NFSv4 Working Group David L. Black
Internet Draft Stephen Fridella
Expires: April 2008 Jason Glasgow
Intended Status: Proposed Standard EMC Corporation
October 3, 2007
pNFS Block/Volume Layout
draft-ietf-nfsv4-pnfs-block-04.txt
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Abstract
Parallel NFS (pNFS) extends NFSv4 to allow clients to directly access
file data on the storage used by the NFSv4 server. This ability to
bypass the server for data access can increase both performance and
parallelism, but requires additional client functionality for data
access, some of which is dependent on the class of storage used. The
main pNFS operations draft specifies storage-class-independent
extensions to NFS; this draft specifies the additional extensions
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(primarily data structures) for use of pNFS with block and volume
based storage.
Conventions used in this document
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 [RFC2119].
Table of Contents
1. Introduction........................................... 3
2. Block Layout Description ................................ 3
2.1. Background and Architecture ......................... 3
2.2. GETDEVICELIST and GETDEVICEINFO...................... 4
2.2.1. Volume Identification.......................... 4
2.2.2. Volume Topology................................ 5
2.2.3. GETDEVICELIST and GETDEVICEINFO deviceid4......... 8
2.3. Data Structures: Extents and Extent Lists............. 9
2.3.1. Layout Requests and Extent Lists.................11
2.3.2. Layout Commits ................................11
2.3.3. Layout Returns ................................12
2.3.4. Client Copy-on-Write Processing..................13
2.3.5. Extents are Permissions.........................14
2.3.6. End-of-file Processing .........................15
2.3.7. Client Fencing ................................16
2.4. Crash Recovery Issues...............................18
2.5. Recalling resources: CB_RECALL_ANY ...................18
2.6. Transient and Permanent Errors.......................19
3. Security Considerations.................................19
4. Conclusions............................................21
5. IANA Considerations.....................................21
6. Revision History .......................................21
7. Acknowledgments........................................22
8. References.............................................23
8.1. Normative References................................23
8.2. Informative References..............................23
Author's Addresses........................................23
Intellectual Property Statement.............................24
Disclaimer of Validity.....................................24
Copyright Statement .......................................25
Acknowledgment............................................25
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1. Introduction
Figure 1 shows the overall architecture of a pNFS system:
+-----------+
|+-----------+ +-----------+
||+-----------+ | |
||| | NFSv4 + pNFS | |
+|| Clients |<------------------------------>| Server |
+| | | |
+-----------+ | |
||| +-----------+
||| |
||| |
||| +-----------+ |
||| |+-----------+ |
||+----------------||+-----------+ |
|+-----------------||| | |
+------------------+|| Storage |------------+
+| Systems |
+-----------+
Figure 1 pNFS Architecture
The overall approach is that pNFS-enhanced clients obtain sufficient
information from the server to enable them to access the underlying
storage (on the Storage Systems) directly. See the pNFS portion of
[NFSV4.1] for more details. This draft is concerned with access from
pNFS clients to Storage Systems over storage protocols based on
blocks and volumes, such as the SCSI protocol family (e.g., parallel
SCSI, FCP for Fibre Channel, iSCSI, SAS). This class of storage is
referred to as block/volume storage. While the Server to Storage
System protocol is not of concern for interoperability here, it will
typically also be a block/volume protocol when clients use block/
volume protocols.
2. Block Layout Description
2.1. Background and Architecture
The fundamental storage abstraction supported by block/volume storage
is a storage volume consisting of a sequential series of fixed size
blocks. This can be thought of as a logical disk; it may be realized
by the Storage System as a physical disk, a portion of a physical
disk or something more complex (e.g., concatenation, striping, RAID,
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and combinations thereof) involving multiple physical disks or
portions thereof.
A pNFS layout for this block/volume class of storage is responsible
for mapping from an NFS file (or portion of a file) to the blocks of
storage volumes that contain the file. The blocks are expressed as
extents with 64 bit offsets and lengths using the existing NFSv4
offset4 and length4 types. Clients must be able to perform I/O to
the block extents without affecting additional areas of storage
(especially important for writes), therefore extents MUST be aligned
to 512-octet boundaries, and SHOULD be aligned to the block size used
by the NFSv4 server in managing the actual filesystem (4 kilobytes
and 8 kilobytes are common block sizes). This block size is
available as the NFSv4.1 layout_blksize attribute. [NFSV4.1]
The pNFS operation for requesting a layout (LAYOUTGET) includes the
"layoutiomode4 loga_iomode" argument which indicates whether the
requested layout is for read-only use or read-write use. A read-only
layout may contain holes that are read as zero, whereas a read-write
layout will contain allocated, but un-initialized storage in those
holes (read as zero, can be written by client). This draft also
supports client participation in copy on write by providing both
read-only and un-initialized storage for the same range in a layout.
Reads are initially performed on the read-only storage, with writes
going to the un-initialized storage. After the first write that
initializes the un-initialized storage, all reads are performed to
that now-initialized writeable storage, and the corresponding read-
only storage is no longer used.
