices (mainly the size
attribute; EOF). It is important to note that the level of
synchronization is from the point of view of the client who issued
the LAYOUTCOMMIT. The updated state on the metadata server need only
reflect the state as of the client's last operation (previous to the
LAYOUTCOMMIT), it need not reflect a globally synchronized state
(e.g., other clients may be performing, or may have performed I/O
since the client's last operation and the LAYOUTCOMMIT).
The management protocol is free to synchronize the attributes 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 comprise that file as of the issuing client's last
operation. Thus, a client that queries the size of a file between a
WRITE to a storage device and the LAYOUTCOMMIT may observe a size
that does not reflects the actual data written.
3.5.1 LAYOUTCOMMIT and mtime/atime/change
The change attribute and the modify/access times may be updated, by
the server, at LAYOUTCOMMIT time; since for some layout types, the
change attribute (and atime/mtime) can not be updated by the
appropriate I/O operation performed at a storage device. The
arguments to LAYOUTCOMMIT allow the client to provide suggested
access and modify time values to the server. Again, depending upon
the layout type, these client provided values may or may not be used.
The server should sanity check the client provided values before they
are used. For example, the server should ensure that time does not
flow backwards. The client always has the option to set these
attributes (according to the NFSv4 specification) through an explicit
SETATTR operation.
As mentioned, for some layout protocols the change attribute and
mtime/atime may be updated at or after the time the I/O occurred
(e.g., if the storage device is able to communicate these attributes
to the metadata server). If, upon receiving a LAYOUTCOMMIT, the
server implementation is able to determine that the file did not
change since the last time the change attribute was updated (e.g., no
WRITEs or over-writes occurred), the implementation need not update
the change attribute (file-based protocols may have enough state to
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make this determination or may update the change attribute upon each
file modification). This also applies for mtime and atime; if the
server implementation is able to determine that the file has not been
modified since the last mtime update, the server need not update
mtime at LAYOUTCOMMIT time. Once LAYOUTCOMMIT completes, the new
change attribute and mtime/atime should be visible if that file was
modified since the latest previous LAYOUTCOMMIT or LAYOUTGET.
3.5.2 LAYOUTCOMMIT and size
The file's size may be updated at LAYOUTCOMMIT time as well. The
LAYOUTCOMMIT operation contains an argument that indicates the last
byte offset to which the client wrote ("lastbytewritten"). Note: for
this offset to be viewed as a file size it must be incremented by one
byte (a write to offset 0 would map into a file size of 1, but the
last byte written is 0). The metadata server may do one of the
following:
1. It may update the file's size based on the last byte written
offset. However, to the extent possible, the metadata server
should sanity check any value to which the file's size is going
to be set. E.g., it must not truncate the file based on the
client presenting a smaller last byte written offset than the
file's current size.
2. If it has sufficient other knowledge of file size (e.g., by
querying the storage devices through the management protocol), it
may ignore the client provided argument and use the query-derived
value.
3. It may use the last byte written offset as a hint, subject to
correction when other information is available as above.
The method chosen to update the file's size will depend on the
storage device's and/or the management protocol's implementation.
For example, if the storage devices are block devices with no
knowledge of file size, the metadata server must rely on the client
to set the size appropriately. A new size flag and length are also
returned in the results of a LAYOUTCOMMIT. This union indicates
whether a new size was set, and to what length it was set. If a new
size is set as a result of LAYOUTCOMMIT, then the metadata server
must reply with the new size. As well, if the size is updated, the
metadata server in conjunction with the management protocol SHOULD
ensure that the new size is reflected by the storage devices
immediately upon return of the LAYOUTCOMMIT operation; e.g., a READ
up to the new file size should succeed on the storage devices
(assuming no intervening truncations). Again, if the client wants to
explicitly grow or shrink a file, SETATTR must be used.
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Since client layout holders may be unaware of changes made to the
file's size (through LAYOUTCOMMIT or SETATTR) by other clients, an
additional callback/notification has been added for pNFS.
CB_SIZECHANGED is a notification that the metadata server sends to
layout holders to notify them of a change in file size. This is
preferred over issuing CB_LAYOUTRECALL to each of the layout holders.
3.5.3 LAYOUTCOMMIT and layoutupdate
The LAYOUTCOMMIT operation contains a "layoutupdate" argument. This
argument is a layout type specific structure. The structure can be
used to pass arbitrary layout type specific information from the
client to the metadata server at LAYOUTCOMMIT time. For example, if
using a block layout, the client can indicate to the metadata server
which reserved or allocated blocks it used and which it did not. The
"layoutupdate" structure need not be the same structure as the layout
returned by LAYOUTGET. The structure is defined by the layout type
and is opaque to LAYOUTCOMMIT.
3.6 Crash Recovery
Crash recovery is complicated due to the distributed nature of the
pNFS protocol. In general, crash recovery for layouts is similar to
crash recovery for delegations in the base NFSv4 protocol. However,
the client's ability to perform I/O without contacting the metadata
server introduces subtleties that must be handled correctly if file
system corruption is to be avoided.
3.6.1 Leases
The layout lease period plays a critical role in crash recovery.
Depending on the capabilities of the storage protocol, it is crucial
that the client is able to maintain an accurate layout lease timer to
ensure that I/Os are not issued to storage devices after expiration
of the layout lease period. In order for the client to do so, it
must know which operations renew a lease.
3.6.1.1 Lease Renewal
The current NFSv4 specification allows for implicit lease renewals to
occur upon receiving an I/O. However, due to the distributed pNFS
architecture, implicit lease renewals are limited to operations
performed at the metadata server (including I/O performed through the
metadata server). So, a client must not assume that READ and WRITE
I/O to storage devices implicitly renew lease state.
If sessions are required for pNFS, as has been suggested, then the
SEQUENCE operation is to be used to explicitly renew leases. It is
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proposed that the SEQUENCE operation be extended to return all the
specific information that RENEW does, but not as an error as RENEW
returns it. Since, when using session, beginning each compound with
the SEQUENCE op allows renews to be performed without an additional
operation and without an additional request. Again, the client must
not rely on any operation to the storage devices to renew a lease.
