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General Network File System Version 4 Internet-Draft Network File System version 4.2 (NFSv4.2) clients commonly perform client-side caching of file data in order to improve performance. On some systems, applications may influence client data caching behavior, but there is no standardized mechanism for a server or administrator to indicate that particular file data should not be cached by clients for reasons of performance or correctness. This document introduces a new file data caching attribute for NFSv4.2. Files marked with this attribute are intended to be accessed with client-side caching of file data suppressed, in order to support workloads that require predictable data visibility. This document extends NFSv4.2. Note to Readers Discussion of this draft takes place on the NFSv4 working group mailing list (nfsv4@ietf.org), which is archived at . Source code and issues list for this draft can be found at . Working Group information can be found at .

Introduction Clients of remote filesystems commonly perform client-side caching of file data in order to improve performance. Such caching may include retaining data read from the server to satisfy subsequent READ requests, as well as retaining data written by applications in order to delay or combine WRITE requests before transmitting them to the server. While these techniques are effective for many workloads, they may be unsuitable for workloads that require predictable data visibility or involve concurrent modification of shared files by multiple clients. In some cases, Network File System version 4.2 (NFSv4.2) (see ) mechanisms such as file delegations can reduce the impact of concurrent access. However, delegations are not always available or effective, particularly for workloads with frequent concurrent writers or rapidly changing access patterns. There have been prior efforts to bypass file data caching in order to address these issues. In High-Performance Computing (HPC) workloads, file data caching is often bypassed to improve predictability and to avoid read-modify-write hazards when multiple clients write disjoint byte ranges of the same file. Applications on some systems can request bypass of the client data cache by opening files with the O_DIRECT flag (see ). However, this approach has limitations, including the requirement that each application be explicitly modified and the lack of a standardized mechanism for communicating this intent between servers and clients. This document introduces the uncacheable file data attribute to NFSv4.2. This attribute allows a server to indicate that client-side caching of file data for a particular file is unsuitable. When both the client and the server support this attribute, the client is advised to suppress client-side caching of file data for that file, in accordance with the semantics defined in this document. The attribute itself is specified in and the client behavior it advises in . Using the process described in Section 6, this document extends NFSv4.2 ; as that section provides, it does not update , which remains a valid description of the base variant of the minor version. The revisions are built on top of the external data representation (XDR) generated from .

Definitions

client-side caching of file data

The retention of file data by a client in a local data cache, commonly referred to as the page cache, for the purpose of satisfying subsequent READ requests or delaying transmission of WRITE data to the server.

write-behind caching

A form of file data caching in which WRITE data is retained by the client and transmission of the data to the server is delayed in order to combine multiple WRITE operations or improve efficiency.

direct I/O

An access mode in which file data is transferred between application buffers and the underlying storage without populating or consulting the client's file data cache. Direct I/O suppresses both read caching and write-behind caching of file data.

write hole

A write hole is an instance of data corruption that arises when multiple clients modify disjoint byte ranges within the same encoded data block without having a consistent view of the existing contents. This can result in stale data overwriting newer updates, particularly in environments that use erasure encoding or striped storage.

revalidation

The procedure of Section 10.3.1 by which a client determines whether data or attributes it holds in a cache are still current: the client fetches the change attribute from the server, compares it with the value it cached, and, if they differ, treats the cached data and attributes as invalid. A client validates data when it fetches it from the server; it revalidates before reusing what it cached.

This document assumes familiarity with the NFSv4 protocol operations, error codes, object types, and attributes as defined in .

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 when, and only when, they appear in all capitals, as shown here.

The Uncacheable File Data Attribute The uncacheable file data attribute, fattr4_uncacheable_file_data (see ), is a RECOMMENDED attribute for NFSv4.2, in the attribute-category sense of Section 5.2 and Section 12 rather than the BCP 14 sense; a server is not required to support it. It is read-write, applies on a per-file basis, and has a data type of boolean. summarizes the attribute using the columns of Section 12.1, where "R W" indicates that GETATTR may retrieve the attribute and SETATTR may set it. New RECOMMENDED Attribute

Name	Id	Data Type	Acc
uncacheable_file_data	87	bool	R W

Support for the uncacheable file data attribute is specific to the exported filesystem and may differ between filesystems served by the same server. A client can determine whether the attribute is supported for a given file by examining the supported_attrs attribute for that file's filesystem or by probing support using the procedures described in . The uncacheable file data attribute applies only to regular files, that is, objects of type NF4REG or NF4NAMEDATTR ( Section 6.1). A named attribute is read and written with the same READ and WRITE operations as a regular file ( Section 5.3), so a client caches its data by the same mechanism and is exposed to the same hazards. A server that receives a GETATTR requesting this attribute for an object that is not a regular file MUST return FALSE: support for an attribute is advertised per file system ( Section 5.8.1.1), so a server that supports this attribute supports it for every object in that file system and owes a value for each ( Section 18.7.3). As with rawdev ( Section 5.8.2.31), the value SHOULD NOT be considered useful for such an object. A server that receives a SETATTR requesting this attribute on an object that is not a regular file MUST return NFS4ERR_WRONG_TYPE ( Section 15.1.2.9).

Client-Side Caching of File Data The uncacheable file data attribute advises the client to limit the use of client-side caching of file data for a file. This includes both write-behind caching and read caching, which are addressed separately below. The server is often in a better position than individual clients to determine sharing patterns, access behavior, or correctness requirements associated with a file. By exposing this information via an attribute, the server can advise clients to limit file data caching in a consistent manner.

