Parallel NFS (pNFS) SCSI Layout
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The Parallel Network File System (pNFS) allows a separation between
the metadata (onto a metadata server) and data (onto a storage device)
for a file. The SCSI Layout Type is defined in this document as an
extension to pNFS to allow the use SCSI based block storage devices.
shows the overall
architecture of a Parallel NFS (pNFS) system:
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 Section 12 of for more details.
This document is concerned with access from pNFS clients to
storage devices over block storage protocols based on
the the SCSI Architecture Model (),
e.g., Fibre Channel Protocol (FCP) for Fibre Channel, Internet SCSI
(iSCSI) or Serial Attached SCSI (SAS). pNFS SCSI layout requires
block based SCSI command sets, for example SCSI Block Commands
(). While SCSI command set for non-block based
access exist these are not supported by the SCSI layout type, and
all future references to SCSI storage devices will imply a block
based SCSI command set.
The Server to Storage System protocol, called the "Control Protocol",
is not of concern for interoperability, although it will typically be
the same SCSI based storage protocol.
This document is based on and makes changes to
the block layout type to provide a better pNFS layout protocol for
SCSI based storage devices. Despite these changes,
remains the defining document for the existing
block layout type. is unnecessary in the context
of the SCSI layout type because the new layout type provides mandatory
disk access protection as part of the layout type definition. In contrast
to , this document uses SCSI protocol features
to provide reliable fencing by using SCSI Persistent Reservations, and it
can provide reliable and efficient device discovery by using SCSI device
identifiers instead of having to rely on probing all devices potentially
attached to a client. This new layout type also optimizes the I/O path
by reducing the size of the LAYOUTCOMMIT payload. Except for these
changes the protocol is identical to , most
importantly there are no changes to way the volume topology is built
, and the data structures that describe
extents are unchanged compared to
as well.
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 .
The following definitions are provided for the purpose of providing
an appropriate context for the reader.
This document defines a byte as an octet, i.e., a datum exactly 8
bits in length.
The "client" is the entity that accesses the NFS server's
resources. The client may be an application that contains the
logic to access the NFS server directly. The client may also be
the traditional operating system client that provides remote file
system services for a set of applications.
The "server" is the entity responsible for coordinating client
access to a set of file systems and is identified by a server
owner.
The external data representation (XDR) description and scripts
for extracting the XDR description are Code Components as
described in Section 4 of "Legal Provisions
Relating to IETF Documents". These Code Components are
licensed according to the terms of Section 4 of "Legal Provisions
Relating to IETF Documents".
This document contains the XDR description
of the NFSv4.1 SCSI layout protocol. The XDR description is
embedded in this document in a way that makes it simple for the
reader to extract into a ready-to-compile form. The reader can
feed this document into the following shell script to produce
the machine readable XDR description of the NFSv4.1 SCSI layout:
That is, if the above script is stored in a file called "extract.sh", and
this document is in a file called "spec.txt", then the reader can do:
The effect of the script is to remove leading white space from each
line, plus a sentinel sequence of "///".
The embedded XDR file header follows.
Subsequent XDR descriptions, with the sentinel sequence are
embedded throughout the document.
Note that the XDR code contained in this document depends on
types from the NFSv4.1 nfs4_prot.x file .
This includes both nfs types that end with a 4, such as
offset4, length4, etc., as well as more generic types such as
uint32_t and uint64_t.
The fundamental storage model supported by SCSI storage devices
is a Logical Unit (LU) consisting of a sequential series of fixed-size
blocks. Logical units used as devices for NFS SCSI layouts,
and the SCSI initiators used for the pNFS Metadata Server and clients
MUST support SCSI persistent reservations as defined in
.
A pNFS layout for this SCSI 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 logical block size boundaries of the underlying logical units
(typically 512 or 4096 bytes). For complex volume topologies the
serves MUST ensure extents are aligned to the logical block size
boundaries of the larges logical block size in the volume topology.
