Internet DRAFT - draft-ietf-nfsv4-pnfs
draft-ietf-nfsv4-pnfs
NFSv4 G. Goodson
Internet-Draft NetApp
Expires: April 10, 2006 B. Welch
B. Halevy
Panasas
D. Black
EMC
A. Adamson
CITI
October 7, 2005
NFSv4 pNFS Extensions
draft-ietf-nfsv4-pnfs-00.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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. General Definitions . . . . . . . . . . . . . . . . . . . . . 7
2.1 Metadata Server . . . . . . . . . . . . . . . . . . . . . 7
2.2 Client . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Storage Device . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Storage Protocol . . . . . . . . . . . . . . . . . . . . . 8
2.5 Control Protocol . . . . . . . . . . . . . . . . . . . . . 8
2.6 Metadata . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.7 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. pNFS protocol semantics . . . . . . . . . . . . . . . . . . . 9
3.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 Layout Types . . . . . . . . . . . . . . . . . . . . . 9
3.1.2 Layout Iomode . . . . . . . . . . . . . . . . . . . . 10
3.1.3 Layout Segments . . . . . . . . . . . . . . . . . . . 10
3.1.4 Device IDs . . . . . . . . . . . . . . . . . . . . . . 11
3.1.5 Aggregation Schemes . . . . . . . . . . . . . . . . . 12
3.2 Guarantees Provided by Layouts . . . . . . . . . . . . . . 12
3.3 Getting a Layout . . . . . . . . . . . . . . . . . . . . . 13
3.4 Committing a Layout . . . . . . . . . . . . . . . . . . . 14
3.4.1 LAYOUTCOMMIT and mtime/atime/change . . . . . . . . . 15
3.4.2 LAYOUTCOMMIT and size . . . . . . . . . . . . . . . . 15
3.4.3 LAYOUTCOMMIT and layoutupdate . . . . . . . . . . . . 16
3.5 Recalling a Layout . . . . . . . . . . . . . . . . . . . . 17
3.5.1 Basic Operation . . . . . . . . . . . . . . . . . . . 17
3.5.2 Recall Callback Robustness . . . . . . . . . . . . . . 18
3.5.3 Recall/Return Sequencing . . . . . . . . . . . . . . . 19
3.6 Metadata Server Write Propagation . . . . . . . . . . . . 21
3.7 Crash Recovery . . . . . . . . . . . . . . . . . . . . . . 21
3.7.1 Leases . . . . . . . . . . . . . . . . . . . . . . . . 21
3.7.2 Client Recovery . . . . . . . . . . . . . . . . . . . 23
3.7.3 Metadata Server Recovery . . . . . . . . . . . . . . . 23
3.7.4 Storage Device Recovery . . . . . . . . . . . . . . . 25
4. Security Considerations . . . . . . . . . . . . . . . . . . . 26
4.1 File Layout Security . . . . . . . . . . . . . . . . . . . 27
4.2 Object Layout Security . . . . . . . . . . . . . . . . . . 27
4.3 Block/Volume Layout Security . . . . . . . . . . . . . . . 29
5. The NFSv4 File Layout Type . . . . . . . . . . . . . . . . . . 29
5.1 File Striping and Data Access . . . . . . . . . . . . . . 29
5.1.1 Sparse and Dense Storage Device Data Layouts . . . . . 31
5.1.2 Metadata and Storage Device Roles . . . . . . . . . . 32
5.1.3 Device Multipathing . . . . . . . . . . . . . . . . . 33
5.1.4 Operations Issued to Storage Devices . . . . . . . . . 34
5.2 Global Stateid Requirements . . . . . . . . . . . . . . . 35
5.3 The Layout Iomode . . . . . . . . . . . . . . . . . . . . 35
5.4 Storage Device State Propagation . . . . . . . . . . . . . 35
5.4.1 Lock State Propagation . . . . . . . . . . . . . . . . 36
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5.4.2 Open-mode Validation . . . . . . . . . . . . . . . . . 36
5.4.3 File Attributes . . . . . . . . . . . . . . . . . . . 37
5.5 Storage Device Component File Size . . . . . . . . . . . . 38
5.6 Crash Recovery Considerations . . . . . . . . . . . . . . 38
5.7 Security Considerations . . . . . . . . . . . . . . . . . 39
5.8 Alternate Approaches . . . . . . . . . . . . . . . . . . . 39
6. pNFS Typed Data Structures . . . . . . . . . . . . . . . . . . 40
6.1 pnfs_layouttype4 . . . . . . . . . . . . . . . . . . . . . 40
6.2 pnfs_deviceid4 . . . . . . . . . . . . . . . . . . . . . . 41
6.3 pnfs_deviceaddr4 . . . . . . . . . . . . . . . . . . . . . 41
6.4 pnfs_devlist_item4 . . . . . . . . . . . . . . . . . . . . 42
6.5 pnfs_layout4 . . . . . . . . . . . . . . . . . . . . . . . 42
6.6 pnfs_layoutupdate4 . . . . . . . . . . . . . . . . . . . . 43
6.7 pnfs_layouthint4 . . . . . . . . . . . . . . . . . . . . . 43
6.8 pnfs_layoutiomode4 . . . . . . . . . . . . . . . . . . . . 43
7. pNFS File Attributes . . . . . . . . . . . . . . . . . . . . . 44
7.1 pnfs_layouttype4<> FS_LAYOUT_TYPES . . . . . . . . . . . . 44
7.2 pnfs_layouttype4<> FILE_LAYOUT_TYPES . . . . . . . . . . . 44
7.3 pnfs_layouthint4 FILE_LAYOUT_HINT . . . . . . . . . . . . 44
7.4 uint32_t FS_LAYOUT_PREFERRED_BLOCKSIZE . . . . . . . . . . 44
7.5 uint32_t FS_LAYOUT_PREFERRED_ALIGNMENT . . . . . . . . . . 44
8. pNFS Error Definitions . . . . . . . . . . . . . . . . . . . . 45
9. pNFS Operations . . . . . . . . . . . . . . . . . . . . . . . 45
9.1 LAYOUTGET - Get Layout Information . . . . . . . . . . . . 46
9.2 LAYOUTCOMMIT - Commit writes made using a layout . . . . . 48
9.3 LAYOUTRETURN - Release Layout Information . . . . . . . . 51
9.4 GETDEVICEINFO - Get Device Information . . . . . . . . . . 53
9.5 GETDEVICELIST - Get List of Devices . . . . . . . . . . . 54
10. Callback Operations . . . . . . . . . . . . . . . . . . . . 55
10.1 CB_LAYOUTRECALL . . . . . . . . . . . . . . . . . . . . . 56
10.2 CB_SIZECHANGED . . . . . . . . . . . . . . . . . . . . . . 58
11. Layouts and Aggregation . . . . . . . . . . . . . . . . . . 58
11.1 Simple Map . . . . . . . . . . . . . . . . . . . . . . . . 59
11.2 Block Extent Map . . . . . . . . . . . . . . . . . . . . . 59
11.3 Striped Map (RAID 0) . . . . . . . . . . . . . . . . . . . 59
11.4 Replicated Map . . . . . . . . . . . . . . . . . . . . . . 59
11.5 Concatenated Map . . . . . . . . . . . . . . . . . . . . . 60
11.6 Nested Map . . . . . . . . . . . . . . . . . . . . . . . . 60
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 60
12.1 Normative References . . . . . . . . . . . . . . . . . . . 60