Using the SEQUENCE operation for renewals, simplifies the client's
perception of lease renewal.
3.6.1.2 Client Lease Timer
Depending on the storage protocol and layout type in use, it may be
crucial that the client not issue I/Os to storage devices if the
corresponding layout's lease has expired. Doing so may lead to file
system corruption if the layout has been given out and used by
another client. In order to prevent this, the client must maintain
an accurate lease timer for all layouts held. RFC3530 has the
following to say regarding the maintenance of a client lease timer:
...the client must track operations which will renew the lease
period. Using the time that each such request was sent and the
time that the corresponding reply was received, the client should
bound the time that the corresponding renewal could have occurred
on the server and thus determine if it is possible that a lease
period expiration could have occurred.
To be conservative, the client should start its lease timer based on
the time that the it issued the operation to the metadata server,
rather than based on the time of the response.
It is also necessary to take propagation delay into account when
requesting a renewal of the lease:
...the client should subtract it from lease times (e.g., if the
client estimates the one-way propagation delay as 200 msec, then
it can assume that the lease is already 200 msec old when it gets
it). In addition, it will take another 200 msec to get a response
back to the server. So the client must send a lock renewal or
write data back to the server 400 msec before the lease would
expire.
Thus, the client must be aware of the one-way propagation delay and
should issue renewals well in advance of lease expiration. Clients,
to the extent possible, should try not to issue I/Os that may extend
past the lease expiration time period. However, since this is not
always possible, the storage protocol must be able to protect against
the effects of inflight I/Os, as is discussed later.
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3.6.2 Client Recovery
Client recovery for layouts works in much the same way as NFSv4
client recovery works for other lock/delegation state. When an NFSv4
client reboots, it will lose all information about the layouts that
it previously owned. There are two methods by which the server can
reclaim these resources and allow otherwise conflicting layouts to be
provided to other clients.
The first is through the expiry of the client's lease. If the client
recovery time is longer than the lease period, the client's lease
will expire and the server will know that state may be released. for
layouts the server may release the state immediately upon lease
expiry or it may allow the layout to persist awaiting possible lease
revival, as long as there are no conflicting requests.
On the other hand, the client may recover in less time than it takes
for the lease period to expire. In such a case, the client will
contact the server through the standard SETCLIENTID protocol. The
server will find that the client's id matches the id of the previous
client invocation, but that the verifier is different. The server
uses this as a signal to release all the state associated with the
client's previous invocation.
3.6.3 Metadata Server Recovery
The server recovery case is slightly more complex. In general, the
recovery process again follows the standard NFSv4 recovery model: the
client will discover that the metadata server has rebooted when it
receives an unexpected STALE_STATEID or STALE_CLIENTID reply from the
server; it will then proceed to try to reclaim its previous
delegations during the server's recovery grace period. However,
layouts are not reclaimable in the same sense as data delegations;
there is no reclaim bit, thus no guarantee of continuity between the
previous and new layout.
[OPEN ISSUE: there is no reclaim bit for getting a layout. Currently
layouts obtained through LAYOUTGET make no guarantee of continuity in
the case of reclaiming an old layout. Recall, a layout is not
required to perform I/O. I/O can always be performed through the
metadata server. If a reclaim bit existed a block layout type might
be happier knowing it got the layout back with the assurance of
continuity. However, this would require the metadata server trusting
the client in telling it the exact layout it had (i.e., the full
block-list); maybe too much trust?]
If the client has dirty data that it needs to write out, or an
outstanding LAYOUTCOMMIT, the client should try to obtain a new
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layout covering the byte range covering the previous layout.
However, the client might not not get the same layout it had. The
range might be different or it might get the same range but the
content of the layout might be different. For example, if using a
block-based layout, the blocks provisionally assigned by the layout
might be different, in which case the client will have to write the
corresponding blocks again (and it might decide in the interests of
simplicity to always write them again). Alternatively, the client
might be unable to obtain a new layout and thus, must write the data
using normal NFSv4 through the metadata server.
There is an important safety concern associated with layouts that
does not come into play in the standard NFSv4 case. If a standard
NFSv4 client makes use of a stale delegation, while reading, the
consequence could be to deliver stale data to an application. If
writing, using a stale delegation or a stale state stateid for an
open or lock would result in the rejection of the client's write with
the appropriate stale stateid error.
However, the pNFS layout enables the client to directly access the
file system storage---if this access is not properly managed by the
NFSv4 server the client can potentially corrupt the file system data
or metadata. Thus, it is vitally important that the client discover
that the metadata server has rebooted, and that the client stops
using stale layouts before the metadata server gives them away to
other clients. To ensure this, the client must be implemented so
that layouts are never used to access the storage after the client's
lease timer has expired. It is crucial that clients have precise
knowledge of the lease periods of their layouts. For specific
details on lease renewal and client lease timers, see Section 3.6.1.
The prohibition on using stale layouts applies to all layout related
accesses, especially the flushing of dirty data to the storage
devices. If the client's lease timer expires because the client
could not contact the server for any reason, the client MUST
immediately stop using the layout until the server can be contacted
and the layout can be officially recovered or reclaimed. However,
this is only part of the solution. It is also necessary to deal with
the consequences of I/Os already in flight.
The issue of the effects of I/Os started before lease expiration and
possibly continuing through lease expiration is the responsibility of
the data storage protocol and as such is layout type specific. There
are two approaches the data storage protocol can take. The protocol
may adopt a global solution which prevents all I/Os from being
executed after the lease expiration (and thus is safe against a
client who issues I/Os after lease expiration). This is the
preferred solution and the solution used by NFSv4 file based layouts
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(see Section 5.6). Alternatively, the storage protocol may rely on
proper client operation and only deal with the effects of lingering
I/Os. These solutions may impact the client layout-driver, the
metadata server layout-driver, and the management protocol.