2.2. GETDEVICELIST and GETDEVICEINFO
2.2.1. Volume Identification
Storage Systems such as storage arrays can have multiple physical
network ports that need not be connected to a common network,
resulting in a pNFS client having simultaneous multipath access to
the same storage volumes via different ports on different networks.
The networks may not even be the same technology - for example,
access to the same volume via both iSCSI and Fibre Channel is
possible, hence network address are difficult to use for volume
identification. For this reason, this pNFS block layout identifies
storage volumes by content, for example providing the means to match
(unique portions of) labels used by volume managers. Any block pNFS
system using this layout MUST support a means of content-based unique
volume identification that can be employed via the data structure
given here.
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struct pnfs_block_sig_component4 { /* disk signature component */
offset4 sig_offset; /* octet offset of component
within signature block */
opaque contents<>; /* contents of this component of the
signature (this is opaque) */
};
Note that the opaque "contents" field in the
"pnfs_block_sig_component4" structure MUST NOT be interpreted as a
zero-terminated string, as it may contain embedded zero-valued
octets. There are no restrictions on alignment (e.g., neither
sig_offset nor the length are required to be multiples of 4). The
sig_offset represents an offset from the start of a signature block
(defined below).
The pNFS client block layout driver uses this volume identification
to map pnfs_block_volume_type4 VOLUME_SIMPLE deviceid4s to its local
view of a LUN.
2.2.2. Volume Topology
The pNFS block server volume topology is expressed as an arbitrary
combination of base volume types enumerated in the following data
structures.
enum pnfs_block_volume_type4 {
VOLUME_SIMPLE = 0, /* volume maps to a single LU */
VOLUME_SLICE = 1, /* volume is a slice of another volume */
VOLUME_CONCAT = 2, /* volume is a concatenation of multiple
volumes */
VOLUME_STRIPE = 3 /* volume is striped across multiple
volumes */
};
struct pnfs_block_simple_volume_info4 {
deviceid4 vol_id; /* this volume id */
int64_t sig_offset; /* offset in 512 octet blocks
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from start of volume if positive
from end of volume if negative
pnfs_block_sig_component4 ds<MAX_SIG_COMP>;
/* disk signature */
};
struct pnfs_block_slice_volume_info4 {
deviceid4 vol_id; /* this volume id */
offset4 start; /* offset of the start of the
slice in 512 octet blocks */
length4 length; /* length of slice in 512 octet blocks
*/
deviceid4 volume; /* volume which is sliced */
};
struct pnfs_block_concat_volume_info4 {
deviceid4 vol_id; /* this volume id */
deviceid4 volumes<>; /* volumes which are concatenated */
};
struct pnfs_block_stripe_volume_info4 {
deviceid4 vol_id; /* this volume id */
length4 stripe_unit; /* size of stripe in 512 octect
blocks */
deviceid4 volumes<>; /* volumes which are striped
across -- MUST be same size */
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};
union pnfs_block_volume4 switch (pnfs_block_volume_type4 type) {
case VOLUME_SIMPLE:
pnfs_block_simple_volume_info4 simple_info;
case VOLUME_SLICE:
pnfs_block_slice_volume_info4 slice_info;
case VOLUME_CONCAT:
pnfs_block_concat_volume_info4 concat_info;
case VOLUME_STRIPE:
pnfs_block_stripe_volume_info4 stripe_info;
};
struct pnfs_block_deviceaddr4 {
pnfs_block_volume4 volumes<>; /* array of volumes */
};
The "pnfs_block_deviceaddr4" data structure is a structure that
allows arbitrarily complex nested volume structures to be encoded.
The types of aggregations that are allowed are stripes,
concatenations, and slices. Note that the volume topology expressed
in the pnfs_block_deviceaddr4 data structure will always resolve to a
set of pnfs_block_volume_type4 VOLUME_SIMPLE. The array of volumes
is ordered such that the root volume is the last element of the
array. Concat, slice and stripe volumes MUST refer to volumes
defined by lower indexed elements of the array.
The "pnfs_block_device_addr4" data structure is returned by the
server as the storage-protocol-specific opaque field da_addr_body in
the "device_addr4" structure by successful GETDEVICELIST and
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GETDEVICEINFO operations. [NFSV4.1]. Typically the server in
response to a GETDEVICELIST request will return a single
"devlist_item4" in the gdlr_devinfo_list array. This is because the
"opaque da_addr_body" field inside the "device_addr4" encodes the
entire volume hierarchy. In the case of copy-on-write file systems,
the "gdlr_devinfo_list" array will contain two devices_item4s, one
describing the read-only volume hierarchy, and one describing the
writable volume hierarchy.
As noted above, all device_addr4 structures eventually resolve to a
set of volumes of type "pnfs_block_volume_type4 VOLUME_SIMPLE" These
volumes are each uniquely identified by a set of signature components
located within respective signature blocks. Each VOLUME_SIMPLE
volume specifies the location of its signature block in terms of 512
octet blocks. The "int64_t sig_offset" is a signed quantity which
when positive represents an offset from the start of the volume, and
when negative represents an offset from the end of the volume.