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 document also
supports client participation in copy-on-write (e.g., for file
systems with snapshots) 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 writable storage, and the corresponding read-only storage
is no longer used.
The SCSI layout solution expands the security responsibilities of the
pNFS clients, and there are a number of environments where the mandatory
to implement security properties for NFS cannot be satisfied. The
additional security responsibilities of the client follow, and a full
discussion is present in ,
"Security Considerations".
Typically, SCSI storage devices provide access control mechanisms
(e.g., Logical Unit Number (LUN) mapping and/or masking), which
operate at the granularity of individual hosts, not individual
blocks. For this reason, block-based protection must be provided
by the client software.
Similarly, SCSI storage devices typically are not able to validate
NFS locks that apply to file regions. 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 SCSI storage devices are generally not capable of
enforcing such file-based security, in environments where pNFS
clients cannot be trusted to enforce such policies, pNFS SCSI
layouts SHOULD NOT be used.
The layout4 type defined in
is extended with a new value as follows:
This document defines structure associated with the layouttype4
value LAYOUT4_SCSI. specifies
the loc_body structure as an XDR type "opaque". The opaque
layout is uninterpreted by the generic pNFS client layers, but
obviously must be interpreted by the Layout Type implementation.
SCSI targets implementing export unique LU
names for each LU through the Device Identification VPD page (page code
0x83), which can be obtained using the INQUIRY command with the EVPD
bit set to one. This document uses a subset of this information to
identify LUs backing pNFS SCSI layouts. Device Identification VPD page
descriptors used to identify LUs for use with pNFS SCSI layouts must
adhere to the following restrictions:
The "ASSOCIATION" MUST be set to 0 (The DESIGNATOR field is
associated with the addressed logical unit).The "DESIGNATOR TYPE" MUST be set to one of four values
that are required for the mandatory logical unit name in
section 7.7.3 of , as explicitly listed
in the "pnfs_scsi_designator_type" enumeration:
T10 vendor ID based
EUI-64-based
NAA
SCSI name string
Any other association or designator type MUST NOT be used.
Use of T10 vendor IDs is discouraged when one of the other types
can be used.
The "CODE SET" VPD page field is stored in the "sbv_code_set" field of
the "pnfs_scsi_base_volume_info4" structure, the "DESIGNATOR TYPE" is
stored in "sbv_designator_type", and the DESIGNATOR is stored in
"sbv_designator". Due to the use of a XDR array the "DESIGNATOR LENGTH"
field does not need to be set separately. Only certain combinations
of "sbv_code_set" and "sbv_designator_type" are valid, please refer to
for details, and note that ASCII may be used
as the code set for UTF-8 text that contains only printable
ASCII characters.
Note that a Device Identification VPD page MAY contain multiple
descriptors with the same association, code set and designator type.
NFS clients thus MUST check all the descriptors for a possible match
to "sbv_code_set", "sbv_designator_type" and "sbv_designator".
Storage devices 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.
Selection of one or multiple ports to access the storage device
is left up to the client.
Additionally the server returns a Persistent Reservation key in
the "sbv_pr_key" field. See for more
details on the use of Persistent Reservations.
The pNFS SCSI layout volume topology is expressed in terms of the
volume types described below. The individual components of the
topology are contained in an array and components may refer to
other components by using array indices.
The "pnfs_scsi_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_scsi_deviceaddr4 data structure will always resolve to a
set of pnfs_scsi_volume_type4 PNFS_SCSI_VOLUME_BASE. The array
of volumes is ordered such that the root of the volume hierarchy 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_scsi_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 a successful GETDEVICEINFO operation
.
As noted above, all device_addr4 structures eventually resolve to a
set of volumes of type PNFS_SCSI_VOLUME_BASE.
Complicated volume hierarchies may be composed of dozens of volumes
each with several components; thus, the device address may
require several kilobytes. The client SHOULD be prepared to allocate
a large buffer to contain the result. In the case of the server
returning NFS4ERR_TOOSMALL, the client SHOULD allocate a buffer of at
least gdir_mincount_bytes to contain the expected result and retry
the GETDEVICEINFO request.