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,
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
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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.]
[Requiring strict equivalence to NFSv4 security mechanisms is the
wrong approach. Will need to lay down a set of statements that each
protocol has to make starting with access check location/properties.]
4.1 File Layout Security
A NFSv4 file layout type is defined in Section 5; see Section 5.7 for
additional security considerations and details. In summary, the
NFSv4 file layout type requires that all I/O access checks MUST be
performed by the storage devices, as defined by the NFSv4
specification. If another file layout type is being used, additional
access checks may be required. But in all cases, the access control
performed by the storage devices must be at least as strict as that
specified by the NFSv4 protocol.
4.2 Object Layout Security
The object storage protocol relies on a cryptographically secure
capability to control accesses at the object storage devices.
Capabilities are generated by the metadata server, returned to the
client, and used by the client as described below to authenticate
their requests to the Object Storage Device (OSD). Capabilities
therefore achieve the required access and open mode checking. They
allow the file server to define and check a policy (e.g., open mode)
and the OSD to check and enforce that policy without knowing the
details (e.g., user IDs and ACLs). Since capabilities are tied to
layouts, and since they are used to enforce access control, the
server should recall layouts and revoke capabilities when the file
ACL or mode changes in order to signal the clients.
Each capability is specific to a particular object, an operation on
that object, a byte range w/in the object, and has an explicit
expiration time. The capabilities are signed with a secret key that
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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:
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
As typically used, block protocols rely on clients to enforce file
access checks since the storage devices are generally unaware of the
files they are storing (and in particular are unaware of which blocks
belongs to which file). In such environments, the physical addresses
of blocks are exported to pNFS clients via layouts. An alternative
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method of block protocol use is for the storage devices to export
virtualized block addresses, which do reflect the files to which
blocks belong. These virtual block addresses are exported to pNFS
clients via layouts. This allows the storage device to make
appropriate access checks, while mapping virtual block addresses to
physical block addresses.
In environments where access control is important and client-only
access checks provide insufficient assurance of access control
enforcement (e.g., there is concern about a malicious of
malfunctioning client skipping the access checks) and where physical
block addresses are exported to clients, the storage devices will
generally be unable to compensate for these client deficiencies.
In such threat environments, block protocols SHOULD NOT be used with
pNFS, unless the data server is able to implement the appropriate
access checks, via use of virtualized block addresses, or other
means. NFSv4 without pNFS or pNFS with a different type of storage
protocol would be a more suitable means to access files in such
environments. Storage-device/protocol-specific methods (e.g. LUN
masking/mapping) may be available to prevent malicious or high-risk
clients from directly accessing storage devices.
5. 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 type describes a method for striping data across
multiple devices. The data for each stripe unit is stored within an
NFSv4 file located on a particular storage device. The structures
used to describe the stripe layout are as follows:
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enum stripetype4 {
STRIPE_SPARSE = 1,
STRIPE_DENSE = 2
};
struct nfsv4_file_layout { /* Per device info */
pnfs_deviceid4 dev_id;
nfs_fh4 fh;
};
struct nfsv4_file_layouttype4 {
stripetype4 stripe_type;
uint64_t stripe_unit;
length4 file_size;
nfsv4_file_layout dev_list<>;
};
The file layout specifies an ordered array of (deviceID, filehandle)
tuples, as well as the stripe size, type of stripe layout (discussed
a little later), and the file's current size (current as of LAYOUTGET
time). The filehandle, "fh", identifies the file on a storage device
identified by "dev_id", that holds a particular stripe of the file.
The stripe width is determined by the stripe unit size multiplied by
the number of devices in the dev_list. The stripe held by (dev_id,
fh) is determined by that tuples position within the device list,
"dev_list". For example, consider a dev_list consisting of the
following (dev_id, fh) pairs:
<(1,0x12), (2,0x13), (1,0x15)> and stripe_unit = 32KB
The stripe width is 32KB * 3 devices = 96KB. The first entry
specifies that on device 1 in the data file with filehandle 0x12
holds the first 32KB of data (and every 32KB stripe beginning where
the file's offset % 96KB == 0).
Notice, devices and filehandles may be repeated multiple times within
the device list array (as is shown where storage device 1 holds both
the first and third stripe of data). Data is striped across the
devices in the order listed in the device list array in increments of
the stripe size. A data file stored on a storage device MUST map to
a single file as defined by the metadata server; i.e., data from two
files as viewed by the metadata server MUST NOT be stored within the
same data file on any storage device.
The "stripe_type" field specifies how the data is laid out within the
data file on a storage device. It allows for two different data
layouts: sparse and dense or packed. The stripe type determines the
calculation that must be made to map the file's offset (as visible to
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the client) to the offset within the data file located on the storage
device.
5.1.1 Sparse and Dense Storage Device Data Layouts
The stripe_type field allows for two storage device data file
representations. Example sparse and dense storage device data
layouts are illustrated below:
Sparse file-layout (stripe_unit = 4KB)
------------------
Is represented by the following file layout on the storage devices:
Offset ID:0 ID:1 ID:2
0 +--+ +--+ +--+ +--+ indicates a
|//| | | | | |//| stripe that
4KB +--+ +--+ +--+ +--+ contains data
| | |//| | |
8KB +--+ +--+ +--+
| | | | |//|
12KB +--+ +--+ +--+
|//| | | | |
16KB +--+ +--+ +--+
| | |//| | |
+--+ +--+ +--+
The sparse file-layout has holes for the byte ranges not exported by
that storage device. This allows clients to access data using the
real offset into the file, regardless of the storage device's
position within the stripe. However, if a client writes to one of
the holes (e.g., offset 4-12KB on device 1), then an error MUST be
returned by the storage device. This requires that the storage
device have knowledge of the layout for each file.
When using a sparse layout, the offset into the storage device data
file is the same as the offset into the main file.