Negative offsets are permitted in order to simplify the client
implementation on systems where the device label is found at a fixed
offset from the end of the volume. If the server uses negative
offsets to describe the signature, then the client and server MUST
NOT see different volume sizes. Negative offsets SHOULD NOT be used
in systems that dynamically resize volumes unless care is taken to
ensure that the device label is always present at the offset from the
end of the volume as seen by the clients.
2.2.3. GETDEVICELIST and GETDEVICEINFO deviceid4
The "deviceid4 dli_id" returned in the devlist_item4 of a successful
GETDEVICELIST operation is a shorthand id used to reference the whole
volume topology. Decoding the "pnfs_block_deviceaddr4" results in a
flat ordering of 512 octet data blocks mapped to VOLUME_SIMPLE
deviceid4s. Combined with the deviceid4 mapping to a client LUN
described in 2.2.1 Volume Identification, a logical volume offset
can be mapped to a 512 block on a pNFS client LUN. [NFSV4.1] With
the exception of the root volume id, the device ids returned in the
volumes array of a pnfs_block_deviceaddr4 data structure should not
be passed as arguments in a GETDEVICEINFO request. These non-root
volume device ids are never returned by LAYOUTGET in the
"pnfs_block_layout4 vol_id" field. If a non-root device id is passed
as an argument in a GETDEVICEINFO request, the server SHOULD return
NFS4ERR_INVAL.
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2.3. Data Structures: Extents and Extent Lists
A pNFS block layout is a list of extents within a flat array of 512-
octet data blocks in a logical volume. The details of the volume
topology can be determined by using the GETDEVICEINFO or
GETDEVICELIST operation (see discussion of volume identification,
section 2.2 above). The block layout describes the individual block
extents on the volume that make up the file. The offsets and length
contained in an extent are specified in units of octets.
enum pnfs_block_extent_state4 {
READ_WRITE_DATA = 0, /* the data located by this extent is valid
for reading and writing. */
READ_DATA = 1, /* the data located by this extent is valid
for reading only; it may not be written.
*/
INVALID_DATA = 2, /* the location is valid; the data is
invalid. It is a newly (pre-) allocated
extent. There is physical space on the
volume. */
NONE_DATA = 3 /* the location is invalid. It is a hole in
the file. There is no physical space on
the volume. */
};
struct pnfs_block_extent4 {
offset4 file_offset; /* the starting octet offset in
the file */
length4 extent_length; /* the size in octets of the
extent */
offset4 storage_offset; /* the starting octet offset in
the volume */
pnfs_block_extent_state4 es; /* the state of this extent */
};
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struct pnfs_block_layout4 {
deviceid4 vol_id; /* id of logical volume on which
file is stored. */
pnfs_block_extent4 extents<>; /* extents which make up this
layout. */
};
The block layout consists of a deviceid4, shorthand for the whole
topology of the logical volume on which the file is stored, followed
by a list of extents which map the logical regions of the file to
physical locations on the volume. The "storage offset" field within
each extent identifies a location on the logical volume described by
the "volume" field in the layout. The client is responsible for
translating this logical offset into an offset on the appropriate
underlying SAN logical unit.
Each extent maps a logical region of the file onto a portion of the
specified logical volume. The file_offset, extent_length, and es
fields for an extent returned from the server are always valid. The
interpretation of the storage_offset field depends on the value of es
as follows (in increasing order):
o READ_WRITE_DATA means that storage_offset is valid, and points to
valid/initialized data that can be read and written.
o READ_DATA means that storage_offset is valid and points to valid/
initialized data which can only be read. Write operations are
prohibited; the client may need to request a read-write layout.
o INVALID_DATA means that storage_offset is valid, but points to
invalid un-initialized data. This data must not be physically read
from the disk until it has been initialized. A read request for
an INVALID_DATA extent must fill the user buffer with zeros. Write
requests must write whole server-sized blocks to the disk; octets
not initialized by the user must be set to zero. Any write to
storage in an INVALID_DATA extent changes the written portion of
the extent to READ_WRITE_DATA; the pNFS client is responsible for
reporting this change via LAYOUTCOMMIT.
o NONE_DATA means that storage_offset is not valid, and this extent
may not be used to satisfy write requests. Read requests may be
satisfied by zero-filling as for INVALID_DATA. NONE_DATA extents
may be returned by requests for readable extents; they are never
returned if the request was for a writeable extent.
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An extent list lists all relevant extents in increasing order of the
file_offset of each extent; any ties are broken by increasing order
of the extent state (es).
2.3.1. Layout Requests and Extent Lists
Each request for a layout specifies at least three parameters: file
offset, desired size, and minimum size. If the status of a request
indicates success, the extent list returned must meet the following
criteria:
o A request for a readable (but not writeable) layout returns only
READ_DATA or NONE_DATA extents (but not INVALID_DATA or
READ_WRITE_DATA extents).
o A request for a writeable layout returns READ_WRITE_DATA or
INVALID_DATA extents (but not NONE_DATA extents). It may also
return READ_DATA extents only when the offset ranges in those
extents are also covered by INVALID_DATA extents to permit writes.
o The first extent in the list MUST contain the starting offset.
o The total size of extents in the extent list MUST cover at least
the minimum size and no more than the desired size. One exception
is allowed: the total size MAY be smaller if only readable extents
were requested and EOF is encountered.
o Extents in the extent list MUST be logically contiguous for a
read-only layout. For a read-write layout, the set of writable
extents (i.e., excluding READ_DATA extents) MUST be logically
contiguous. Every READ_DATA extent in a read-write layout MUST be
covered by an INVALID_DATA extent. This overlap of READ_DATA and
INVALID_DATA extents is the only permitted extent overlap.
o Extents MUST be ordered in the list by starting offset, with
READ_DATA extents preceding INVALID_DATA extents in the case of
equal file_offsets.