12.2 Informative References . . . . . . . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 61
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 62
Intellectual Property and Copyright Statements . . . . . . . . 63
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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 |+-----------+ |
||+----------------||+-----------+ Control|
|+-----------------||| | 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/volume protocols such as iSCSI ([4]), and FCP ([5]). The
block/volume protocol support can be independent of the addressing
structure of the block/volume protocol used, allowing more than
one protocol to access the same file data and enabling
extensibility to other block/volume protocols.
o Object protocols such as OSD over iSCSI or Fibre Channel [6].
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 others, 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 can be stored on the storage devices. However, similar to
NFSv4, data may also be written through the metadata server. 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.
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2.3 Storage Device
This is a device, or server, that controls the file's data, but
leaves other metadata management up to the metadata server. A
storage device could be another NFS server, or an Object Storage
Device (OSD) or a block device accessed over a SAN (e.g., 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 (e.g., NFSv4), object protocols (e.g.,
OSD), and block/volume protocols (e.g., based on SCSI-block
commands). These protocols are in turn realizable over a variety of
transport stacks. 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 Control 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 control 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 control 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 control protocol, in practice
the control 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 control
protocol. Instead, it just describes the requirements on the control
protocol for maintaining attributes like modify time, the change
attribute, and the end-of-file position.
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2.6 Metadata
This is information about a file, like its name, owner, where it
stored, and so forth. The information is managed by the exported
file system server (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. pNFS protocol semantics
This section describes the semantics of the pNFS protocol extension
to NFSv4; this is the protocol between the client and the metadata
server.
3.1 Definitions
This sub-section defines a number of terms necessary for describing
layouts and their semantics. In addition, it more precisely defines
how layouts are identified and how they can be composed of smaller
granularity layout segments.
3.1.1 Layout Types
A layout describes the mapping of a file's data to the storage
devices that hold the data. A layout is said to belong to a specific
"layout type" (see Section 6.1 for its RPC definition). The layout
type allows for variants to handle different storage protocols (e.g.,
block/volume [7], object [8], and file [Section 5] layout types). A
metadata server, along with its control protocol, must support at
least one layout type. A private sub-range of the layout type name
space is also defined. Values from the private layout type range can
be used for internal testing or experimentation.
As an example, a file layout type could be an array of tuples (e.g.,
deviceID, file_handle), along with a definition of how the data is
stored across the devices (e.g., striping). A block/volume 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
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the first block. An object layout might be 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. Note, the actual
layouts are more complex than these simple expository examples.
This document defines a NFSv4 file layout type using a stripe-based
aggregation scheme (see Section 5). Adjunct specifications are being
drafted that precisely define other layout formats (e.g., block/
volume [7], and object [8] layouts) to allow interoperability among
clients and metadata servers.
3.1.2 Layout Iomode
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 as READs (i.e., READ/WRITE). For certain layout
types, it is useful for a client to specify this intent at LAYOUTGET
time. E.g., for block/volume based protocols, block allocation could
occur when a READ/WRITE iomode is specified. A special
LAYOUTIOMODE_ANY iomode is defined and can only be used for
LAYOUTRETURN and LAYOUTRECALL, not for LAYOUTGET. It specifies that
layouts pertaining to both READ and RW iomodes are being returned or
recalled, respectively.
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;
open mode checks and lock enforcement are always enforced, and are
logically separate from the pNFS layout level. As well, open modes
and locks 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.
3.1.3 Layout Segments
Until this point, layouts have been defined in a fairly vague manner.
A layout is more precisely identified by the following tuple:
<ClientID, FH, layout type>; the FH refers to the FH of the file on
the metadata server. Note, layouts describe a file, not a byte-range
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of a file.
Since a layout that describes an entire file may be very large, there
is a desire to manage layouts in smaller chunks that correspond to
byte-ranges of the file. For example, the entire layout need not be
returned, recalled, or committed. These chunks are called "layout
segments" and are further identified by the byte-range they
represent. Layout operations require the identification of the
layout segment (i.e., clientID, FH, layout type, and byte-range), as
well as the iomode. This structure allows clients and metadata
servers to aggregate the results of layout operations into a singly
maintained layout.