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Dense/packed file-layout (stripe_unit = 4KB)
------------------------
Is represented by the following file layout on the storage devices:
Offset ID:0 ID:1 ID:2
0 +--+ +--+ +--+
|//| |//| |//|
4KB +--+ +--+ +--+
|//| |//| |//|
8KB +--+ +--+ +--+
|//| |//| |//|
12KB +--+ +--+ +--+
|//| |//| |//|
16KB +--+ +--+ +--+
|//| |//| |//|
+--+ +--+ +--+
The dense or packed file-layout does not leave holes on the storage
devices. Each stripe unit is spread across the storage devices. As
such, the storage devices need not know the file's layout since the
client is allowed to write to any offset.
The calculation to determine the byte offset within the data file for
dense storage device layouts is:
stripe_width = stripe_unit * N; where N = |dev_list|
dev_offset = floor(file_offset / stripe_width) * stripe_unit +
file_offset % stripe_unit
Regardless of the storage device data file layout, the calculation to
determine the index into the device array is the same:
dev_idx = floor(file_offset / stripe_unit) mod N
5.1.2 Operations Issued to Storage Devices
Clients MUST use the filehandle described within the layout when
accessing data on the storage devices. When using the layout's
filehandle, the client MUST only issue READ, WRITE, PUTFH, COMMIT,
and NULL operations to the storage device associated with that
filehandle. If a client issues an operation other than those
specified above, using the filehandle and storage device listed in
the client's layout, that storage device SHOULD return an error to
the client. The client MUST follow the instruction implied by the
layout (which filehandles to use on which devices). As described in
Section 3.2, a client MUST NOT issue I/Os to storage devices for
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which it does not hold a valid layout. The storage devices may
reject such requests.
[OPEN ISSUE: Should SHOULD be should]
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 3.5.2, 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.
5.2 Global Stateid Requirements
Note, there are no stateids returned embedded within the layout. The
client MUST use the stateid representing open or lock state as
returned by an earlier metadata operation (e.g., OPEN, LOCK), or a
special stateid to perform I/O on the data-servers (as in regular
NFSv4). Special stateid usage for I/O is subject to the NFSv4
protocol specification. The stateid used for I/O MUST have the same
effect and be subject to the same validation on storage device as it
would if the I/O was being performed on the metadata server itself
(in the absence of pNFS). This has the implication that stateids are
globally valid on both the metadata and storage devices. This
requires the metadata server to propagate changes in lock and open
state to the data-servers, so that the data-servers can validate I/O
accesses. This is discussed further in Section 5.4.
5.3 The Layout Iomode
The layout iomode need not used by the metadata server when servicing
NFSv4 file-based layouts, although in some circumstances it may be
useful to use. For example, if the server implementation supports
reading from read-only replicas or mirrors, it would be useful for
the server to return a layout enabling the client to do so. As such,
the client should set the iomode based on its intent to read or write
the data. The client may default to an iomode of READ/WRITE. 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.
[OPEN ISSUE: Again, requiring storage devices to validate that
clients hold valid layouts, requires propagating layouts to storage
devices. This is not possible if using vanilla NFSv4 storage
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devices.]
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
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. Immediate propagation refers to
the synchronous propagation of state from the metadata server to the
storage device(s); the propagation must be complete before returning
to the client.
5.4.1 Lock State Propagation
Mandatory locks MUST be made effective at the storage devices before
the request that establishes them returns to the caller. Thus,
mandatory lock state MUST be synchronously propagated to the storage
devices. On the other hand, since advisory lock state is not used
for checking I/O accesses at the storage devices, there is no
semantic reason for propagating advisory lock state to the storage
devices. However, since all lock, unlock, open downgrades and
upgrades affect the sequence ID stored within the stateid, the
stateid changes which may cause difficulty if this state is not
propagated. Thus, when a client uses a stateid on a storage device
for I/O with a newer sequence number than the one the storage device
has, the storage device should query the metadata server and get any
pending updates to that stateid. This allows stateid sequence number
changes to be propagated lazily, on-demand.
[OPEN ISSUE: How does the requirement of sessions affect the
propagation of stateid sequence numbers?]
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 stateids returned by open are sufficient and
eliminate overhead for this kind of state propagation.
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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.
5.4.3 File Attributes
Since the SETATTR operation has the ability to modify state that is
visible on both the metadata and storage devices (e.g., the size),
care must be taken to ensure that the resultant state (across the set
of storage devices) is consistent; especially when truncating or
growing the file.
As described earlier, the LAYOUTCOMMIT operation is used to ensure
that the metadata is synced with changes made to the storage devices.
For the file-based protocol, it is necessary to re-sync state such as
the size attribute, and the setting of mtime/atime. See Section 3.5
for a full description of the semantics regarding LAYOUTCOMMIT and
attribute synchronization. It should be noted, that by using a file-
based layout type, it is possible to synchronize this state before
LAYOUTCOMMIT occurs. For example, the management protocol can be
used to query the attributes present on the storage devices.
Any changes to file attributes that control authorization (or access)
as reflected by ACCESS calls or READs and WRITEs on the metadata
server, MUST be propagated to the storage devices for enforcement on
READ and WRITE I/O calls. If the changes made on the metadata server
result in more restrictive access permissions for any user, those
changes MUST be propagated to the storage devices synchronously.
Recall that the NFSv4 protocol [2] specifies that:
...since the NFS version 4 protocol does not impose any
requirement that READs and WRITEs issued for an open file have the
same credentials as the OPEN itself, the server still must do
appropriate access checking on the READs and WRITEs themselves.
This also includes changes to ACLs. The propagation of access right
changes due to changes in ACLs may be asynchronous only if the server
implementation is able to determine that the updated ACL is not more
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restrictive for any user specified in the old ACL. Due to the
relative infrequency of ACL updates, it is suggested that all changes
be propagated synchronously.
[OPEN ISSUE: it has been suggested that the NFSv4 specification is in
error with regard to allowing principles other than those used for
OPEN to be used for file I/O. It has been suggested that it should be
fixed here by pNFS].