2.3.2. Layout Commits
struct pnfs_block_layoutupdate4 {
pnfs_block_extent4 commit_list<>;/* list of extents to which now
contain valid data. */
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bool make_version; /* client requests server to
create copy-on-write image of
this file. */
};
The "pnfs_block_layoutupdate4" structure is used by the client as the
block-protocol specific argument in a LAYOUTCOMMIT operation. The
"commit_list" field is an extent list covering regions of the file
layout that were previously in the INVALID_DATA state, but have been
written by the client and should now be considered in the
READ_WRITE_DATA state. The es field of each extent in the
commit_list MUST be set to READ_WRITE_DATA. Implementers should be
aware that a server may be unable to commit regions at a granularity
smaller than a file-system block (typically 4KB or 8KB). As noted
above, the block-size that the server uses is available as an NFSv4
attribute, and any extents included in the "commit_list" MUST be
aligned to this granularity and have a size that is a multiple of
this granularity. If the client believes that its actions have moved
the end-of-file into the middle of a block being committed, the
client MUST write zeroes from the end-of-file to the end of that
block before committing the block. Failure to do so may result in
junk (uninitialized data) appearing in that area if the file is
subsequently extended by moving the end-of-file.
The "make_version" field of the structure is a flag that the client
may set to request that the server create a copy-on-write image of
the file (pNFS clients may be involved in this operation - see
section 2.2.4, below). In anticipation of this operation the client
which sets the "make_version" flag in the LAYOUTCOMMIT operation
should immediately mark all extents in the layout that is possesses
as state READ_DATA. Future writes to the file require a new
LAYOUTGET operation to the server with an "iomode" set to
LAYOUTIOMODE_RW.
2.3.3. Layout Returns
struct pnfs_block_layoutreturn4 {
pnfs_block_extent4 rel_list<>; /* list of extents the client
will no longer use. */
};
The "rel_list" field is an extent list covering regions of the file
layout that are no longer needed by the client. Including extents in
the "rel_list" for a LAYOUTRETURN operation represents an explicit
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release of resources by the client, usually done for the purpose of
avoiding unnecessary CB_LAYOUTRECALL operations in the future.
Note that the block/volume layout supports unilateral layout
revocation. When a layout is unilaterally revoked by the server,
usually due to the client's lease timer expiring or the client
failing to return a layout in a timely manner, it is important for
the sake of correctness that any in-flight I/Os that the client
issued before the layout was revoked are rejected at the storage.
For the block/volume protocol, this is possible by fencing a client
with an expired layout timer from the physical storage. Note,
however, that the granularity of this operation can only be at the
host/logical-unit level. Thus, if one of a client's layouts is
unilaterally revoked by the server, it will effectively render
useless *all* of the client's layouts for files located on the
storage units comprising the logical volume. This may render useless
the client's layouts for files in other filesystems.
2.3.4. Client Copy-on-Write Processing
Distinguishing the READ_WRITE_DATA and READ_DATA extent types in
combination with the allowed overlap of READ_DATA extents with
INVALID_DATA extents allows copy-on-write processing to be done by
pNFS clients. In classic NFS, this operation would be done by the
server. Since pNFS enables clients to do direct block access, it is
useful for clients to participate in copy-on-write operations. All
block/volume pNFS clients MUST support this copy-on-write processing.
When a client wishes to write data covered by a READ_DATA extent, it
MUST have requested a writable layout from the server; that layout
will contain INVALID_DATA extents to cover all the data ranges of
that layout's READ_DATA extents. More precisely, for any file_offset
range covered by one or more READ_DATA extents in a writable layout,
the server MUST include one or more INVALID_DATA extents in the
layout that cover the same file_offset range. When performing a write
to such an area of a layout, the client MUST effectively copy the
data from the READ_DATA extent for any partial blocks of file_offset
and range, merge in the changes to be written, and write the result
to the INVALID_DATA extent for the blocks for that file_offset and
range. That is, if entire blocks of data are to be overwritten by an
operation, the corresponding READ_DATA blocks need not be fetched,
but any partial-block writes must be merged with data fetched via
READ_DATA extents before storing the result via INVALID_DATA extents.
For the purposes of this discussion, "entire blocks" and "partial
blocks" refer to the server's file-system block size. Storing of
data in an INVALID_DATA extent converts the written portion of the
INVALID_DATA extent to a READ_WRITE_DATA extent; all subsequent reads
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MUST be performed from this extent; the corresponding portion of the
READ_DATA extent MUST NOT be used after storing data in an
INVALID_DATA extent.