A pNFS SCSI layout is a list of extents within a flat array of data
blocks in a volume. The details of the volume topology can
be determined by using the GETDEVICEINFO operation. The SCSI 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 bytes.
The SCSI layout consists of a list of extents that map the regions
of the file to locations on a volume. The "se_storage_offset" field
within each extent identifies a location on the volume specified by
the "se_vol_id" field in the extent.
The se_vol_id itself is shorthand for the whole topology of the
volume on which the file is stored. The client is responsible for
translating this volume-relative offset into an offset on the
appropriate underlying SCSI LU.
Each extent maps a region of the file onto a portion of the
specified LU. The se_file_offset, se_length, and se_state fields for
an extent returned from the server are valid for all extents. In
contrast, the interpretation of the se_storage_offset field depends on
the value of se_state as follows (in increasing order):
means that se_storage_offset is valid, and points to
valid/initialized data that can be read and written.
means that se_storage_offset is valid and points to valid/initialized
data that can only be read. Write operations are prohibited; the
client may need to request a read-write layout.
means that se_storage_offset is valid, but points to invalid
un-initialized data. This data must not be read from the
disk until it has been initialized. A read request for a
PNFS_SCSI_INVALID_DATA extent must fill the user buffer with zeros,
unless the extent is covered by a PNFS_SCSI_READ_DATA extent of a
copy-on-write file system. Write requests must write whole
server-sized blocks to the disk; bytes not initialized by the user
must be set to zero. Any write to storage in a
PNFS_SCSI_INVALID_DATA extent changes the written portion of the
extent to PNFS_SCSI_READ_WRITE_DATA; the pNFS client is responsible
for reporting this change via LAYOUTCOMMIT.
means that se_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 PNFS_SCSI_INVALID_DATA. PNFS_SCSI_NONE_DATA
extents may be returned by requests for readable extents; they are
never returned if the request was for a writable extent.
An extent list contains all relevant extents in increasing order of
the se_file_offset of each extent; any ties are broken by increasing
order of the extent state (se_state).
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:
A request for a readable (but not writable) layout MUST return
either PNFS_SCSI_READ_DATA or PNFS_SCSI_NONE_DATA extents. It
SHALL NOT return PNFS_SCSI_INVALID_DATA or
PNFS_SCSI_READ_WRITE_DATA extents.
A request for a writable layout MUST return
PNFS_SCSI_READ_WRITE_DATA or PNFS_SCSI_INVALID_DATA extents,
and it MAY return addition PNFS_SCSI_READ_DATA extents for
ranges covered by PNFS_SCSI_INVALID_DATA extents to allow
client side copy-on-write operations. A request for a writable
layout SHALL NOT return PNFS_SCSI_NONE_DATA extents.
The first extent in the list MUST contain the requested starting
offset.
The total size of extents within the requested range MUST cover at
least the minimum size. One exception is allowed: the total size
MAY be smaller if only readable extents were requested and EOF is
encountered.
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 PNFS_SCSI_READ_DATA extents) MUST be
logically contiguous. Every PNFS_SCSI_READ_DATA extent in a
read-write layout MUST be covered by one or more
PNFS_SCSI_INVALID_DATA extents. This overlap of
PNFS_SCSI_READ_DATA and PNFS_SCSI_INVALID_DATA extents is the
only permitted extent overlap.
Extents MUST be ordered in the list by starting offset, with
PNFS_SCSI_READ_DATA extents preceding PNFS_SCSI_INVALID_DATA
extents in the case of equal se_file_offsets.
According to , if the minimum requested
size, loga_minlength, is zero, this is an indication to the
metadata server that the client desires any layout at offset
loga_offset or less that the metadata server has "readily
available". Given the lack of a clear definition of this phrase,
in the context of the SCSI layout type, when loga_minlength is
zero, the metadata server SHOULD:
when processing requests for readable layouts, return all
such, even if some extents are in the PNFS_SCSI_NONE_DATA
state.
when processing requests for writable layouts, return
extents which can be returned in the PNFS_SCSI_READ_WRITE_DATA
state.
