Internet Engineering Task Force (IETF) T. Haynes
Request for Comments: 8434 Hammerspace
Updates: 5661 August 2018
Category: Standards Track
Requirements for Parallel NFS (pNFS) Layout Types
This document defines the requirements that individual Parallel NFS
(pNFS) layout types need to meet in order to work within the pNFS
framework as defined in RFC 5661. In so doing, this document aims to
clearly distinguish between requirements for pNFS as a whole and
those specifically directed to the pNFS file layout. The lack of a
clear separation between the two sets of requirements has been
troublesome for those specifying and evaluating new layout types. In
this regard, this document updates RFC 5661.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction ....................................................22. Definitions .....................................................32.1. Use of the Terms "Data Server" and "Storage Device" ........52.2. Requirements Language ......................................63. The Control Protocol ............................................63.1. Control Protocol Requirements ..............................83.2. Previously Undocumented Protocol Requirements ..............93.3. Editorial Requirements ....................................104. Specifications of Original Layout Types ........................114.1. File Layout Type ..........................................114.2. Block Layout Type .........................................124.3. Object Layout Type ........................................135. Summary ........................................................146. Security Considerations ........................................157. IANA Considerations ............................................158. References .....................................................168.1. Normative References ......................................168.2. Informative References ....................................16
Author's Address ..................................................171. Introduction
The concept of "layout type" has a central role in the definition and
implementation of Parallel NFS (pNFS) (see [RFC5661]). Clients and
servers implementing different layout types behave differently in
many ways while conforming to the overall pNFS framework defined in
[RFC5661] and this document. Layout types may differ as to:
o The method used to do I/O operations directed to data storage
o The requirements for communication between the metadata server
(MDS) and the storage devices.
o The means used to ensure that I/O requests are only processed when
the client holds an appropriate layout.
o The format and interpretation of nominally opaque data fields in
pNFS-related NFSv4.x data structures.
Each layout type will define the needed details for its usage in the
specification for that layout type; layout type specifications are
always Standards Track RFCs. Except for the file layout type defined
in Section 13 of [RFC5661], existing layout types are defined in
their own Standards Track documents, and it is anticipated that new
layout types will be defined in similar documents.
The file layout type was defined in the Network File System (NFS)
version 4.1 protocol specification [RFC5661]. The block layout type
was defined in [RFC5663], and the object layout type was defined in
[RFC5664]. Subsequently, the Small Computer System Interface (SCSI)
layout type was defined in [RFC8154].
Some implementers have interpreted the text in Sections 12 ("Parallel
NFS (pNFS)") and 13 ("NFSv4.1 as a Storage Protocol in pNFS: the File
Layout Type") of [RFC5661] as applying only to the file layout type.
Because Section 13 was not covered in a separate Standards Track
document such as those for both the block and object layout types,
there was some confusion as to the responsibilities of both the
metadata server and the data servers (DSs) that were laid out in
As a consequence, authors of new specifications (see [RFC8435] and
[Lustre]) may struggle to meet the requirements to be a pNFS layout
type. This document gathers the requirements from all of the
original Standards Track documents regarding layout type and then
specifies the requirements placed on all layout types independent of
the particular type chosen.
control communication requirement: the specification for information
on layouts, stateids, file metadata, and file data that must be
communicated between the metadata server and the storage devices.
There is a separate set of requirements for each layout type.
control protocol: the particular mechanism that an implementation of
a layout type would use to meet the control communication
requirement for that layout type. This need not be a protocol as
normally understood. In some cases, the same protocol may be used
as both a control protocol and storage protocol.
storage protocol: the protocol used by clients to do I/O operations
to the storage device. Each layout type specifies the set of
loose coupling: when the control protocol is a storage protocol.
tight coupling: an arrangement in which the control protocol is one
designed specifically for control communication. It may be either
a proprietary protocol adapted specifically to a particular
metadata server or a protocol based on a Standards Track document.
(file) data: that part of the file system object that contains the
data to be read or written. It is the contents of the object
rather than the attributes of the object.
data server (DS): a pNFS server that provides the file's data when
the file system object is accessed over a file-based protocol.
Note that this usage differs from that in [RFC5661], which applies
the term in some cases even when other sorts of protocols are
being used. Depending on the layout, there might be one or more
data servers over which the data is striped. While the metadata
server is strictly accessed over the NFSv4.1 protocol, the data
server could be accessed via any file access protocol that meets
the pNFS requirements.