In the LAYOUTCOMMIT operation that normally sends updated layout
information back to the server, for writable data, some INVALID_DATA
extents may be committed as READ_WRITE_DATA extents, signifying that
the storage at the corresponding storage_offset values has been
stored into and is now to be considered as valid data to be read.
READ_DATA extents are not committed to the server. For extents that
the client receives via LAYOUTGET as INVALID_DATA and returns via
LAYOUTCOMMIT as READ_WRITE_DATA, the server will understand that the
READ_DATA mapping for that extent is no longer valid or necessary for
that file.
2.3.5. Extents are Permissions
Layout extents returned to pNFS clients grant permission to read or
write; READ_DATA and NONE_DATA are read-only (NONE_DATA reads as
zeroes), READ_WRITE_DATA and INVALID_DATA are read/write,
(INVALID_DATA reads as zeros, any write converts it to
READ_WRITE_DATA). This is the only client means of obtaining
permission to perform direct I/O to storage devices; a pNFS client
MUST NOT perform direct I/O operations that are not permitted by an
extent held by the client. Client adherence to this rule places the
pNFS server in control of potentially conflicting storage device
operations, enabling the server to determine what does conflict and
how to avoid conflicts by granting and recalling extents to/from
clients.
Block/volume class storage devices are not required to perform read
and write operations atomically. Overlapping concurrent read and
write operations to the same data may cause the read to return a
mixture of before-write and after-write data. Overlapping write
operations can be worse, as the result could be a mixture of data
from the two write operations; data corruption can occur if the
underlying storage is striped and the operations complete in
different orders on different stripes. A pNFS server can avoid these
conflicts by implementing a single writer XOR multiple readers
concurrency control policy when there are multiple clients who wish
to access the same data. This policy SHOULD be implemented when
storage devices do not provide atomicity for concurrent read/write
and write/write operations to the same data.
If a client makes a layout request that conflicts with an existing
layout delegation, the request will be rejected with the error
NFS4ERR_LAYOUTTRYLATER. This client is then expected to retry the
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request after a short interval. During this interval the server
SHOULD recall the conflicting portion of the layout delegation from
the client that currently holds it. This reject-and-retry approach
does not prevent client starvation when there is contention for the
layout of a particular file. For this reason a pNFS server SHOULD
implement a mechanism to prevent starvation. One possibility is that
the server can maintain a queue of rejected layout requests. Each
new layout request can be checked to see if it conflicts with a
previous rejected request, and if so, the newer request can be
rejected. Once the original requesting client retries its request,
its entry in the rejected request queue can be cleared, or the entry
in the rejected request queue can be removed when it reaches a
certain age.
NFSv4 supports mandatory locks and share reservations. These are
mechanisms that clients can use to restrict the set of I/O operations
that are permissible to other clients. Since all I/O operations
ultimately arrive at the NFSv4 server for processing, the server is
in a position to enforce these restrictions. However, with pNFS
layout delegations, I/Os will be issued from the clients that hold
the delegations directly to the storage devices that host the data.
These devices have no knowledge of files, mandatory locks, or share
reservations, and are not in a position to enforce such restrictions.
For this reason the NFSv4 server MUST NOT grant layout delegations
that conflict with mandatory locks or share reservations. Further,
if a conflicting mandatory lock request or a conflicting open request
arrives at the server, the server MUST recall the part of the layout
delegation in conflict with the request before granting the request.
2.3.6. End-of-file Processing
The end-of-file location can be changed in two ways: implicitly as
the result of a WRITE or LAYOUTCOMMIT beyond the current end-of-file,
or explicitly as the result of a SETATTR request. Typically, when a
file is truncated by an NFSv4 client via the SETATTR call, the server
frees any disk blocks belonging to the file which are beyond the new
end-of-file octet, and may write zeros to the portion of the new end-
of-file block beyond the new end-of-file octet. These actions render
any pNFS layouts which refer to the blocks that are freed or written
semantically invalid. Therefore, the server MUST recall from clients
the portions of any pNFS layouts which refer to blocks that will be
freed or written by the server before processing the truncate
request. These recalls may take time to complete; as explained in
[NFSv4.1], if the server cannot respond to the client SETATTR request
in a reasonable amount of time, it SHOULD reply to the client with
the error NFS4ERR_DELAY.
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Blocks in the INVALID_DATA state which lie beyond the new end-of-file
block present a special case. The server has reserved these blocks
for use by a pNFS client with a writable layout for the file, but the
client has yet to commit the blocks, and they are not yet a part of
the file mapping on disk. The server MAY free these blocks while
processing the SETATTR request. If so, the server MUST recall any
layouts from pNFS clients which refer to the blocks before processing
the truncate. If the server does not free the INVALID_DATA blocks
while processing the SETATTR request, it need not recall layouts
which refer only to the INVALID DATA blocks.
When a file is extended implicitly by a WRITE or LAYOUTCOMMIT beyond
the current end-of-file, or extended explicitly by a SETATTR request,
the server need not recall any portions of any pNFS layouts.