5.5 Extending file size
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
(size == 0) and writes to byte 128KB; the client then seeks to the
beginning of the file and reads byte 100. The client should receive
0s back as a result of the read. However, if the read falls on a
different storage device to the client's original write, the storage
device servicing the READ may still believe that the file's size is
at 0 and return no data (with the EOF flag set). The storage device
can only return 0s if it knows that the file's size has been
extended. This would require the immediate propagation of the file's
size to all storage devices, which is potentially very costly,
instead, another approach as outlined below.
First, the file's size is returned within the layout by LAYOUTGET.
This size must reflect the latest size at the metadata server as set
by the most recent of either the last LAYOUTCOMMIT or SETATTR;
however, it may be more recent. Second, if a client performs a read
that is returned short (i.e., is fully within the file's size, but
the storage device indicates EOF and returns partial or no data), the
client must assume that it is a hole and substitute 0s for the data
not read (up until its known local file size). If a client extends
the file, it must update its local file size. Third, if the metadata
server receives a SETATTR of the size or a LAYOUTCOMMIT that alters
the file's size, the metadata server MUST send out CB_SIZECHANGED
messages with the new size to clients holding layouts (it need not
send a notification to the client that performed the operation that
resulted in the size changing). Upon reception of the CB_SIZECHANGED
notification, clients must update their local size for that file. As
well, if a new file size is returned as a result to LAYOUTCOMMIT, the
client must update their local file size.
5.5.2 LAYOUTCOMMIT and EOF
Another complication can arise due to EOF. If a file has been grown
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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.
5.6 Crash Recovery Considerations
As described in Section 3.6, the layout type specific storage
protocol is responsible for handling the effects of I/Os started
before lease expiration, extending through lease expiration. The
NFSv4 file layout type prevents all I/Os from being executed after
lease expiration, without relying on a precise client lease timer and
without requiring storage devices to maintain lease timers.
It works as follows. In the presence of sessions, each compound
begins with a SEQUENCE operation that contains the "clientID". On
the storage device, the clientID can be used to validate that the
client has a valid layout for the I/O being performed, if it does
not, the I/O is rejected. Before the metadata server takes any
action to invalidate a layout given out by a previous instance, it
must make sure that all layouts from that previous instance are
invalidated at the storage devices. Note: it is sufficient to
invalidate the stateids associated with the layout only if special
stateids are not being used for I/O at the storage devices, otherwise
the layout itself must be invalidated.
This means that a metadata server may not restripe a file until it
has contacted all of the storage devices to invalidate the layouts
from the previous instance nor may it give out locks that conflict
with locks embodied by the stateids associated with any layout from
the previous instance without either doing a specific invalidation
(as it would have to do anyway) or doing a global storage device
invalidation.
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5.7 Security Considerations
The NFSv4 file layout type MUST adhere to the security considerations
outlined in Section 4. More specifically, storage devices must make
all of the required access checks on each READ or WRITE I/O as
determined by the NFSv4 protocol [2]. This impacts the management
protocol and the propagation of state from the metadata server to the
storage devices; see Section 5.4 for more details.
5.8 Alternate Approaches
Two alternate approaches exist for file-based layouts and the method
used by clients to obtain stateids used for I/O. Both approaches
embed stateids within the layout.
However, before examining these approaches it is important to
understand the distinction between clients and owners. Delegations
belong to clients, while locks (record and share reservations) are
held by owners (who in turn belong to a specific client). As such,
delegations can only protect against inter-client conflicts, not
intra-client conflicts. Layouts are held by clients and SHOULD NOT
be associated with state held by owners. Therefore, if stateids used
for data access are embedded within a layout, these stateids can only
act as delegation stateids, protecting against inter-client
conflicts; stateids pertaining to an owner can not be embedded within
the layout. This has the implication that the client MUST arbitrate
among all intra-client conflicts (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.
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
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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. As well, a private range of layout types should exist and
also be well defined. This would allow custom installations to
introduce new layout types.
[OPEN ISSUE: It must be decided whether IANA should control the
namespace of layout types, or whether each new layout type must go
through the specification process (probably as a minor version
extension)]
The LAYOUT_NFSV4_FILES enumeration specifies that the NFSv4 file
layout type is to be used.
6.2 pnfs_deviceid4
typedef 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. A client must not assume
that device IDs are valid across metadata server reboots. The device
ID is qualified by the layout type. This allows different layout
drivers to generate device IDs without the need for co-ordination.
See Section 3.1.1 for more details.
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6.3 pnfs_devaddr4
union pnfs_devaddr4 switch (pnfs_layouttype4 layout_type) {
case LAYOUT_NFSV4_FILES:
string r_netid<>; /* network ID */
string r_addr<>; /* universal address */
default:
opaque devaddr<>; /* For other layouts */
};
The device address is used to set up a communication channel with the
storage device. Different layout types will require different types
of structures to define how they communicate with storage devices.
The union is switched on the layout type. Currently, the only device
address defined is that for the NFSv4 file layout, which identifies a
storage device by network IP address and port number. This is
sufficient for the clients to communicate with the NFSv4 storage
devices, and may also be sufficient for object-based storage drivers
to communicate with OSDs.
6.4 pnfs_devlist_item4
struct pnfs_devlist_item4 {
pnfs_deviceid4 id;
pnfs_deviceaddr4 addr;
};
An array of these values is returned by the GETDEVICELIST operation.
They define the set of devices associated with a file system.
6.5 pnfs_layout4
union pnfs_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_layoutiomode4 iomode;
pnfs_layouttypes4 layout;
};
The pnfs_layout4 structure defines a layout for a file. The
pnfs_layouttypes4 union contains the portion of the layout specific
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to the layout type. Currently, only the NFSv4 file layout type is
defined; see Section 5.1 for its definition. Since layouts are sub-
dividable, the offset and length (together with the file's
filehandle, the clientid, iomode, and layout type), identifies the
layout.