See Section 2.1 for a comparison of this term and "storage
storage device: the target to which clients may direct I/O requests
when they hold an appropriate layout. Note that each data server
is a storage device but that some storage device are not data
servers. See Section 2.1 for further discussion.
fencing: the process by which the metadata server prevents the
storage devices from processing I/O from a specific client to a
layout: the information a client uses to access file data on a
storage device. This information includes specification of the
protocol (layout type) and the identity of the storage devices to
The bulk of the contents of the layout are defined in [RFC5661] as
nominally opaque, but individual layout types are responsible for
specifying the format of the layout data.
layout iomode: a grant of either read-only or read/write I/O to the
layout stateid: a 128-bit quantity returned by a server that
uniquely defines the layout state provided by the server for a
specific layout that describes a layout type and file (see
Section 12.5.2 of [RFC5661]). Further, Section 12.5.3 of
[RFC5661] describes differences in handling between layout
stateids and other stateid types.
layout type: a specification of both the storage protocol used to
access the data and the aggregation scheme used to lay out the
file data on the underlying storage devices.
recalling a layout: a graceful recall, via a callback, of a specific
layout by the metadata server to the client. Graceful here means
that the client would have the opportunity to flush any WRITEs,
etc., before returning the layout to the metadata server.
revoking a layout: an invalidation of a specific layout by the
metadata server. Once revocation occurs, the metadata server will
not accept as valid any reference to the revoked layout, and a
storage device will not accept any client access based on the
(file) metadata: the part of the file system object that contains
various descriptive data relevant to the file object, as opposed
to the file data itself. This could include the time of last
modification, access time, EOF position, etc.
metadata server (MDS): the pNFS server that provides metadata
information for a file system object. It is also responsible for
generating, recalling, and revoking layouts for file system
objects, for performing directory operations, and for performing
I/O operations to regular files when the clients direct these to
the metadata server itself.
stateid: a 128-bit quantity returned by a server that uniquely
defines the set of locking-related state provided by the server.
Stateids may designate state related to open files, byte-range
locks, delegations, or layouts.
2.1. Use of the Terms "Data Server" and "Storage Device"
In [RFC5661], the terms "data server" and "storage device" are used
o In Section 12, where pNFS in general is discussed, the term
"storage device" is used.
o In Section 13, where the file layout type is discussed, the term
"data server" is used.
o In other sections, the term "data server" is used, even in
contexts where the storage access type is not NFSv4.1 or any other
file access protocol.
As this document deals with pNFS in general, it uses the more generic
term "storage device" in preference to "data server". The term "data
server" is used only in contexts in which a file server is used as a
storage device. Note that every data server is a storage device, but
storage devices that use protocols that are not file access protocols
(such as NFS) are not data servers.
Since a given storage device may support multiple layout types, a
given device can potentially act as a data server for some set of
storage protocols while simultaneously acting as a storage device for
2.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document differs from most Standards Track documents in that it
specifies requirements for those defining future layout types rather
than defining the requirements for implementations directly. This
document makes clear whether:
(1) any particular requirement applies to implementations.
(2) any particular requirement applies to those defining layout
(3) the requirement is a general requirement that implementations
need to conform to, with the specific means left to layout type
definitions type to specify.
3. The Control Protocol
A layout type has to meet the requirements that apply to the
interaction between the metadata server and the storage device such
that they present to the client a consistent view of stored data and
locking state (Section 12.2.6 of [RFC5661]). Particular
implementations may satisfy these requirements in any manner they
choose, and the mechanism chosen need not be described as a protocol.
Specifications defining layout types need to clearly show how
implementations can meet the requirements discussed below, especially
with respect to those that have security implications. In addition,
such specifications may find it necessary to impose requirements on
implementations of the layout type to ensure appropriate
In some cases, there may be no control protocol other than the
storage protocol. This is often described as using a "loosely
coupled" model. In such cases, the assumption is that the metadata
server, storage devices, and client may be changed independently and
that the implementation requirements in the layout type specification
need to ensure this degree of interoperability. This model is used
in the block and object layout type specification.
In other cases, it is assumed that there will be a purpose-built
control protocol that may be different for different implementations
of the metadata server and data server. The assumption here is that
the metadata server and data servers are designed and implemented as
a unit and interoperability needs to be assured between clients and
metadata-data server pairs, developed independently. This is the
model used for the file layout.