The "pnfs_scsi_layoutupdate4" structure is used by the client as the
SCSI layout-specific argument in a LAYOUTCOMMIT operation. The
"slu_commit_list" field is a list covering regions of the file layout
that were previously in the PNFS_SCSI_INVALID_DATA state, but have
been written by the client and should now be considered in the
PNFS_SCSI_READ_WRITE_DATA state. The extents in the commit list MUST
be disjoint and MUST be sorted by sr_file_offset. Implementors should
be aware that a server may be unable to commit regions at a granularity
smaller than a file-system block (typically 4 KB or 8 KB). As noted
above, the block-size that the server uses is available as an NFSv4
attribute, and any extents included in the "slu_commit_list" MUST be
aligned to this granularity and have a size that is a multiple of
this granularity.
Since the block in question is in state PNFS_SCSI_INVALID_DATA,
byte ranges not written should be filled with zeros. This applies
even if it appears that the area being written is beyond what the
client believes to be the end of file.
A LAYOUTRETURN operation represents an explicit release of
resources by the client. This may be done in response to a
CB_LAYOUTRECALL or before any recall, in order to avoid a future
CB_LAYOUTRECALL. When the LAYOUTRETURN operation specifies a
LAYOUTRETURN4_FILE return type, then the layoutreturn_file4 data
structure specifies the region of the file layout that is no
longer needed by the client.
The LAYOUTRETURN operation is done without any SCSI layout
specific data. The opaque "lrf_body" field of the
"layoutreturn_file4" data structure MUST have length zero.
Layouts may be unilaterally revoked by the server, due to the
client's lease time expiring, or the client failing to return a
layout which has been recalled in a timely manner. For the SCSI
layout type this is accomplished by fencing off the client from
access to storage as described in .
When this is done, it is necessary that all I/Os issued by the
fenced-off client be rejected by the storage This includes any
in-flight I/Os that the client issued before the layout was
revoked.
Note, that the granularity of this operation can only be at the
host/LU 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 volume. This may render
useless the client's layouts for files in other file systems.
See for a discussion of
recovery from from fencing.
Copy-on-write is a mechanism used to support file and/or file system
snapshots. When writing to unaligned regions, or to regions smaller
than a file system block, the writer must copy the portions of the
original file data to a new location on disk. This behavior can
either be implemented on the client or the server. The paragraphs
below describe how a pNFS SCSI layout client implements access to a
file that requires copy-on-write semantics.
Distinguishing the PNFS_SCSI_READ_WRITE_DATA and
PNFS_SCSI_READ_DATA extent types in combination with the allowed
overlap of PNFS_SCSI_READ_DATA extents with PNFS_SCSI_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 SCSI
pNFS clients MUST support this copy-on-write processing.
When a client wishes to write data covered by a PNFS_SCSI_READ_DATA
extent, it MUST have requested a writable layout from the server;
that layout will contain PNFS_SCSI_INVALID_DATA extents to cover all
the data ranges of that layout's PNFS_SCSI_READ_DATA extents. More
precisely, for any se_file_offset range covered by one or more
PNFS_SCSI_READ_DATA extents in a writable layout, the server MUST
include one or more PNFS_SCSI_INVALID_DATA extents in the layout
that cover the same se_file_offset range. When performing a write
to such an area of a layout, the client MUST effectively copy the
data from the PNFS_SCSI_READ_DATA extent for any partial blocks of
se_file_offset and range, merge in the changes to be written, and
write the result to the PNFS_SCSI_INVALID_DATA extent for the blocks
for that se_file_offset and range. That is, if entire blocks of
data are to be overwritten by an operation, the corresponding
PNFS_SCSI_READ_DATA blocks need not be fetched, but any partial-
block writes must be merged with data fetched via
PNFS_SCSI_READ_DATA extents before storing the result via
PNFS_SCSI_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 a
PNFS_SCSI_INVALID_DATA extent converts the written portion of the
PNFS_SCSI_INVALID_DATA extent to a PNFS_SCSI_READ_WRITE_DATA
extent; all subsequent reads MUST be performed from this extent; the
corresponding portion of the PNFS_SCSI_READ_DATA extent MUST NOT be
used after storing data in a PNFS_SCSI_INVALID_DATA extent. If a
client writes only a portion of an extent, the extent may be split at
block aligned boundaries.