[OPEN ISSUE: it has been suggested that the layout type include a
generic striping layer, as defined in Section 5.1. This has not yet
been done.]
6.6 pnfs_layoutupdate4
union pnfs_layoutupdate4 switch (pnfs_layouttype4 layout_type) {
case LAYOUT_NFSV4_FILES:
void;
default:
opaque layout_data<>;
};
The pnfs_layoutupdate4 structure is used by the client to return
'updated' layout information to the metadata server at LAYOUTCOMMIT
time. This provides a channel to pass layout type specific
information back to the metadata server. E.g., for block layout
types this could include the list of reserved blocks that were
written. The contents of the structure are determined by the layout
type and are defined in their context.
6.7 pnfs_layoutiomode4
enum pnfs_layoutiomode4 {
LAYOUTIOMODE_READ = 1,
LAYOUTIOMODE_RW = 2,
LAYOUTIOMODE_ANY = 3,
};
The iomode specifies whether the client intends to read or write
(with the possibility of reading) the data represented by the layout.
The ANY iomode MUST NOT be used for LAYOUTGET, however, it can be
used for LAYOUTRETURN and LAYOUTRECALL. The ANY iomode specifies
that layouts pertaining to both READ and RW are being returned or
recalled, respectively. The metadata server's use of the iomode may
depend on the layout type being used. The storage devices may
validate I/O accesses against the iomode (and reject invalid
accesses).
7. pNFS File Attributes
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7.1 pnfs_layouttype4<> FS_LAYOUT_TYPES
This attribute applies to a file system and indicates what layout
types are supported by the file system. We expect this attribute to
be queried when a client encounters a new fsid. This attribute is
used by the client to determine if it has applicable layout drivers.
7.2 pnfs_layouttype4<> FILE_LAYOUT_TYPE
This attribute indicates the particular layout type(s) used for a
file. This is for informational purposes only. The client needs to
use the LAYOUTGET operation in order to get enough information (e.g.,
specific device information) in order to perform I/O.
7.3 pnfs_layouttypes4 FILE_LAYOUT_HINT
This attribute may be set on newly created files to influence the
metadata server's choice for the file's layout. It is suggested that
this attribute is set as one of the initial attributes within the
OPEN call. The metadata server may ignore this attribute. This
attribute is a sub-set of the layout 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.
7.4 uint32_t FS_LAYOUT_PREFERRED_BLOCKSIZE
This attribute is a file system wide attribute and indicates the
preferred block size for direct storage device access.
7.5 uint32_t FS_LAYOUT_PREFERRED_ALIGNMENT
This attribute is a file system wide attribute and indicates the
preferred alignment for direct storage device access.
8. pNFS Error Definitions
NFS4ERR_BADLAYOUT Layout specified is invalid.
NFS4ERR_BADIOMODE Layout iomode is invalid.
NFS4ERR_LAYOUTUNAVAILABLE Layouts are not available for the file or
its containing file system.
NFS4ERR_LAYOUTTRYLATER Layouts are temporarily unavailable for the
file, client should retry later.
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NFS4ERR_UNKNOWN_LAYOUTTYPE Layout type is unknown.
9. pNFS Operations
9.1 LAYOUTGET - Get Layout Information
SYNOPSIS
(cfh), clientid, layout_type, iomode, offset, length,
maxcount -> layout
ARGUMENT
struct LAYOUTGET4args {
/* CURRENT_FH: file */
clientid4 clientid;
pnfs_layouttype4 layout_type;
pnfs_layoutiomode4 iomode;
offset4 offset;
length4 length;
count4 maxcount;
};
RESULT
struct LAYOUTGET4resok {
pnfs_layout4 layout;
};
union LAYOUTGET4res switch (nfsstat4 status) {
case NFS4_OK:
LAYOUTGET4resok resok4;
default:
void;
};
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, byte range (offset,
length pair), and iomode. The use of the iomode depends upon the
layout type, but should reflect the client's data access intent.
The LAYOUTGET operation returns layout information for the specified
byte range. To get a layout from a specific offset through the end-
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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 "maxcount" field specifies the
maximum layout size (in bytes) that the client can handle. If the
layout exceeds the size specified by maxcount, the metadata server
will return the NFS4ERR_TOOSMALL error.
As well, the metadata server may adjust the range of the returned
layout 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 metadata server may also return a layout with an iomode other
than that requested by the client. If it does so, it must ensure
that the iomode is more permissive than the iomode requested. E.g.,
this allows an implementation to upgrade read-only requests to read/
write requests at its discretion (within the limits of the layout
type specific protocol). An iomode of either LAYOUTIOMODE_READ or
LAYOUTIOMODE_RW must be returned.
The format of the returned layout is specific to the underlying file
system. Layout types other than the NFSv4 file layout type should be
specified outside of this document.
If layouts are not supported for the requested file or its containing
file system the server SHOULD return NFS4ERR_LAYOUTUNAVAILABLE. If
the layout type is not supported, the metadata server should return
NFS4ERR_UNKNOWN_LAYOUTTYPE. If layouts are supported but no layout
matches the client provided layout identification, the server should
return NFS4ERR_BADLAYOUT. If an invalid iomode is specified, or an
iomode of LAYOUTIOMODE_ANY is specified, the server should return
NFS4ERR_BADIOMODE.
If the layout for the file is unavailable due to transient
conditions, e.g. file sharing prohibits layouts, the server should
return NFS4ERR_LAYOUTTRYLATER.
If the layout conflicts with a mandatory byte range lock held on the
file, and if the storage devices have no method of enforcing
mandatory locks, other than through the restriction of layouts, the
metadata server should return NFS4ERR_LOCKED.
On success, the current filehandle retains its value.
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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.