Another possibility is for the definition of a control protocol to be
specified in a Standards Track document. There are two subcases to
o A new layout type includes a definition of a particular control
protocol whose use is obligatory for metadata servers and storage
devices implementing the layout type. In this case, the
interoperability model is similar to the first case above, and the
defining document should assure interoperability among metadata
servers, storage devices, and clients developed independently.
o A control protocol is defined in a Standards Track document that
meets the control protocol requirements for one of the existing
layout types. In this case, the new document's job is to assure
interoperability between metadata servers and storage devices
developed separately. The existing definition document for the
selected layout type retains the function of assuring
interoperability between clients and a given collection of
metadata servers and storage devices. In this context,
implementations that implement the new protocol are treated in the
same way as those that use an internal control protocol or a
An example of this last case is the SCSI layout type [RFC8154], which
extends the block layout type. The block layout type had a
requirement for fencing of clients but did not present a way for the
control protocol (in this case, the SCSI storage protocol) to fence
the client. The SCSI layout type remedies that in [RFC8154] and, in
effect, has a tightly coupled model.
3.1. Control Protocol Requirements
The requirements of interactions between the metadata server and the
storage devices are:
(1) The metadata server MUST be able to service the client's I/O
requests if the client decides to make such requests to the
metadata server instead of to the storage device. The metadata
server must be able to retrieve the data from the constituent
storage devices and present it back to the client. A corollary
to this is that even though the metadata server has successfully
given the client a layout, the client MAY still send I/O
requests to the metadata server.
(2) The metadata server MUST be able to restrict access to a file on
the storage devices when it revokes a layout. The metadata
server typically would revoke a layout whenever a client fails
to respond to a recall or a client's lease is expired due to
non-renewal. It might also revoke the layout as a means of
enforcing a change in locking state or access permissions that
the storage device cannot directly enforce.
Effective revocation may require client cooperation in using a
particular stateid (file layout) or principal (e.g., flexible
file layout) when performing I/O.
In contrast, there is no requirement to restrict access to a
file on the storage devices when a layout is recalled. It is
only after the metadata server determines that the client is not
gracefully returning the layout and starts the revocation that
this requirement is enforced.
(3) A pNFS implementation MUST NOT allow the violation of NFSv4.1's
access controls: Access Control Lists (ACLs) and file open
modes. Section 12.9 of [RFC5661] specifically lays this burden
on the combination of clients, storage devices, and the metadata
server. However, the specification of the individual layout
type might create requirements as to how this is to be done.
This may include a possible requirement for the metadata server
to update the storage device so that it can enforce security.
The file layout requires the storage device to enforce access
whereas the flexible file layout requires both the storage
device and the client to enforce security.
(4) Interactions between locking and I/O operations MUST obey
existing semantic restrictions. In particular, if an I/O
operation would be invalid when directed at the metadata server,
it is not to be allowed when performed on the storage device.
For the block and SCSI layouts, as the storage device is not
able to reject the I/O operation, the client is responsible for
enforcing this requirement.
(5) Any disagreement between the metadata server and the data server
as to the value of attributes such as modify time, the change
attribute, and the EOF position MUST be of limited duration with
clear means of resolution of any discrepancies being provided.
Note the following:
(a) Discrepancies need not be resolved unless any client has
accessed the file in question via the metadata server,
typically by performing a GETATTR.
(b) A particular storage device might be striped, and as such,
its local view of the EOF position does not match the
global EOF position.
(c) Both clock skew and network delay can lead to the metadata
server and the storage device having different values of
the time attributes. As long as those differences can be
accounted for in what is presented to the client in a
GETATTR, then no violation results.
(d) A LAYOUTCOMMIT requires that changes in attributes
resulting from operations on the storage device need to be
reflected in the metadata server by the completion of the
These requirements may be satisfied in different ways by different
layout types. As an example, while the file layout type uses the
stateid to fence off the client, there is no requirement that other
layout types use this stateid approach.
Each new Standards Track document for a layout type MUST address how
the client, metadata server, and storage devices are to interact to
meet these requirements.
3.2. Previously Undocumented Protocol Requirements
While not explicitly stated as requirements in Section 12 of
[RFC5661], the existing layout types do have more requirements that
they need to enforce.
The client has these obligations when making I/O requests to the
(1) Clients MUST NOT perform I/O to the storage device if they do
not have layouts for the files in question.
(2) Clients MUST NOT perform I/O operations outside of the specified
ranges in the layout segment.
(3) Clients MUST NOT perform I/O operations that would be
inconsistent with the iomode specified in the layout segments it
Under the file layout type, the storage devices are able to reject
any request made not conforming to these requirements. This may not
be possible for other known layout types, which puts the burden of
enforcing such violations solely on the client. For these layout
(1) The metadata server MAY use fencing operations to the storage
devices to enforce layout revocation against the client.
(2) The metadata server MUST allow the clients to perform data I/O
against it, even if it has already granted the client a layout.
A layout type might discourage such I/O, but it cannot forbid
(3) The metadata server MUST be able to do storage allocation,
whether that is to create, delete, extend, or truncate files.