It is important to define when layout segments overlap and/or
conflict with each other. For a layout segment to overlap another
layout segment both segments must be of the same layout type,
correspond to the same filehandle, and have the same iomode; in
addition, the byte-ranges of the segments must overlap. Layout
segments conflict, when they overlap and differ in the content of the
layout (i.e., the storage device/file mapping parameters differ).
Note, differing iomodes do not lead to conflicting layouts. It is
permissible for layout segments with different iomodes, pertaining to
the same byte range, to be held by the same client.
3.1.4 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 operation (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 and unique per file
system (FSID). 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 cannot expect the mapping between device ID and storage
device address to persist across server reboots, hence a client MUST
fetch new mappings on startup or upon detection of a metadata server
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reboot unless it can revalidate its existing mappings. Not all
layout types support such revalidation, and the means of doing so is
layout specific. 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.
3.1.5 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.
3.2 Guarantees Provided by Layouts
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.
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 (i.e., those that would be used in the absence of
pNFS). However, if a valid layout for a file is not held by the
client, the storage device should 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/volume 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 must recall the layout
when those permissions become invalid for any reason; for example
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when a file becomes unwritable or inaccessible to a client. 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
observed by users of files. If a storage device is unable to
restrict 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.
3.3 Getting a Layout
A client obtains a layout through a new operation, LAYOUTGET. The
metadata server will give out layouts of a particular type (e.g.,
block/volume, object, or file) and aggregation as requested by the
client. 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. A field within the LAYOUTGET request, "minlength",
specifies the minimum overlap that MUST exist between the requested
layout and the layout returned by the metadata server. The
"minlength" field should specify a size of at least one. A metadata
server may give-out multiple overlapping, non-conflicting layout
segments to the same client in response to a LAYOUTGET.
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
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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 iSCSI (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.
3.4 Committing a Layout
Due to the nature of the protocol, the file attributes, and data
location mapping (e.g., which offsets store data vs. store holes)
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.
The LAYOUTCOMMIT operation is responsible for committing a modified
layout segment to the metadata server. Note: the data should be
written and committed to the appropriate storage devices before the
LAYOUTCOMMIT occurs. Note, if the data is being written
asynchronously through the metadata server a COMMIT to the metadata
server is required to sync the data and make it visible on the
storage devices (see Section 3.6 for more details). The scope of
this operation depends on the storage protocol in use. For block/
volume-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 (i.e., 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 control protocol is free to synchronize the attributes before it
receives a LAYOUTCOMMIT, however upon successful completion of a
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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.
3.4.1 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. According to the NFSv4 specification, The client
always has the option to set these attributes 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
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.
3.4.2 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 ("last_write_offset"). Note:
for this offset to be viewed as a file size it must be incremented by
one byte (e.g., a write to offset 0 would map into a file size of 1,
but the last write offset is 0). The metadata server may do one of
the following:
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1. It may update the file's size based on the last write 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 write 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 control protocol), it
may ignore the client provided argument and use the query-derived
value.
3. It may use the last write 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 control 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 control 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 zero-extend
or truncate a file, SETATTR must be used; it need not be used when
simply writing past EOF.
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.
3.4.3 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/volume 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
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the layout returned by LAYOUTGET. The structure is defined by the
layout type and is opaque to LAYOUTCOMMIT.
3.5 Recalling a Layout
3.5.1 Basic Operation
Since a layout protects a client's access to a file via a direct
client-storage-device 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
CB_LAYOUTRECALL callback operation (see Section 10.1). This callback
can either recall a layout segment identified by a byte range, or all
the layouts associated with a file system (FSID). However, there is
no single operation to return all layouts associated with an FSID;
multiple layout segments may be returned in a single compound
operation. Section 3.5.3 discusses sequencing issues surrounding the
getting, returning, and recalling of layouts.
The iomode is also specified when recalling a layout or layout
segment. Generally, the iomode in the recall request must match the
layout, or segment, being returned; e.g., a recall with an iomode of
RW should cause the client to only return RW layout segments (not R
segments). However, 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
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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 MUST NOT 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 SHOULD
reject the I/O.
3.5.2 Recall Callback Robustness
For simplicity, the discussion thus far has assumed that pNFS client
state for a file exactly matches the pNFS server state for that file
and client regarding layout ranges and permissions. This assumption
leads to the implicit assumption that any callback results in a
LAYOUTRETURN or set of LAYOUTRETURNs that exactly match the range in
the callback, since both client and server agree about the state
being maintained. However, it can be useful if this assumption does
not always hold. For example:
o It may be useful for clients to be able to discard layout
information without calling LAYOUTRETURN. If conflicts that
require callbacks are very rare, and a server can use a multi-file
callback to recover per-client resources (e.g., via a FSID recall,
or a multi-file recall within a single compound), the result may
be significantly less client-server pNFS traffic.
o It may be similarly useful for servers to enhance information
about what layout ranges are held by a client beyond what a client
actually holds. In the extreme, a server could manage conflicts
on a per-file basis, only issuing whole-file callbacks even though
clients may request and be granted sub-file ranges.
o As well, the synchronized state assumption is not robust to minor
errors. A more robust design would allow for divergence between
client and server and the ability to recover. It is vital that a
client not assign itself layout permissions beyond what the server
has granted and that the server not forget layout permissions that
have been granted in order to avoid errors. On the other hand, if
a server believes that a client holds a layout segment that the
client does not know about, it's useful for the client to be able
to issue the LAYOUTRETURN that the server is expecting in response
to a recall.