2.3.7. Client Fencing
The pNFS block protocol must handle situations in which a system
failure, typically a network connectivity issue, requires the server
to unilaterally revoke extents from one client in order to transfer
the extents to another client. The pNFS server implementation MUST
ensure that when resources are transferred to another client, they
are not used by the client originally owning them, and this must be
ensured against any possible combination of partitions and delays
among all of the participants to the protocol (server, storage and
client). Several approaches to guaranteeing this isolation are
possible and are discussed below.
One server based implementation choice for fencing is to use the
STOMITH (Shoot The Other Machine In The Head) protocol, i.e., turn
off the power to the client machine that needs to be isolated. This
is possible if the server has access to either an IPMI interface to
power cycle the client, or an alternate method of turning off power
to a non-communicative client. The client SHOULD be kept powered off
for at least the duration of the server lease time, as it is
possible, although untypical, that the client caches the layout
information on persistent storage. This approach can in some
instances guarantee that the rogue client no longer is capable of
accessing the storage. However, in other situations, for example
lack of TCP/IP access to the client's IPMI network address, this
approach cannot guarantee anything.
Another implementation choice for fencing the block client from the
block storage is the use of LUN (Logical Unit Number) masking or
mapping at the storage systems or storage area network to disable
access by the client to be isolated. In contrast to the STOMITH
approach, this requires server access to a management interface for
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the storage system and authorization to perform LUN masking and
management operations. For example, SMI-S [SMIS] provides a means to
discover and mask LUNs, including a means of associating clients with
the necessary World Wide Names or Initiator names to be masked.
In the absence of support for other mechanisms, the server MUST
choose to rely on the clients to implement a timed lease I/O fencing
mechanism. Because clients do not know if the server is using
STOMITH or LUN masking. in all cases the client MUST implement timed
lease fencing. In timed lease fencing we define two time periods,
the first, "lease_time" is the length of a lease as defined by the
server's lease_time attribute (see Section 5.4 of [NFSV4.1]), and the
second, "maximum_io_time" is the maximum time it can take for a
client I/O to the storage system to either complete or fail; this
value is often 30 seconds or 60 seconds, but may be longer in some
environments. If the maximum client I/O time cannot be bounded, this
timed lease mechanism MUST NOT be used. The client can use GETATTR
to query the server's default setting of "maximum_io_time". The
server must respond with the maximum I/O time in seconds. If the
client's maximum I/O time is greater than the server's default, then
the client MUST use SETATTR to inform the server of its maximum_I/O
time. Using these two time span values, we specify the behavior of
the client and server as follows.
When a client receives layout information via a LAYOUTGET operation,
those layouts are valid for at most "lease_time" seconds from when
the server granted them. A layout is renewed by any successful
SEQUEUNCE operation, or whenever a new stateid is created or updated
(see the section "Lease Renewal" of [NFSV4.1]). If the layout lease
is not renewed prior to expiration, the client MUST cease to use the
layout after "lease_time" seconds from when it either sent the
original LAYOUTGET command, or sent the last operation renewing the
lease. In other words, the client may not issue any I/O to blocks
specified by an expired layout. In the presence of large
communication delays between the client and server it is even
possible for the lease to expire prior to the server response
arriving at the client. In such a situation the client MUST NOT use
the expired layouts, and SHOULD revert to using standard NFSv41 READ
and WRITE operations. Furthermore, the client must be configured
such that I/O operations complete within the "maximum_io_time" even
in the presence of multipathing drivers that will retry I/Os via
multiple paths. If a client cannot guarantee a bounded maximum I/O
time, it MUST NOT use pNFS.
As stated in the section "Dealing with Lease Expiration on the
Client" of [NFSV4.1], if any SEQUENCE operation is successful, but
sr_status_flag has SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED,
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SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED, or
SEQ4_STATUS_ADMIN_STATE_REVOKED set, the client MUST immediately
cease to use all layouts and device id to device address mappings
associated with the corresponding server.
In the absence of known two way communication between the client and
the server on the fore channel, the server must wait for at least the
time period "lease_time" plus "maximum_io_time" before transferring
layouts from the original client to any other client. The server,
like the client, must take a conservative approach, and start the
lease expiration timer from the time that it received the operation
which last renewed the lease.
2.4. Crash Recovery Issues
When the server crashes while the client holds a writable layout, and
the client has written data to blocks covered by the layout, and the
blocks are still in the INVALID_DATA state, the client has two
options for recovery. If the data that has been written to these
blocks is still cached by the client, the client can simply re-write
the data via NFSv4, once the server has come back online. However,
if the data is no longer in the client's cache, the client MUST NOT
attempt to source the data from the data servers. Instead, it should
attempt to commit the blocks in question to the server during the
server's recovery grace period, by sending a LAYOUTCOMMIT with the
"loca_reclaim" flag set to true. This process is described in detail
in [NFSv4.1] section 18.42.4.
2.5. Recalling resources: CB_RECALL_ANY
The server may decide that it cannot hold all of the state for
layouts without running out of resources. In such a case, it is free
to recall individual layouts using CB_LAYOUTRECALL to reduce the
load, or it may choose to request that the client return any layout.