The means to address these requirements will vary with the layout
type. A control protocol will be used to effect these; the control
protocol could be a purpose-built one, one identical to the storage
protocol, or a new Standards Track control protocol.
3.3. Editorial Requirements
This section discusses how the protocol requirements discussed above
need to be addressed in documents specifying a new layout type.
Depending on the interoperability model for the layout type in
question, this may involve the imposition of layout-type-specific
requirements that ensure appropriate interoperability of pNFS
components that are developed separately.
The specification of the layout type needs to make clear how the
client, metadata server, and storage device act together to meet the
protocol requirements discussed previously. If the document does not
impose implementation requirements sufficient to ensure that these
semantic requirements are met, it is not appropriate for publication
as an RFC from the IETF stream.
Some examples include:
o If the metadata server does not have a means to invalidate a
stateid issued to the storage device to keep a particular client
from accessing a specific file, then the layout type specification
has to document how the metadata server is going to fence the
client from access to the file on that storage device.
o If the metadata server implements mandatory byte-range locking
when accessed directly by the client, then the layout type
specification must require that this also be done when data is
read or written using the designated storage protocol.
4. Specifications of Original Layout Types
This section discusses how the original layout types interact with
Section 12 of [RFC5661], which enumerates the requirements of pNFS
layout type specifications. It is not normative with regards to the
file layout type presented in Section 13 of [RFC5661], the block
layout type [RFC5663], and the object layout type [RFC5664]. These
are discussed here only to illuminate the updates Section 3 of this
document makes to Section 12 of [RFC5661].
4.1. File Layout Type
Because the storage protocol is a subset of NFSv4.1, the semantics of
the file layout type comes closest to the semantics of NFSv4.1 in the
absence of pNFS. In particular, the stateid and principal used for
I/O MUST have the same effect and be subject to the same validation
on a data server as it would have if the I/O were being performed on
the metadata server itself. The same set of validations are applied
whether or not pNFS is in effect.
While for most implementations, the storage devices can do the
following validations that are each presented as a "SHOULD" and not a
"MUST" in [RFC5661]:
(1) client holds a valid layout,
(2) client I/O matches the layout iomode, and
(3) client does not go out of the byte ranges,
Actually, the first point is presented in [RFC5661] as both:
"MUST": in Section 13.6
As described in Section 12.5.1, a client MUST NOT send an I/O to a
data server for which it does not hold a valid layout; the data
server MUST reject such an I/O.
"SHOULD": in Section 13.8
The iomode need not be checked by the data servers when clients
perform I/O. However, the data servers SHOULD still validate that
the client holds a valid layout and return an error if the client
It should be noted that it is just these layout-specific checks that
are optional, not the normal file access semantics. The storage
devices MUST make all of the required access checks on each READ or
WRITE I/O as determined by the NFSv4.1 protocol. If the metadata
server would deny a READ or WRITE operation on a file due to its ACL,
mode attribute, open access mode, open deny mode, mandatory byte-
range locking state, or any other attributes and state, the storage
device MUST also deny the READ or WRITE operation. Also, while the
NFSv4.1 protocol does not mandate export access checks based on the
client's IP address, if the metadata server implements such a policy,
then that counts as such state as outlined above.
The data filehandle provided by the PUTFH operation to the data
server provides sufficient context to enable the data server to
ensure that the client has a valid layout for the I/O being performed
for the subsequent READ or WRITE operation in the compound.
Finally, the data server can check the stateid presented in the READ
or WRITE operation to see if that stateid has been rejected by the
metadata server; if so, the data server will cause the I/O to be
fenced. Whilst it might just be the open owner or lock owner on that
client being fenced, the client should take the NFS4ERR_BAD_STATEID
error code to mean it has been fenced from the file and contact the
4.2. Block Layout Type
With the block layout type, the storage devices are generally not
able to enforce file-based security. Typically, storage area network
(SAN) disk arrays and SAN protocols provide coarse-grained access
control mechanisms (e.g., Logical Unit Number (LUN) mapping and/or
masking), with a target granularity of disks rather than individual
blocks and a source granularity of individual hosts rather than of
users or owners. Access to block storage is logically at a lower
layer of the I/O stack than NFSv4. Since NFSv4 security is not
directly applicable to protocols that access such storage directly,
Section 2.1 of [RFC5663] specifies that:
in environments where pNFS clients cannot be trusted to enforce
such policies, pNFS block layout types SHOULD NOT be used.