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Thus, in light of the above, it is useful for a server to be able to
issue callbacks for layout ranges it has not granted to a client, and
for a client to return ranges it does not hold. A pNFS client must
always return layout segments that comprise the full range specified
by the recall. Note, the full recalled layout range need not be
returned as part of a single operation, but may be returned in
segments. This allows the client to stage the flushing of dirty
data, layout commits, and returns. Also, it indicates to the
metadata server that the client is making progress.
In order to ensure client/server convergence on the layout state, the
final LAYOUTRETURN operation in a sequence of returns for a
particular recall, SHOULD specify the entire range being recalled,
even if layout segments pertaining to partial ranges were previously
returned. In addition, if the client holds no layout segment that
overlaps the range being recalled, the client should return the
NFS4ERR_NOMATCHING_LAYOUT error code. This allows the server to
update its view of the client's layout state.
3.5.3 Recall/Return Sequencing
As with other stateful operations, pNFS requires the correct
sequencing of layout operations. This proposal assumes that sessions
will precede or accompany 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. Also, this
specification is reliant on the sessions protocol to provide the
correct sequencing between regular operations and callbacks. It is
the server's responsibility to avoid inconsistencies regarding the
layouts it hands out and the client's responsibility to properly
serialize its layout requests.
One critical issue with operation sequencing concerns callbacks. The
protocol must defend against races between the reply to a LAYOUTGET
operation and a subsequent CB_LAYOUTRECALL. It MUST NOT be possible
for a client to process the CB_LAYOUTRECALL for a layout that it has
not received in a reply message to a LAYOUTGET.
3.5.3.1 Client Side Considerations
Consider a pNFS client that has issued a LAYOUTGET and then receives
an overlapping recall callback for the same file. There are two
possibilities, which the client cannot distinguish when the callback
arrives:
1. The server processed the LAYOUTGET before issuing the recall, so
the LAYOUTGET response is in flight, and must be waited for
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because it may be carrying layout info that will need to be
returned to deal with the recall callback.
2. The server issued the callback before receiving the LAYOUTGET.
The server will not respond to the LAYOUTGET until the recall
callback is processed.
This can cause deadlock, as the client must wait for the LAYOUTGET
response before processing the recall in the first case, but that
response will not arrive until after the recall is processed in the
second case. This deadlock can be avoided by adhering to the
following requirements:
o A LAYOUTGET MUST be rejected with an error (i.e.,
NFS4ERR_RECALLCONFLICT) if there's an overlapping outstanding
recall callback to the same client
o When processing a recall, the client MUST wait for a response to
all conflicting outstanding LAYOUTGETs before performing any
RETURN that could be affected by any such response.
o The client SHOULD wait for responses to all operations required to
complete a recall before sending any LAYOUTGETs that would
conflict with the recall because the server is likely to return
errors for them.
Now the client can wait for the LAYOUTGET response, as it will be
received in both cases.
3.5.3.2 Server Side Considerations
Consider a related situation from the pNFS server's point of view.
The server has issued a recall callback and receives an overlapping
LAYOUTGET for the same file before the LAYOUTRETURN(s) that respond
to the recall callback. Again, there are two cases:
1. The client issued the LAYOUTGET before processing the recall
callback.
2. The client issued the LAYOUTGET after processing the recall
callback, but it arrived before the LAYOUTRETURN that completed
that processing.
The simplest approach is to always reject the overlapping LAYOUTGET.
The client has two ways to avoid this result - it can issue the
LAYOUTGET as a subsequent element of a COMPOUND containing the
LAYOUTRETURN that completes the recall callback, or it can wait for
the response to that LAYOUTRETURN.
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This leads to a more general problem; in the absence of a callback if
a client issues concurrent overlapping LAYOUTGET and LAYOUTRETURN
operations, it is possible for the server to process them in either
order. Again, a client must take the appropriate precautions in
serializing its actions.
[ASIDE: HighRoad forbids a client from doing this, as the per-file
layout stateid will cause one of the two operations to be rejected
with a stale layout stateid. This approach is simpler and produces
better results by comparison to allowing concurrent operations, at
least for this sort of conflict case, because server execution of
operations in an order not anticipated by the client may produce
results that are not useful to the client (e.g., if a LAYOUTRETURN is
followed by a concurrent overlapping LAYOUTGET, but executed in the
other order, the client will not retain layout extents for the
overlapping range).]
3.6 Metadata Server Write Propagation
Asynchronous writes written through the metadata server may be
propagated lazily to the storage devices. For data written
asynchronously through the metadata server, a client performing a
read at the appropriate storage device is not guaranteed to see the
newly written data until a COMMIT occurs at the metadata server.
While the write is pending, reads to the storage device can give out
either the old data, the new data, or a mixture thereof. After
either a synchronous write completes, or a COMMIT is received (for
asynchronously written data), the metadata server must ensure that
storage devices give out the new data and that the data has been
written to stable storage. If the server implements its storage in
any way such that it cannot obey these constraints, then it must
recall the layouts to prevent reads being done that cannot be handled
correctly.
3.7 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.
3.7.1 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
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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.
3.7.1.1 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; this includes 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
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.
3.7.1.2 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:
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...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.
3.7.2 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.
3.7.3 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;
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there is no reclaim bit, thus no guarantee of continuity between the
previous and new layout. This is not necessarily required since a
layout is not required to perform I/O; I/O can always be performed
through the metadata server.
[NOTE: there is no reclaim bit for getting a layout. Thus, in the
case of reclaiming an old layout obtained through LAYOUTGET, there is
no guarantee of continuity. If a reclaim bit existed a block/volume
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); however, divergence is avoided by having
the server tell the client what is contained within the layout.]
If the client has dirty data that it needs to write out, or an
outstanding LAYOUTCOMMIT, the client should try to obtain a new
layout segment covering the byte range covered by the previous layout
segment. However, the client might not not get the same layout
segment 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/volume-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; in
the interest of simplicity, the client might decide 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 3.7.1.