For the block layout we define the following bit
const RCA4_BLK_LAYOUT_RECALL_ANY_LAYOUTS = 4
When the server sends a CB_RECALL_ANY request to a client specifying
the RCA4_BLK_LAYOUT_RECALL_ANY_LAYOUTS bit in craa_type_mask, the
client should immediately respond with NFS4_OK, and then
asynchronously return complete file layouts until the number of files
with layouts cached on the client is less the craa_object_to_keep.
The block layout does not currently use bits 5, 6 or 7. If any of
these bits are set, the client should return NFS4ERR_INVAL.
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2.6. Transient and Permanent Errors
The server may respond to LAYOUTGET with a variety of error statuses.
These errors can convey transient conditions or more permanent
conditions that are unlikely to be resolved soon.
The transient errors, NFS4ERR_RECALLCONFLICT and NFS4ERR_TRYLATER are
used to indicate that the server cannot immediately grant the layout
to the client. In the former case this is because the server has
recently issued a CB_LAYOUTRECALL to the requesting client, whereas
in the case of NFS4ERR_TRYLATER, the server cannot grant the request
possibly due to sharing conflicts with other clients. In either
case, a reasonable approach for the client is to wait several
milliseconds and retry the request. The client SHOULD track the
number of retries, and if forward progress is not made, the client
SHOULD send the READ or WRITE operation directly to the server.
The error NFS4ERR_LAYOUTUNAVAILABLE may be returned by the server if
layouts are not supported for the requested file or its containing
file system. The server may also return this error code if the
server is the progress of migrating the file from secondary storage,
or for any other reason which causes the server to be unable to
supply the layout. As a result of receiving
NFS4ERR_LAYOUTUNAVAILABLE, the client SHOULD send future READ and
WRITE requests directly to the server. It is expected that a client
will not cache the file's layoutunavailable state forever, particular
if the file is closed, and thus eventually, the client MAY reissue a
LAYOUTGET operation.
3. Security Considerations
Typically, SAN disk arrays and SAN protocols provide access control
mechanisms (access-logics, lun masking, etc.) which operate at the
granularity of individual hosts. The functionality provided by such
mechanisms makes it possible for the server to "fence" individual
client machines from certain physical disks---that is to say, to
prevent individual client machines from reading or writing to certain
physical disks. Finer-grained access control methods are not
generally available. For this reason, certain security
responsibilities are delegated to pNFS clients for block/volume
layouts. Block/volume storage systems generally control access at a
volume granularity, and hence pNFS clients have to be trusted to only
perform accesses allowed by the layout extents they currently hold
(e.g., and not access storage for files on which a layout extent is
not held). In general, the server will not be able to prevent a
client which holds a layout for a file from accessing parts of the
physical disk not covered by the layout. Similarly, the server will
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not be able to prevent a client from accessing blocks covered by a
layout that it has already returned. This block-based level of
protection must be provided by the client software.
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 the
security requirements are such that client-side protection from
access to storage outside of the layout is not sufficient pNFS
block/volume storage layouts for pNFS SHOULD NOT be used, unless the
storage device is able to implement the appropriate access checks,
via use of virtualized block addresses, or other means.
This also has implications for some NFSv4 functionality outside pNFS.
For instance, if a file is covered by a mandatory read-only lock, the
server can ensure that only readable layouts for the file are granted
to pNFS clients. However, it is up to each pNFS client to ensure
that the readable layout is used only to service read requests, and
not to allow writes to the existing parts of the file. Since
block/volume storage systems are generally not capable of enforcing
such file-based security, in environments where pNFS clients cannot
be trusted to enforce such policies, pNFS block/volume storage
layouts SHOULD NOT be used.
Access to block/volume storage is logically at a lower layer of the
I/O stack than NFSv4, and hence NFSv4 security is not directly
applicable to protocols that access such storage directly. Depending
on the protocol, some of the security mechanisms provided by NFSv4
(e.g., encryption, cryptographic integrity) may not be available, or
may be provided via different means. At one extreme, pNFS with
block/volume storage can be used with storage access protocols (e.g.,
parallel SCSI) that provide essentially no security functionality.
At the other extreme, pNFS may be used with storage protocols such as
iSCSI that provide significant functionality. It is the
responsibility of those administering and deploying pNFS with a
block/volume storage access protocol to ensure that appropriate
protection is provided to that protocol (physical security is a
common means for protocols not based on IP). In environments where
the security requirements for the storage protocol cannot be met,
pNFS block/volume storage layouts SHOULD NOT be used.
When security is available for a storage protocol, it is generally at
a different granularity and with a different notion of identity than
NFSv4 (e.g., NFSv4 controls user access to files, iSCSI controls
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initiator access to volumes). The responsibility for enforcing
appropriate correspondences between these security layers is placed
upon the pNFS client. As with the issues in the first paragraph of
this section, in environments where the security requirements are
such that client-side protection from access to storage outside of
the layout is not sufficient, pNFS block/volume storage layouts
SHOULD NOT be used.
4. Conclusions
This draft specifies the block/volume layout type for pNFS and
associated functionality.
5. IANA Considerations
There are no IANA considerations in this document. All pNFS IANA
Considerations are covered in [NFSV4.1].