Due to these granularity issues, the security burden has been shifted
from the storage devices to the client. Those deploying
implementations of this layout type need to be sure that the client
implementation can be trusted. This is not a new sort of requirement
in the context of SAN protocols. In such environments, the client is
expected to provide block-based protection.
This shift of the burden also extends to locks and layouts. The
storage devices are not able to enforce any of these, and the burden
is pushed to the client to make the appropriate checks before sending
I/O to the storage devices. For example, the server may use a layout
iomode only allowing reading to enforce a mandatory read-only lock.
In such cases, the client has to support that use by not sending
WRITEs to the storage devices. The fundamental issue here is that
the storage device is treated by this layout type in the same fashion
as a local disk device. Once the client has access to the storage
device, it is able to perform both READ and WRITE I/O to the entire
storage device. The byte ranges in the layout, any locks, the layout
iomode, etc., can only be enforced by the client. Therefore, the
client is required to provide that enforcement.
In the context of fencing off of the client upon revocation of a
layout, these limitations come into play again, i.e., the granularity
of the fencing can only be at the level of the host and logical unit.
Thus, if one of a client's layouts is revoked by the server, it will
effectively revoke all of the client's layouts for files located on
the storage units comprising the logical volume. This may extend to
the client's layouts for files in other file systems. Clients need
to be prepared for such revocations and reacquire layouts as needed.
4.3. Object Layout Type
With the object layout type, security checks occur during the
allocation of the layout. The client will typically ask for layouts
covering all of the file and may do so for either READ or READ/WRITE.
This enables it to do subsequent I/O operations without the need to
obtain layouts for specific byte ranges. At that time, the metadata
server should verify permissions against the layout iomode, the file
mode bits or ACLs, etc. As the client may be acting for multiple
local users, it MUST authenticate and authorize the user by issuing
respective OPEN and ACCESS calls to the metadata server, similar to
having NFSv4 data delegations.
Upon successful authorization, the client receives within the layout
a set of object capabilities allowing it I/O access to the specified
objects corresponding to the requested iomode. These capabilities
are used to enforce access control and locking semantics at the
storage devices. Whenever one of the following occurs on the
metadata server, then the metadata server MUST change the capability
version attribute on all objects comprising the file in order to
invalidate any outstanding capabilities before committing to one of
o the permissions on the object change,
o a conflicting mandatory byte-range lock is granted, or
o a layout is revoked and reassigned to another client.
When the metadata server wishes to fence off a client to a particular
object, then it can use the above approach to invalidate the
capability attribute on the given object. The client can be informed
via the storage device that the capability has been rejected and is
allowed to fetch a refreshed set of capabilities, i.e., reacquire the
In the three original layout types, the burden of enforcing the
security of NFSv4.1 can fall to either the storage devices (files),
the client (blocks), or the metadata server (objects). Such choices
are conditioned by the native capabilities of the storage devices --
if a control protocol can be implemented, then the burden can be
shifted primarily to the storage devices.
In the context of this document, we treat the control protocol as a
set of requirements. As new layout types are published, the defining
documents MUST address:
(1) The fencing of clients after a layout is revoked.
(2) The security implications of the native capabilities of the
storage devices with respect to the requirements of the NFSv4.1
In addition, these defining documents need to make clear how other
semantic requirements of NFSv4.1 (e.g., locking) are met in the
context of the proposed layout type.
6. Security Considerations
This section does not deal directly with security considerations for
existing or new layout types. Instead, it provides a general
framework for understating security-related issues within the pNFS
framework. Specific security considerations will be addressed in the
Security Considerations sections of documents specifying layout
types. For example, in Section 3 of [RFC5663], the lack of finer-
than-physical disk access control necessitates that the client is
delegated the responsibility to enforce the access provided to them
in the layout extent that they were granted by the metadata server.
The layout type specification must ensure that only data access
consistent with the NFSV4.1 security model is allowed. It may do
this directly, by providing that appropriate checks be performed at
the time each access is performed. It may do it indirectly by
allowing the client or the storage device to be responsible for
making the appropriate checks. In the latter case, I/O access rights
are reflected in layouts, and the layout type must provide a way to
prevent inappropriate access due to permissions changes between the
time a layout is granted and the time the access is performed.
The metadata server MUST be able to fence off a client's access to
the data file on a storage device. When it revokes the layout, the
client's access MUST be terminated at the storage devices. The
client has a subsequent opportunity to reacquire the layout and
perform the security check in the context of the newly current access
7. IANA Considerations
This document has no IANA actions.
Dave Noveck provided an early review that sharpened the clarity of
the definitions. He also provided a more comprehensive review of the
Both Chuck Lever and Christoph Helwig provided insightful comments
during the working group last call.
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