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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 (see
Section 5.6); as well, the object storage device protocol allows
storage to fence clients after lease expiration. 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
control protocol.
3.7.4 Storage Device Recovery
Storage device crash recovery is mostly dependent upon the layout
type in use. However, there are a few general techniques a client
can use if it discovers a storage device has crashed while holding
asynchronously written, non-committed, data. First and foremost, it
is important to realize that the client is the only one who has the
information necessary to recover asynchronously written data; since,
it holds the dirty data and most probably nobody else does. Second,
the best solution is for the client to err on the side or caution and
attempt to re-write the dirty data through another path.
The client, rather than hold the asynchronously written data
indefinitely, is encouraged to, and can make sure that the data is
written by using other paths to that data. The client may write the
data to the metadata server, either synchronously or asynchronously
with a subsequent COMMIT. Once it does this, there is no need to
wait for the original storage device. In the event that the data
range to be committed is transferred to a different storage device,
as indicated in a new layout, the client may write to that storage
device. Once the data has been committed at that storage device,
either through a synchronous write or through a commit to that
storage device (e.g., through the NFSv4 COMMIT operation for the
NFSv4 file layout), the client should consider the transfer of
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responsibility for the data to the new server as strong evidence that
this is the intended and most effective method for the client to get
the data written. In either case, once the write is on stable
storage (through either the storage device or metadata server), there
is no need to continue either attempting to commit or attempting to
synchronously write the data to the original storage device or wait
for that storage device to become available. That storage device may
never be visible to the client again.
This approach does have a "lingering write" problem, similar to
regular NFSv4. Suppose a WRITE is issued to a storage device for
which no response is received. The client breaks the connection,
trying to re-establish a new one, and gets a recall of the layout.
The client issues the I/O for the dirty data through an alternative
path, for example, through the metadata server and it succeeds. The
client then goes on to perform additional writes that all succeed.
If at some time later, the original write to the storage device
succeeds, data inconsistency could result. The same problem can
occur in regular NFSv4. For example, a WRITE is held in a switch for
some period of time while other writes are issued and replied to, if
the original WRITE finally succeeds, the same issues can occur.
However, this is solved by sessions in NFSv4.x.
4. Security Considerations
The pNFS extension partitions the NFSv4 file system protocol into two
parts, the control path and the data path (i.e., 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
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subject to the same threats as client-server communication, the
protocols used for that communication need to provide security
mechanisms comparable to those available via RPSEC_GSS for NFSv4.
Many situations in which pNFS is likely to be used will not be
subject to the overall threat profile for which NFSv4 is required to
provide countermeasures.
pNFS implementations MUST NOT remove NFSv4's access controls. The
combination of clients, storage devices, and the server are
responsible for ensuring that all client to storage device file data
access respects NFSv4 ACLs and file open modes. This entails
performing both of these checks on every access in the client, the
storage device, or both. If a pNFS configuration performs these
checks only in the client, the risk of a misbehaving client obtaining
unauthorized access is an important consideration in determining when
it is appropriate to use such a pNFS configuration. Such
configurations SHOULD NOT be used when client- only access checks do
not provide sufficient assurance that NFSv4 access control is being
applied correctly.
The following subsections describe security considerations
specifically applicable to each of the three major storage device
protocol types supported for pNFS.
[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 MUST implement the security aspects
described in version 1 of the T10 OSD protocol definition [6]. The
remainder of this section gives an overview of the security mechanism
described in that standard. The goal is to give the reader a basic
understanding of the object security model. Any discrepancies
between this text and the actual standard are obviously to be
resolved in favor of the OSD standard.
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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
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 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
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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/Volume Layout Security
As typically used, block/volume 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 method of block/volume 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/volume protocols SHOULD NOT be
used with pNFS, unless the storage device 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. The 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_layouthint {
stripetype4 stripe_type;
length4 stripe_unit;
uint32_t stripe_width;
};
struct nfsv4_file_layout { /* Per data stripe */
pnfs_deviceid4 dev_id<>;
nfs_fh4 fh;
};
struct nfsv4_file_layouttype4 { /* Per file */
stripetype4 stripe_type;
length4 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 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 "dev_id" array can be used for multipathing and is discussed
further in Section 5.1.3. 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).
Devices may be repeated multiple times within the device list array;
this is shown where storage device 1 holds both the first and third
stripe of data. Filehandles can only be repeated if a sparse stripe
type is used. 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
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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 client visible file offset
to the offset within the data file located on the storage device.
The layout hint structure is described in more detail in Section 6.7.
It is used, by the client, as by the FILE_LAYOUT_HINT attribute to
specify the type of layout to be used for a newly created file.
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
Section 5.5 describe the semantics for dealing with reads to holes
within the striped file. This is of particular concern, since each
individual component stripe file (i.e., the component of the striped
file that lives on a particular storage device) may be of different
length. Thus, clients may experience 'short' reads when reading off
the end of one of these component files.
5.1.2 Metadata and Storage Device Roles
In many cases, the metadata server and the storage device will be
separate pieces of physical hardware. The specification text is
written as if that were always case. However, it can be the case
that the same physical hardware is used to implement both a metadata
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and storage device and in this case, the specification text's
references to these two entities are to be understood as referring to
the same physical hardware implementing two distinct roles and it is
important that it be clearly understood on behalf of which role the
hardware is executing at any given time.
Two sub-cases can be distinguished. In the first sub-case, the same
physical hardware is used to implement both a metadata and data
server in which each role is addressed through a distinct network
interface (e.g., IP addresses for the metadata server and storage
device are distinct). As long as the storage device address is
obtained from the layout and is distinct from the metadata server's
address, using the device ID therein to obtain the appropriate
storage device address, it is always clear, for any given request, to
what role it is directed, based on the destination IP address.