[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_BADLAYOUT
NFS4ERR_BADIOMODE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_LAYOUTUNAVAILABLE
NFS4ERR_LAYOUTTRYLATER
NFS4ERR_LOCKED
NFS4ERR_NOFILEHANDLE
NFS4ERR_NOTSUPP
NFS4ERR_STALE
NFS4ERR_STALE_CLIENTID
NFS4ERR_TOOSMALL
NFS4ERR_UNKNOWN_LAYOUTTYPE
9.2 LAYOUTCOMMIT - Commit writes made using a layout
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SYNOPSIS
(cfh), clientid, offset, length, lastbytewritten,
time_modify, time_access, layoutupdate -> newsize
ARGUMENT
union newtime4 switch (bool timechanged) {
case TRUE:
nfstime4 time;
case FALSE:
void;
};
union newsize4 switch (bool sizechanged) {
case TRUE:
length4 size;
case FALSE:
void;
};
struct LAYOUTCOMMIT4args {
/* CURRENT_FH: file */
clientid4 clientid;
offset4 offset;
length4 length;
length4 lastbytewritten;
newtime4 time_modify;
newtime4 time_access;
pnfs_layoutupdate4 layoutupdate;
};
RESULT
struct LAYOUTCOMMIT4resok {
newsize4 newsize;
};
union LAYOUTCOMMIT4res switch (nfsstat4 status) {
case NFS4_OK:
LAYOUTCOMMIT4resok resok4;
default:
void;
};
DESCRIPTION
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Commits changes in the layout portion represented by the current
filehandle, clientid, and byte range. Since layouts are sub-
dividable, a smaller portion of a layout, retrieved via LAYOUTGET,
may be committed. The region being committed is specified through
the byte range (length and offset). Note: the "layoutupdate"
structure does not include the length and offset, as they are already
specified in the arguments.
The LAYOUTCOMMIT operation indicates that the client has completed
writes using a layout obtained by a previous LAYOUTGET. The client
may have only written a subset of the data range it previously
requested. LAYOUTCOMMIT allows it to commit or discard provisionally
allocated space and to update the server with a new end of file. The
layout referenced by LAYOUTCOMMIT is still valid after the operation
completes and can be continued to be referenced by the clientid,
filehandle, byte range, and layout type.
The "lastbytewritten" field specifies the offset of the last byte
written by the client previous to the LAYOUTCOMMIT. Note: this value
is never equal to the file's size (at most it is 1 byte less than the
file's size). The metadata server may use this information to
determine whether the file's size needs to be updated. If the
metadata server updates the file's size as the result of the
LAYOUTCOMMIT operation, it must return the new size as part of the
results.
The "time_modify" and "time_access" fields allow the client to
suggest times it would like the metadata server to set. The metadata
server may use these time values or it may use the time of the
LAYOUTCOMMIT operation to set these time values. If the metadata
server uses the client provided times, it should sanity check the
values (e.g., to ensure time does not flow backwards). If the client
wants to force the metadata server to set an exact time, the client
should use a SETATTR operation in a compound right after
LAYOUTCOMMIT. See Section 3.5 for more details.
The "layoutupdate" argument to LAYOUTCOMMIT provides a mechanism for
a client to provide layout specific updates to the metadata server.
For example, the layout update can describe what regions of the
original layout have been used and what regions can be deallocated.
There is no NFSv4 file layout specific layoutupdate structure.
The layout information is more verbose for block devices than for
objects and files because the latter hide the details of block
allocation behind their storage protocols. At the minimum, the
client needs to communicate changes to the end of file location back
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
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LAYOUTCOMMIT). For blocks, it needs to specify precisely which
blocks have been used.
If the layout identified in the arguments does not exist, the error
NFS4ERR_BADLAYOUT is returned. The layout being committed may also
be rejected if it does not correspond to an existing layout with an
iomode of RW.
On success, the current filehandle retains its value.
[OPEN ISSUE: is it good enough to allow the metadata server to update
the change attribute, or should the client be able to direct the
metadata server to update it.]
ERRORS
NFS4ERR_BADLAYOUT
NFS4ERR_BADIOMODE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_NOFILEHANDLE
NFS4ERR_STALE
NFS4ERR_STALE_CLIENTID
NFS4ERR_UNKNOWN_LAYOUTTYPE
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9.3 LAYOUTRETURN - Release Layout Information
SYNOPSIS
(cfh), clientid, offset, length, iomode, layout_type -> -
ARGUMENT
struct LAYOUTRETURN4args {
/* CURRENT_FH: file */
clientid4 clientid;
offset4 offset;
length4 length;
pnfs_layoutiomode4 iomode;
pnfs_layouttype4 layout_type;
};
RESULT
struct LAYOUTRETURN4res {
nfsstat4 status;
};
DESCRIPTION
Returns the layout represented by the current filehandle, clientid,
byte range, iomode, and layout type. After this call, the client
MUST NOT use the layout and the associated storage protocol to access
the file data. The layout being returned may be a subdivision of a
layout previously fetched through LAYOUTGET. If the length is all
1s, the layout covers the range from offset to EOF. An iomode of ANY
specifies that all layouts that match the other arguments to
LAYOUTRETURN (i.e., clientid, byte range, and type) are being
returned.
Layouts may be returned when recalled or voluntarily (i.e., before
the server has recalled them). In either case the client must
properly propagate state changed under the context of the layout to
storage or to the server before returning the layout.
If a client fails to return a layout in a timely manner, then the
metadata server should use its 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.
If the layout identified in the arguments does not exist, the error
NFS4ERR_BADLAYOUT is returned. If a layout exists, but the iomode
does not match, NFS4ERR_BADIOMODE is returned.
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On success, the current filehandle retains its value.
[OPEN ISSUE: Should LAYOUTRETURN be modified to handle FSID
callbacks?]