When a client wishes to write data to a PNFS_SCSI_INVALID_DATA
extent that is not covered by a PNFS_SCSI_READ_DATA extent, it MUST
treat this write identically to a write to a file not involved with
copy-on-write semantics. Thus, data must be written in at least
block-sized increments, aligned to multiples of block-sized offsets,
and unwritten portions of blocks must be zero filled.
Layout extents returned to pNFS clients grant permission to read or
write; PNFS_SCSI_READ_DATA and PNFS_SCSI_NONE_DATA are read-only
(PNFS_SCSI_NONE_DATA reads as zeroes), PNFS_SCSI_READ_WRITE_DATA
and PNFS_SCSI_INVALID_DATA are read/write, (PNFS_SCSI_INVALID_DATA
reads as zeros, any write converts it to PNFS_SCSI_READ_WRITE_DATA).
This is the only means a client has 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.
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
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
layouts, I/Os will be issued from the clients that hold the layouts
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 layouts 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 in conflict
with the request before granting the request.
SCSI storage devices do not provide byte granularity access and can
only perform read and write operations atomically on a block
granularity. WRITES to SCSI storage devices thus require
read-modify-write cycles to write data smaller than the block size
or which is otherwise not block-aligned.
Write operations from multiple clients to the same block can thus
lead to data corruption even if the byte range written by the
applications does not overlap.
When there are multiple clients who wish to access the same
block, a pNFS server MUST avoid these conflicts by implementing a
concurrency control policy of single writer XOR multiple readers for
a given data block.
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 that are
beyond the new end-of-file byte, and MUST write zeros to the
portion of the new end-of-file block beyond the new end-of-file
byte. These actions render any pNFS layouts that refer to the
blocks that are freed or written semantically invalid. Therefore,
the server MUST recall from clients the portions of any pNFS
layouts that refer to blocks that will be freed or written by
the server before effecting the file truncation. These recalls
may take time to complete; as explained in ,
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.
Blocks in the PNFS_SCSI_INVALID_DATA state that 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 that refer to the blocks before
processing the truncate. If the server does not free the
PNFS_SCSI_INVALID_DATA blocks while processing the SETATTR request,
it need not recall layouts that refer only to the
PNFS_SCSI_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.
The layout hint attribute specified in
is not supported by the SCSI layout, and the pNFS server MUST
reject setting a layout hint attribute with a loh_type value
of LAYOUT4_SCSI_VOLUME during OPEN or SETATTR operations. On a
file system only supporting the SCSI layout a server MUST NOT
report the layout_hint attribute in the supported_attrs attribute.
The pNFS SCSI protocol must handle situations in which a system
failure, typically a network connectivity issue, requires the server
to unilaterally revoke extents from a client after the client fails
to respond to a CB_LAYOUTRECALL request. This is implemented by
fencing off a non-responding client from access to the storage
device.
The pNFS SCSI protocol implements fencing using Persistent
Reservations (PRs), similar to the fencing method used by existing
shared disk file systems. By placing a PR of type
"Exclusive Access – Registrants Only" on each SCSI LU exported to
pNFS clients the MDS prevents access from any client that
does not have an outstanding device device ID that gives the client
a reservation key to access the LU, and allows the MDS to
revoke access to the logic unit at any time.
To allow fencing individual systems, each system must use a unique
Persistent Reservation key. does not specify
a way to generate keys. This document assigns the burden to generate
unique keys to the MDS, which must generate a key for itself before
exporting a volume, and a key for each client that accesses
SCSI layout volumes. Individuals keys for each volume that a client
can access are permitted but not required.