However, it may also be the case that even though the metadata server
and storage device are distinct from one client's point of view, the
roles may be reversed according to another client's point of view.
For example, in the cluster file system model a metadata server to
one client, may be a storage device to another client. Thus, it is
safer to always mark the filehandle so that operations addressed to
storage devices can be distinguished.
The second sub-case is where both the metadata and storage device
have the same network address. This requires us to make the
distinction as to which role each request is directed, on a another
basis. Since the network address is the same, the request is
understood as being directed at one or the other, based on the
filehandle of the first current filehandle value for the request. If
the first current file handle is one derived from a layout (i.e., it
is specified within the layout) (and it is recommended that these be
distinguishable), then the request is to be considered as executed by
a storage device. Otherwise, the operation is to be understood as
executed by the metadata server.
If a current filehandle is set that is inconsistent with the role to
which it is directed, then the error NFS4ERR_BADHANDLE should result.
For example, if a request is directed at the storage device, because
the first current handle is from a layout, any attempt to set the
current filehandle to be a value not from a layout should be
rejected. Similarly, if the first current file handle was for a
value not from a layout, a subsequent attempt to set the current file
handle to a value obtained from a layout should be rejected.
5.1.3 Device Multipathing
The NFSv4 file layout supports multipathing to 'equivalent' devices.
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Device-level multipathing is primarily of use in the case of a data
server failure --- it allows the client to switch to another storage
device that is exporting the same data stripe, without having to
contact the metadata server for a new layout.
To support device multipathing, an array of device IDs is encoded
within the data stripe portion of the file's layout. This array
represents an ordered list of devices where the first element has the
highest priority. Each device in the list MUST be 'equivalent' to
every other device in the list and each device must be attempted in
the order specified.
Equivalent devices MUST export the same system image (e.g., the
stateids and filehandles that they use are the same) and must provide
the same consistency guarantees. Two equivalent storage devices must
also have sufficient connections to the storage, such that writing to
one storage device is equivalent to writing to another, this also
applies to reading. Also, if multiple copies of the same data exist,
reading from one must provide access to all existing copies. As
such, it is unlikely that multipathing will provide additional
benefit in the case of an I/O error.
[NOTE: the error cases in which a client is expected to attempt an
equivalent storage device should be specified.]
5.1.4 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 (i.e., which filehandles to use on which devices). As
described in Section 3.2, a client MUST NOT issue I/Os to storage
devices for which it does not hold a valid layout. The storage
devices may reject such requests.
GETATTR and SETATTR MUST be directed to the metadata server. In the
case of a SETATTR of the size attribute, the control 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 3.4.2). Section 5.5, describes the
mechanism by which the client is to handle storage device file's that
do not reflect the metadata server's size.
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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 storage devices, 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 storage devices, so that the storage devices can
validate I/O accesses. This is discussed further in Section 5.4.
Depending on when stateids are propagated, the existence of a valid
stateid on the storage device may act as proof of a valid layout.
[NOTE: a number of proposals have been made that have the possibility
of limiting the amount of validation performed by the storage device,
if any of these proposals are accepted or obtain consensus, the
global stateid requirement can be revisited.]
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
(LAYOUTIOMODE_RW). The iomode need not be checked by the storage
devices when clients perform I/O. However, the storage devices SHOULD
still validate that the client holds a valid layout and return an
error if the client does not.
5.4 Storage Device State Propagation
Since the metadata server, which handles lock and open-mode state
changes, as well as ACLs, may not be collocated with the storage
devices where I/O access are validated, as such, 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 control
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protocol, even though the control 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.
[NOTE: With the reliance on the sessions protocol, there is no real
need for sequence ID portion of the stateid to be validated on I/O
accesses. It is proposed that the seq. ID checking is obsoleted.]
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 (i.e., 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.
5.4.2 Open-mode Validation
Open-mode validation MUST be performed against the open mode(s) held
by the storage devices. However, the server implementation may not
always require the immediate propagation of changes. Reduction in
access because of CLOSEs or DOWNGRADEs do not have to be propagated
immediately, but SHOULD be propagated promptly; whereas changes due
to revocation MUST be propagated immediately. On the other hand,
changes that expand access (e.g., new OPEN's and upgrades) don't have
to be propagated immediately but the storage device SHOULD NOT reject
a request because of mode issues without making sure that the upgrade
is not in flight.
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5.4.3 File Attributes
Since the SETATTR operation has the ability to modify state that is
visible on 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 3.4
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 control 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
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.
[NOTE: 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. If changes within a minor version alter the
behavior of NFSv4 with regard to OPEN principals and stateids some
access control checking at the storage device can be made less
expensive. pNFS should be altered to take full advantage of these
changes.]
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5.5 Storage Device Component File Size
A potential problem exists when a component data file on a particular
storage device is grown past EOF; the problem 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.
5.6 Crash Recovery Considerations
As described in Section 3.7, 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
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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.
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 control
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 (e.g., record and share reservations)
are held by owners which 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 (e.g., 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
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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, 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
to the last successful OPEN/LOCK should be used. This approach is
recommended if it is decided that using NFSv4 as a control 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 control 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,
LAYOUT_OSD2_OBJECTS = 2,
LAYOUT_BLOCK_VOLUME = 3
};
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 is to be defined
by this document. This would allow custom installations to introduce
new layout types.
[OPEN ISSUE: Determine private range of layout types]
New layout types must be specified in RFCs approved by the IESG
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before becoming part of the pNFS specification.
The LAYOUT_NFSV4_FILES enumeration specifies that the NFSv4 file
layout type is to be used. The LAYOUT_OSD2_OBJECTS enumeration
specifies that the object layout, as defined in [8], is to be used.
Similarly, the LAYOUT_BLOCK_VOLUME enumeration that the block/volume
layout, as defined in [7], is to be used.