6. Revision History
-00: Initial Version as draft-black-pnfs-block-00
-01: Rework discussion of extents as locks to talk about extents
granting access permissions. Rewrite operation ordering section to
discuss deadlocks and races that can cause problems. Add new section
on recall completion. Add client copy-on-write based on text from
Craig Everhart.
-02: Fix glitches in extent state descriptions. Describe most issues
as RESOLVED. Most of Section 3 has been incorporated into the the
main PNFD draft, add NOTE to that effect and say that it will be
deleted in the next version of this draft (which should be a draft-
ietf-nfsv4 draft). Cleaning up a number of things have been left to
that draft revision, including the interlocks with the types in the
main pNFS draft, layout striping support, and finishing the Security
Considerations section.
-00: New version as draft-ietf-nfsv4-pnfs-block. Removed resolved
operations issues (Section 3). Align types with main pNFS draft
(which is now part of the NFSv4.1 minor version draft), add volume
striping and slicing support. New operations issues are in Section 3
- the need for a "reclaim bit" and EOF concerns are the two major
issues. Extended and improved the Security Considerations section,
but it still needs work. Added 1-sentence conclusion that also still
needs work.
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-01: Changed definition of pnfs_block_deviceaddr4 union to allow more
concise representation of aggregated volume structures. Fixed typos
to make both pnfs_block_layoutupdate and pnfs_block_layoutreturn
structures contain extent lists instead of a single extent. Updated
section 2.1.6 to remove references to CB_SIZECHANGED. Moved
description of recovery from "Issues" section to "Block Layout
Description" section. Removed section 3.2 "End-of-file handling
issues". Merged old "block/volume layout security considerations"
section from previous version of [NFSv4.1] with section 4. Moved
paragraph on lingering writes to the section which describes layout
return. Removed Issues section (3) as the remaining issues are all
resolved.
02: Changed pnfs_deviceaddr4 to deviceaddr4 to match [NFSv4.1].
Updated section 2.2.2 to clarify that the es fields must be
READ_WRITE_DATA in pnfs_block_layoutupdate requests. Updated section
2.2.5 to specify that data corruption can occur; that requests, not
the client, are rejected; that server "SHOULD" recall conflicting
portions of layouts. Clarified that unilateral revocation may affect
layouts from other filesystems. Changed signature offset to be a
signed quantity to allow for labels at a fixed location from the end
of a volume. Changed all data structures to have suffix "4", changed
extentState4 to pnfs_block_extent_state4 and sigComponent to
pnfs_block_sig_component4, to conform to [NFSv4.1].
03: Moved sections GETDEVICELIST and GETDEVICEINFO earlier in
document for better readability. Added pnfs_block_simple_volume4
data structure, and added volume_id fields to all pnfs_block volume
info data structures.
04: Added information about device ids to clarify their usage.
Described where the pnfs_block_deviceaddr4 data structure is found to
be accurate with draft-ietf-nfsv4-minorversion1-14. Updated
references from -08 to -14. Removed root_id from
pnfs_block_deviceaddr4. Changed 'byte' to 'octet'. Clarify the
block size and stripe size in volume data structures. Rename
'volume' and 'id' to be 'vol_id' consistently. Added sections on
CB_RECALL_ANY and fencing.
7. Acknowledgments
This draft draws extensively on the authors' familiarity with the
mapping functionality and protocol in EMC's HighRoad system
[HighRoad]. The protocol used by HighRoad is called FMP (File
Mapping Protocol); it is an add-on protocol that runs in parallel
with filesystem protocols such as NFSv3 to provide pNFS-like
functionality for block/volume storage. While drawing on HighRoad
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FMP, the data structures and functional considerations in this draft
differ in significant ways, based on lessons learned and the
opportunity to take advantage of NFSv4 features such as COMPOUND
operations. The design to support pNFS client participation in copy-
on-write is based on text and ideas contributed by Craig Everhart
(formerly with IBM).
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[NFSV4.1] Shepler, S., Eisler, M., and Noveck, D. ed., "NFSv4 Minor
Version 1", draft-ietf-nfsv4-minorversion1-14.txt, Internet
Draft, July 2007.
8.2. Informative References
[HighRoad] EMC Corporation, "EMC Celerra HighRoad", EMC C819.1 white
paper, available at:
http://www.emc.com/pdf/products/celerra_file_server/HighRoad_wp.pdf
link checked 29 August 2006.
[SMIS] SNIA, "SNIA Storage Management Initiative Specification",
version 1.0.2, available at:
http://www.snia.org/smi/tech_activities/smi_spec_pr/spec/SMIS_1_0_2_f
inal.pdf
Author's Addresses
David L. Black
EMC Corporation
176 South Street
Hopkinton, MA 01748
Phone: +1 (508) 293-7953
Email: black_david@emc.com
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Stephen Fridella
EMC Corporation
228 South Street
Hopkinton, MA 01748
Phone: +1 (508) 249-3528
Email: fridella_stephen@emc.com
Jason Glasgow
EMC Corporation
32 Coslin Drive
Southboro, MA 01772
Phone: +1 (508) 305 8831
Email: glasgow_jason@emc.com
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