Before returning a PNFS_SCSI_VOLUME_BASE volume to the client, the
MDS needs to prepare the volume for fencing using PRs.
This is done by registering the reservation generated for the MDS with
the device using the "PERSISTENT RESERVE OUT" command with a service
action of "REGISTER", followed by a "PERSISTENT RESERVE OUT" command,
with a service action of "RESERVE" and the type field set to 8h
(Exclusive Access – Registrants Only).
To make sure all I_T nexuses (see section 3.1.45 of )
are registered, the MDS SHOULD set the
"All Target Ports" (ALL_TG_PT) bit when registering the key, or
otherwise ensure the registration is performed for each target port,
and MUST perform registration for each initiator port.
Before performing the first I/O to a device returned from a GETDEVICEINFO
operation the client will register the registration key
returned in sbv_pr_key with the storage device
by issuing a "PERSISTENT RESERVE OUT" command with a service action
of REGISTER with the "SERVICE ACTION RESERVATION KEY" set to the
reservation key returned in sbv_pr_key.
To make sure all I_T nexuses are registered, the client SHOULD set the
"All Target Ports" (ALL_TG_PT) bit when registering the key, or
otherwise ensure the registration is performed for each target
port, and MUST perform registration for each initiator port.
When a client stops using a device earlier returned by
GETDEVICEINFO it MUST unregister the earlier registered key by
issuing a "PERSISTENT RESERVE OUT" command with a service action of
"REGISTER" with the "RESERVATION KEY" set to the earlier registered
reservation key.
In case of a non-responding client the MDS fences the client
by issuing a "PERSISTENT RESERVE OUT" command with the service
action set to "PREEMPT" or "PREEMPT AND ABORT", the reservation key
field set to the server's reservation key, the service action
reservation key field set to the reservation key associated with
the non-responding client, and the type field set to 8h (Exclusive
Access – Registrants Only).
After the MDS preempts a client, all client I/O to the LU
fails. The client should at this point return any layout that
refers to the device ID that points to the LU. Note that
the client can distinguish I/O errors due to fencing from other
errors based on the "RESERVATION CONFLICT" SCSI status. Refer to
for details.
A client that detects a "RESERVATION CONFLICT" SCSI status
(I/O error) on the storage devices MUST commit all layouts that
use the storage device through the MDS, return all outstanding
layouts for the device, forget the device ID and unregister the
reservation key.
Future GETDEVICEINFO calls may refer to the storage device
again, in which case the client will perform a new registration
based on the key provided (via sbv_pr_key) at that time.
A critical requirement in crash recovery is that both the client and
the server know when the other has failed. Additionally, it is
required that a client sees a consistent view of data across
server restarts. These requirements and a full discussion of
crash recovery issues are covered in the "Crash Recovery" section
of the NFSv41 specification . This
document contains additional crash recovery material specific
only to the SCSI layout.
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 PNFS_SCSI_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 Section 18.42.4 of .
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.
The NFSv4.1 spec defines the following types:
When the server sends a CB_RECALL_ANY request to a client specifying
the RCA4_TYPE_MASK_BLK_LAYOUT 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 than craa_object_to_keep.
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 error NFS4ERR_RECALLCONFLICT indicates that the server has
recently issued a CB_LAYOUTRECALL to the requesting client, making it
necessary for the client to respond to the recall before processing
the layout request. A client can wait for that recall to be receive
and processe or it can retry as for NFS4ERR_TRYLATER, as described
below.
The error NFS4ERR_TRYLATER is used to indicate that the server cannot
immediately grant the layout to the client. This may be due to
constraints on writable sharing of blocks by multiple clients or to a
conflict with a recallable lock (e.g. a delegation). 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 abandon the
attempt to get a layout and perform READ and WRITE operations by
sending them 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, there is
a conflicting lock that would prevent the layout from being granted,
or for any other reason that causes the server to be unable to supply
the layout. As a result of receiving NFS4ERR_LAYOUTUNAVAILABLE, the
client should abandon the attempt to get a layout and perform READ and
WRITE operations by sending them to the MDS.