6.2 pnfs_deviceid4
typedef uint64_t pnfs_deviceid4; /* 64-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 and are unique per file system
(FSID). This allows different layout drivers to generate device IDs
without the need for co-ordination. See Section 3.1.4 for more
details.
6.3 pnfs_deviceaddr4
struct pnfs_netaddr4 {
string r_netid<>; /* network ID */
string r_addr<>; /* universal address */
};
union pnfs_deviceaddr4 switch (pnfs_layouttype4 layout_type) {
case LAYOUT_NFSV4_FILES:
pnfs_netaddr4 netaddr;
default:
opaque device_addr<>; /* 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. The other device
address we expect to support is a SCSI volume identifier. The final
protocol specification will detail the allowed values for device_type
and the format of their associated location information.
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[NOTE: other device addresses will be added as the respective
specifications mature. It has been suggested that a separate
device_type enumeration is used as a switch to the pnfs_deviceaddr4
structure (e.g., if multiple types of addresses exist for the same
layout type). Until such a time as a real case is made and the
respective layout types have matured, the device address structure
will be left as is.]
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_layoutdata4 switch (pnfs_layouttype4 layout_type) {
case LAYOUT_NFSV4_FILES:
nfsv4_file_layouttype4 file_layout;
default:
opaque layout_data<>;
};
struct pnfs_layout4 {
offset4 offset;
length4 length;
pnfs_layoutiomode4 iomode;
pnfs_layoutdata4 layout;
};
The pnfs_layout4 structure defines a layout for a file. The
pnfs_layoutdata4 union contains the portion of the layout specific to
the layout type. Currently, only the NFSv4 file layout type is
defined; see Section 5.1 for its definition. Since layouts are sub-
dividable, the offset and length together with the file's filehandle,
the clientid, iomode, and layout type, identifies the layout.
[OPEN ISSUE: there is a discussion of moving the striping
information, or more generally the "aggregation scheme", up to the
generic layout level. This creates a two-layer system where the top
level is a switch on different data placement layouts, and the next
level down is a switch on different data storage types. This lets
different layouts (e.g., striping or mirroring or redundant servers)
to be layered over different storage devices. This would move
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geometry information out of nfsv4_file_layouttype4 and up into a
generic pnfs_striped_layout type that would specify a set of
pnfs_deviceid4 and pnfs_devicetype4 to use for storage. Instead of
nfsv4_file_layouttype4, there would be pnfs_nfsv4_devicetype4.]
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/volume
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_layouthint4
union pnfs_layouthint4 switch (pnfs_layouttype4 layout_type) {
case LAYOUT_NFSV4_FILES:
nfsv4_file_layouthint layout_hint;
default:
opaque layout_hint_data<>;
};
The pnfs_layouthint4 structure is used by the client to pass in a
hint about the type of layout it would like created for a particular
file. It is the structure specified by the FILE_LAYOUT_HINT
attribute described below. The metadata server may ignore the hint,
or may selectively ignore fields within the hint. This hint should
be provided at create time as part of the initial attributes within
OPEN. The "nfsv4_file_layouthint" structure is defined in
Section 5.1.
6.8 pnfs_layoutiomode4
enum pnfs_layoutiomode4 {
LAYOUTIOMODE_READ = 1,
LAYOUTIOMODE_RW = 2,
LAYOUTIOMODE_ANY = 3
};
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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 iomodes 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
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_TYPES
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_layouthint4 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 structure 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.
[OPEN ISSUE: it has been suggested that the HINT is a well defined
type other than pnfs_layoutdata4, similar to pnfs_layoutupdate4.]
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
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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.
NFS4ERR_NOMATCHING_LAYOUT Client has no matching layout (segment) to
return.
NFS4ERR_RECALLCONFLICT Layout is unavailable due to a conflicting
LAYOUTRECALL that is in progress.
NFS4ERR_UNKNOWN_LAYOUTTYPE Layout type is unknown.
9. pNFS Operations
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9.1 LAYOUTGET - Get Layout Information
SYNOPSIS
(cfh), clientid, layout_type, iomode, offset, length,
minlength, maxcount -> layout
ARGUMENT
struct LAYOUTGET4args {
/* CURRENT_FH: file */
clientid4 clientid;
pnfs_layouttype4 layout_type;
pnfs_layoutiomode4 iomode;
offset4 offset;
length4 length;
length4 minlength;
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 (and reading) the file given
by the filehandle at the byte range specified by offset and length.
Layouts are identified by the clientid, filehandle, and layout type.
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, a layout segment. To get a layout segment from a
specific offset through the end-of-file, regardless of the file's
length, a length field with all bits set to 1 (one) should be used.
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
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result.
The "minlength" field specifies the minimum size overlap with the
requested offset and length that is to be returned. If this
requirement cannot be met, no layout must be returned; the error
NFS4ERR_LAYOUTTRYLATER can be returned.
The "maxcount" field specifies the maximum layout size (in bytes)
that the client can handle. If the size of the layout structure
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 segment 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 an overlap
of "minlength" 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 segment 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 must
return NFS4ERR_LAYOUTTRYLATER.
If the layout request is rejected due to an overlapping layout
recall, the server must return NFS4ERR_RECALLCONFLICT. See
Section 3.5.3 for details.
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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.
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.
ERRORS
NFS4ERR_BADLAYOUT
NFS4ERR_BADIOMODE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_LAYOUTUNAVAILABLE
NFS4ERR_LAYOUTTRYLATER
NFS4ERR_LOCKED
NFS4ERR_NOFILEHANDLE
NFS4ERR_NOTSUPP
NFS4ERR_RECALLCONFLICT
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, last_write_offset,
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 last_write_offset;
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 segment 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 "last_write_offset" 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 one 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 3.4 for more details. If the new client
desires the resultant mtime or atime, it should issue a GETATTR
following the LAYOUTCOMMIT; e.g., later in the same compound.