ERRORS
NFS4ERR_BADLAYOUT
NFS4ERR_BADIOMODE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_NOFILEHANDLE
NFS4ERR_STALE
NFS4ERR_STALE_CLIENTID
NFS4ERR_UNKNOWN_LAYOUTTYPE
9.4 GETDEVICEINFO - Get Device Information
SYNOPSIS
(cfh), device_id, layout_type, maxcount -> device_addr
ARGUMENT
struct GETDEVICEINFO4args {
/* CURRENT_FH: file */
pnfs_deviceid4 device_id;
pnfs_layouttype4 layout_type;
count4 maxcount;
};
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
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device. The returned device_addr includes a type that indicates how
to interpret the addressing information for that device. The current
filehandle (cfh) is used to identify the file system; device IDs are
unique per file system (FSID) and are qualified by the layout type.
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.
See Section 3.1.1 for more details on device ID assignment.
If the size of the device address exceeds maxcount bytes, the
metadata server will return the error NFS4ERR_TOOSMALL. If an
invalid device ID is given, the metadata server will respond with
NFS4ERR_INVAL.
ERRORS
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_TOOSMALL
NFS4ERR_UNKNOWN_LAYOUTTYPE
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9.5 GETDEVICELIST - Get List of Devices
SYNOPSIS
(cfh), layout_type, maxcount, cookie, cookie_verf ->
device_addrs<>
ARGUMENT
struct GETDEVICELIST4args {
/* CURRENT_FH: file */
pnfs_layouttype4 layout_type;
count4 maxcount;
nfs_cookie4 cookie;
verifier4 cookieverf;
};
RESULT
struct GETDEVICELIST4resok {
pnfs_devlist_item4 device_addrs<>;
};
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, for a particular layout type. This operation may not be
able to fetch all device information at once, thus it uses a cookie
based approach, similar to READDIR, to fetch additional device
information. As in GETDEVICEINFO, the current filehandle (cfh) is
used to identify the file system.
If the metadata server is unable to return a single device address,
it will return the error NFS4ERR_TOOSMALL. If an invalid device ID
is given, the metadata server will respond with NFS4ERR_INVAL.
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ERRORS
NFS4ERR_BAD_COOKIE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_TOOSMALL
NFS4ERR_UNKNOWN_LAYOUTTYPE
10. Callback Operations
10.1 CB_LAYOUTRECALL
SYNOPSIS
layout_type, iomode, layoutrecall -> -
ARGUMENT
enum layoutrecall_type4 {
RECALL_FILE = 1,
RECALL_FSID = 2
};
union layoutrecall4 switch(layoutrecall_type4 recalltype) {
case RECALL_FILE:
nfs_fh4 fh;
offset4 offset;
length4 length;
case RECALL_FSID:
fsid4 fsid;
};
struct CB_LAYOUTRECALLargs {
pnfs_layouttype4 layout_type;
pnfs_layoutiomode4 iomode;
layoutrecall4 layoutrecall;
};
RESULT
struct CB_LAYOUTRECALLres {
nfsstat4 status;
};
DESCRIPTION
The CB_LAYOUTRECALL operation is used to begin the process of
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recalling a layout, a portion thereof, or all layouts pertaining to a
particular file system (FSID). If RECALL_FILE is specified, the
offset and length fields specify the portion of the layout to be
returned. The iomode specifies the set of layouts to be returned.
An iomode of ANY specifies that all matching layouts, regardless of
iomode, must be returned; otherwise, only layouts that exactly match
the iomode must be returned.
If RECALL_FSID is specified, the fsid specifies the file system for
which any outstanding layouts must be returned. Layouts are returned
through the LAYOUTRETURN operation.
If RECALL_FILE is specified and 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. If a length of all 1s
is specified then the layout corresponding to the byte range from
"offset" to the end-of-file MUST be returned.
If the layout specified is not held by the client, an
NFS4ERR_BADLAYOUT error is returned. If the layout type is unknown
to the client, then an NFS4ERR_UNKNOWN_LAYOUTTYPE is returned. If a
layout exists, but the iomode does not match, then an
NFS4ERR_BADIOMODE is returned.
IMPLEMENTATION
The client should reply to the callback immediately. Replying does
not complete the recall except when an error is returned. The recall
is not complete until the layout(s) are returned using a
LAYOUTRETURN.
The client should complete any in-flight I/O operations using the
recalled layout(s) before returning it/them via LAYOUTRETURN. If the
client has buffered dirty data, it may chose to write it directly to
storage before calling LAYOUTRETURN, or to write it later using
normal NFSv4 WRITE operations to the metadata server. If a large
amount of dirty data is outstanding, the client may issue
LAYOUTRETURNs for portions of the layout being recalled; this allows
the server to monitor the client's progress and adherence to the
callback.
ERRORS
NFS4ERR_BADLAYOUT
NFS4ERR_BADIOMODE
NFS4ERR_UNKNOWN_LAYOUTTYPE
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[OPEN ISSUE: Should we add a callback type for returning some
percentage of the layouts held by a client (something in between all
and one)?]
10.2 CB_SIZECHANGED
SYNOPSIS
fh, size -> -
ARGUMENT
struct CB_SIZECHANGEDargs {
nfs_fh4 fh;
length4 size;
};
RESULT
struct CB_SIZECHANGEDres {
nfsstat4 status;
};
DESCRIPTION
The CB_SIZECHANGED operation is used to notify the client that the
size pertaining to the filehandle associated with "fh", has changed.
The new size is specified. Upon reception of this notification
callback, the client should update its internal size for the file.
If the layout being held for the file is of the NFSv4 file layout
type, then the size field within that layout should be updated (see
Section 5.5.1). For other layout types see Section 3.5.2 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
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.]
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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
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
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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
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.
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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).
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.
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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.
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.
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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 Storage Errors
There may be a need for the client to communicate about errors it has
when accessing storage directly. The client may do so in a layout
type dependent way through the layoutupdate field in LAYOUTCOMMIT.
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/
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/
Welch, et al. Expires January 16, 2006 [Page 57]
Internet-Draft pNFS Operations July 2005
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
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.
Welch, et al. Expires January 16, 2006 [Page 58]
Internet-Draft pNFS Operations July 2005
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