It is expected that a client will not cache the file's layoutunavailable
state forever. In particular, when the file is closed or opened by the
client, issuing a new LAYOUTGET is appropriate.
Many storage devices implement volatile write caches that require an
explicit flush to persist the data from write operations to stable
storage. Storage devices implementing should
indicate a volatile write cache by setting the WCE bit to 1 in the
Caching mode page.
When a volatile write cache is used, the pNFS server must ensure
the volatile write cache has been committed to stable storage
before the LAYOUTCOMMIT operation returns by using one of the
SYNCHRONIZE CACHE commands.
The functionality provided by SCSI Persistent Reservations makes it
possible for the MDS to control access by individual client machines
to specific LUs. Individual client machines may be allowed to or
prevented from reading or writing to certain block devices.
Finer-grained access control methods are not generally available.
For this reason, certain responsibilities for enforcing NFSv4
semantics, including security and locking, are delegated to pNFS
clients when SCSI layouts are being used. The metadata server's role
is to only grant layouts appropriately and the 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 that holds a layout for a file from accessing
parts of the physical disk not covered by the layout. Similarly, the
server will not be able to prevent a client from accessing blocks
covered by a layout that it has already returned. The pNFS client
must respect the layout model for this mapping type to appropriately
respect NFSv4 semantics.
Furthermore, there is no way for the storage to determine the specific
NFSv4 entity (principal, openowner, lockowner) on whose behalf the I/O
operation is being done. This fact may limit the functionality to be
supported and require the pNFS client to implement server policies
other than those describable by layouts.
In cases in which layouts previously granted become invalid, the
server has the option of recalling them. In situations in which
communication difficulties prevent this from happening, layouts may be
revoked by the server. This revocation is accompanied by changes in
persistent reservation which have the effect of preventing SCSI access
to the LUs in question by the client.
The effective implementation of these NFSv4 semantic constraints is
complicated by the different granularities of the actors for the
different types of the functionality to be enforced:
To enforce security constraints for particular principals.
To enforce locking constraints for particular owners (openowners
and lockowners)
Fundamental to enforcing both of these sorts of constraints is the
principle that a pNFS client must not issue a SCSI I/O operation
unless it possesses both:
A valid open stateid for the file in question, performing the I/O
that allows I/O of the type in question, which is associated with the
openowner and principal on whose behalf the I/O is to be done.
A valid layout stateid for the file in question that covers the
byte range on which the I/O is to be done and that allows I/O of that
type to be done.
As a result, if the equivalent of I/O with an anonymous or write-bypass
stateid is to be done, it MUST NOT by done using the pNFS SCSI layout
type. The client MAY attempt such I/O using READs and WRITEs that do
not use pNFS and are directed to the MDS.
When open stateids are revoked, due to lease expiration or any form of
administrative revocation, the server MUST recall all layouts that
allow I/O to be done on any of the files for which open revocation
happens. When there is a failure to successfully return those
layouts, the client MUST be fenced.
The restriction noted above provides adequate enforcement of
appropriate security restriction when the principal issuing the I/O is
the same as that opening the file. The server is responsible for
checking that the I/O mode requested by the open is allowed for the
principal doing the OPEN. If the correct sort of I/O is done on behalf
of the same principal, then the security restriction is thereby
enforced.
If I/O is done by a principal different from the one that opened the
file, the client SHOULD send the I/O to be performed by the metadata
server rather than doing it directly to the storage device.
Mandatory enforcement of whole-file locking by means of share
reservations is provided when the pNFS client obeys the requirement
set forth in Section 2.1 above. Since performing I/O requires a valid
open stateid an I/O that violates an existing share reservation would
only be possible when the server allows conflicting open stateids to
exist.