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
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to the server, and, if desired, its view of the file modify and
access time. For block/volume layouts, 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.
ERRORS
NFS4ERR_BADLAYOUT
NFS4ERR_BADIOMODE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_NOFILEHANDLE
NFS4ERR_STALE
NFS4ERR_STALE_CLIENTID
NFS4ERR_UNKNOWN_LAYOUTTYPE
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
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Returns the layout segment 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. As well, it may be
a subset or superset of a layout specified by CB_LAYOUTRECALL.
However, if it is a subset, the recall is not complete until the full
byte range has been returned. It is also permissible, and no error
should result, for a client to return a byte range covering a layout
it does not hold. 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 control protocol with the storage
devices to fence the client from accessing the data referenced by the
layout. See Section 3.5 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.
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
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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_deviceaddr4 device_addr;
};
union GETDEVICEINFO4res switch (nfsstat4 status) {
case NFS4_OK:
GETDEVICEINFO4resok resok4;
default:
void;
};
DESCRIPTION
Returns device type and device address information for a specified
device. The returned device_addr includes a type that indicates how
to interpret the addressing information for that device. 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.
See Section 3.1.4 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
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NFS4ERR_UNKNOWN_LAYOUTTYPE
9.5 GETDEVICELIST - Get List of Devices
SYNOPSIS
(cfh), layout_type, maxcount, cookie, cookieverf ->
cookie, cookieverf, device_addrs<>
ARGUMENT
struct GETDEVICELIST4args {
/* CURRENT_FH: file */
pnfs_layouttype4 layout_type;
count4 maxcount;
nfs_cookie4 cookie;
verifier4 cookieverf;
};
RESULT
struct GETDEVICELIST4resok {
nfs_cookie4 cookie;
verifier4 cookieverf;
pnfs_devlist_item4 device_addrs<>;
};
union GETDEVICELIST4res switch (nfsstat4 status) {
case NFS4_OK:
GETDEVICELIST4resok 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 an array of items (pnfs_devlist_item4) 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 (see [2], section 14.2.24). As
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in GETDEVICEINFO, the current filehandle (cfh) is used to identify
the file system.
As in GETDEVICEINFO, maxcount specifies the maximum number of bytes
to return. 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.
ERRORS
NFS4ERR_BAD_COOKIE
NFS4ERR_FHEXPIRED
NFS4ERR_INVAL
NFS4ERR_TOOSMALL
NFS4ERR_UNKNOWN_LAYOUTTYPE
10. Callback Operations
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10.1 CB_LAYOUTRECALL
SYNOPSIS
layout_type, iomode, layoutrecall -> -
ARGUMENT
enum layoutrecall_type4 {
RECALL_FILE = 1,
RECALL_FSID = 2
};
struct layoutrecall_file4 {
nfs_fh4 fh;
offset4 offset;
length4 length;
};
union layoutrecall4 switch(layoutrecall_type4 recalltype) {
case RECALL_FILE:
layoutrecall_file4 layout;
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
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.
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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 the client does not hold any layout segment either matching or
overlapping with the requested layout, it returns
NFS4ERR_NOMATCHING_LAYOUT. 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.
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 dirty data is flushed while the layout is held, the client must
still issue LAYOUTCOMMIT operations at the appropriate time,
especially before issuing the LAYOUTRETURN. 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.
However, the last LAYOUTRETURN in a sequence of returns, SHOULD
specify the full range being recalled (see Section 3.5.2 for
details).
ERRORS
NFS4ERR_NOMATCHING_LAYOUT
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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). For other layout types see Section 3.4.2 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. Layouts and Aggregation
This section describes several aggregation schemes in a semi-formal
way to provide context for layout formats. 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
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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.
11.1 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).
11.2 Block Extent Map
The data is located on a LUN in the SAN. The layout consists of an
array of (deviceID, blockID, offset, length) tuples. Each entry
describes a block extent.
11.3 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.
11.4 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.
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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 control protocol.
11.5 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.
11.6 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.
12. References
12.1 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] Gibson, G., "pNFS Problem Statement", July 2004, <ftp://
www.ietf.org/internet-drafts/
draft-gibson-pnfs-problem-statement-01.txt>.
12.2 Informative References
[4] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC 3720, April 2004.
[5] Snively, R., "Fibre Channel Protocol for SCSI, 2nd Version
(FCP-2)", ANSI/INCITS 350-2003, Oct 2003.
[6] Weber, R., "Object-Based Storage Device Commands (OSD)", ANSI/
INCITS 400-2004, July 2004,
<http://www.t10.org/ftp/t10/drafts/osd/osd-r10.pdf>.
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[7] Black, D., "pNFS Block/Volume Layout", July 2005, <ftp://
www.ietf.org/internet-drafts/draft-black-pnfs-block-01.txt>.
[8] Zelenka, J., Welch, B., and B. Halevy, "Object-based pNFS
Operations", July 2005, <ftp://www.ietf.org/internet-drafts/
draft-zelenka-pnfs-obj-01.txt>.
Authors' Addresses
Garth Goodson
Network Appliance
495 E. Java Dr
Sunnyvale, CA 94089
USA
Phone: +1-408-822-6847
Email: goodson@netapp.com
Brent Welch
Panasas, Inc.
6520 Kaiser Drive
Fremont, CA 95444
USA
Phone: +1-650-608-7770
Email: welch@panasas.com
URI: http://www.panasas.com/
Benny Halevy
Panasas, Inc.
1501 Reedsdale St., #400
Pittsburgh, PA 15233
USA
Phone: +1-412-323-3500
Email: bhalevy@panasas.com
URI: http://www.panasas.com/
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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.
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