The nature of the SCSI layout type is such implementation/enforcement of
mandatory byte-range locks is very difficult. Given that layouts are
granted to clients rather than owners, the pNFS client is in no position to
successfully arbitrate among multiple lockowners on the same client. Suppose
lockowner A is doing a write and, while the I/O is pending, lockowner B
requests a mandatory byte-range for a byte range potentially overlapping
the pending I/O. In such a situation, the lock request cannot be granted
while the I/O is pending. In a non-pNFS environment, the server would have
to wait for pending I/O before granting the mandatory byte-range lock. In
the pNFS environment the server does not issue the I/O and is thus in no
position to wait for its completion. The server may recall such layouts but
in doing so, it has no way of distinguishing those being used by lockowners
A and B, making it difficult to allow B to perform I/O while forbidding A
from doing so. Given this fact, the MDS need to successfully recall all
layouts that overlap the range being locked before returning a successful
response to the LOCK request. While the lock is in effect, the server
SHOULD respond to requests for layouts which overlap a currently locked
area with NFS4ERR_LAYOUTUNAVAILABLE. To simplify the required logic a
server MAY do this for all layout requests on the file in question as long
as there are any byte-range locks in effect.
Given these difficulties it may be difficult for servers supporting
mandatory byte-range locks to also support SCSI layouts. Servers can
support advisory byte-range locks instead. The NFSv4 protocol currently has
no way of determining whether byte-range lock support on a particular file
system will be mandatory or advisory, except by trying operation which
would conflict if mandatory locking is in effect. Therefore, to avoid
confusion, servers SHOULD NOT switch between mandatory and advisory
byte-range locking based on whether any SCSI layouts have been obtained or
whether a client that has obtained a SCSI layout has requested a byte-range
lock.
Access to SCSI storage devices 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
SCSI layouts can be used with storage access protocols (e.g., serial
attached SCSI () that provide essentially no
security functionality. At the other extreme, pNFS may be used with
storage protocols such as iSCSI () that can
provide significant security functionality. It is the responsibility
of those administering and deploying pNFS with a SCSI 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 SCSI 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
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 SCSI layouts
SHOULD NOT be used.
IANA is requested to assign a new pNFS layout type in the pNFS Layout
Types Registry as follows (the value 5 is suggested):
Layout Type Name: LAYOUT4_SCSI
Value: 0x00000005
RFC: RFCTBD10
How: L (new layout type)
Minor Versions: 1
Key words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.eduLegal Provisions Relating to IETF DocumentsIETF TrustXDR: External Data Representation StandardNetwork Appliance, Inc.Network File System (NFS) Version 4 Minor Version 1 ProtocolSun Microsystems, Inc.Network Appliance, Inc.Network Appliance, Inc.Network File System (NFS) Version 4 Minor Version 1 External Data Representation Standard (XDR) DescriptionSun Microsystems, Inc.Network Appliance, Inc.Network Appliance, Inc.Parallel NFS (pNFS) Block/Volume LayoutEMC CorporationNasuni IncGoogleParallel NFS (pNFS) Block Disk ProtectionEMC CorporationGoogleEMC CorporationInternet Small Computer System Interface (iSCSI) Protocol (Consolidated)MicrosoftIBM Haifa Research LabEMC CorporationSCSI Architecture Model - 5 (SAM-5)INCITS Technical Committee T10SCSI Primary Commands-4INCITS Technical Committee T10SCSI Block Commands-3INCITS Technical Committee T10Serial Attached Scsi-3INCITS Technical Committee T10
Large parts of this document were copied verbatim, and others were inspired by
. Thank to David Black, Stephen Fridella and
Jason Glasgow for their work on the pNFS block/volume layout protocol.
David Black, Robert Elliott and Tom Haynes provided a throughout
review of drafts of this document, and their input led to the current
form of the document.
David Noveck provided ample feedback to various drafts of this document,
wrote the section on enforcing NFSv4 semantics and rewrote various
sections to better catch the intent.
[RFC Editor: please remove this section prior to publishing
this document as an RFC]
[RFC Editor: prior to publishing this document as an RFC, please replace all occurrences of RFCTBD10
with RFCxxxx where xxxx is the RFC number of this document]