Write-Behind Caching The uncacheable file data attribute inhibits write-behind caching, in which multiple pending WRITEs are combined and transmitted to the server at a later time for efficiency. When honoring the uncacheable file data attribute, clients MUST NOT delay transmission of WRITE data for the purpose of combining multiple WRITE operations or improving efficiency. When application data spans a data block in a client cache, delayed transmission of WRITE data can result in clients modifying stale data and overwriting updates written by others. Prompt transmission of WRITE data enables the prompt detection of write holes and reduces the risk of data corruption.

WRITE Durability The uncacheable file data attribute does not, by itself, dictate the stable_how value a client uses on WRITE operations. The protocol-level requirement is the following durability invariant: when the application's write call returns successfully, the WRITE data MUST be durable on the server. A client honoring the uncacheable file data attribute MAY satisfy this invariant by either:

• issuing WRITEs with stable_how of FILE_SYNC4 or DATA_SYNC4, in which case the data is durable on the WRITE response, or
• issuing WRITEs with stable_how of UNSTABLE4 and a COMMIT that completes before the application's write call returns. If the COMMIT response indicates a changed write verifier, the client MUST re-issue the affected WRITEs from the application's buffer, which remains available for the duration of the write call.

Clients MUST NOT defer COMMIT past the point at which the application's write call returns, because no client-side copy of the WRITE data is retained beyond that point and the data could not otherwise be re-driven after a server reboot. The transient retention of WRITE data needed to complete an in-flight UNSTABLE4 and COMMIT exchange is not considered "caching" for the purposes of this attribute. The attribute concerns the long-lived retention of file data for the purpose of satisfying future READs or combining future WRITEs.

Read Caching The uncacheable file data attribute may also influence the use of read caching. Retaining cached READ data while other clients concurrently modify disjoint byte ranges of the same file can result in read-modify-write operations based on stale data. Clients SHOULD ensure that cached file data is not reused without first revalidating it. At a minimum, clients MUST revalidate metadata necessary to ensure correctness of cached file data, including the change attribute and file size. These attributes provide the primary mechanism for detecting modification of file contents. Meeting this MUST requirement satisfies the general SHOULD obligation above. Clients MAY revalidate additional attributes (e.g., modification time or change time) as required by their local semantics or application requirements. Failure to perform such revalidation can result in the client presenting stale or inconsistent file state (e.g., incorrect size or timestamps) to the application. Suppressing read caching in addition to suppressing write-behind caching can further reduce the risk of stale-data overwrite in multi-writer workloads. However, in some cases read caching may remain appropriate when another NFSv4.2 mechanism ensures a consistent view of the file, such as a delegation.

Relationship to Direct I/O While similar in intent to O_DIRECT () and forcedirectio (), the uncacheable file data attribute operates at the protocol level and is advisory. Clients retain flexibility in how they satisfy the requirements described above.

Setting the Uncacheable File Data Attribute In some deployments, applications or administrative tools may request that the uncacheable file data attribute be set on a file in order to influence client behavior. For example, applications that require predictable data visibility or that would otherwise rely on mechanisms such as O_DIRECT may use this attribute as a protocol-visible hint to the server. However, the setting of this attribute is subject to server policy. The server is responsible for determining whether a request to set or clear the attribute is permitted. This may depend on factors such as administrative configuration, export policy, or access control mechanisms. Requests that are not permitted MUST be rejected using existing NFSv4 error codes (e.g., NFS4ERR_INVAL or NFS4ERR_PERM). This document does not require a server to implement any particular policy, nor any particular means of configuring one. A server that always permits, or always refuses, requests to set or clear the attribute conforms to this document; what the protocol requires is only the error returned when a request is refused. One possible deployment model is for a server or administrator to configure a mount (see ) option such that newly created files under a given export are marked as uncacheable file data. In such a configuration, a client may request setting of the attribute at file creation time, via the createattrs of OPEN. This approach is conceptually similar in intent to the Solaris forcedirectio mount option (see ), but differs in scope and visibility in that it allows DIRECT-I/O-like behavior to be applied without requiring changes to individual applications. Unlike local mechanisms such as forcedirectio, the NFSv4.2 attribute is visible to all clients accessing the file and is intended to convey server-side knowledge or policy in a distributed environment. Changes to the uncacheable file data attribute while a file is actively in use may not be immediately reflected in client behavior. A client that has already opened a file MAY continue to operate based on its existing caching behavior and is not required to immediately alter its behavior in response to a change in the attribute. Clients are expected to observe attribute changes through normal NFSv4 mechanisms (e.g., GETATTR or revalidation) and apply updated behavior as appropriate for subsequent operations.

Implementation Status Note to RFC Editor: please remove this section prior to publication. There is a prototype Hammerspace server which implements the uncacheable file data attribute and a prototype Linux client which treats the attribute as an indication to use O_DIRECT-like behavior for file access. For the prototype, all files created under the mount point have the fattr4_uncacheable_file_data set to be true. Experience with the prototype indicates that the uncacheable file data attribute can provide many of the practical benefits of O_DIRECT without requiring application modification. For applications that issue well-formed I/O requests, this approach has been observed to improve performance in many cases, while also reducing memory pressure and CPU utilization in the NFS client.

XDR for Uncacheable Attribute

Extraction of XDR This document contains the external data representation (XDR) description of the uncacheable file attribute. The XDR description is presented in a manner that facilitates easy extraction into a ready-to-compile format. To extract the machine-readable XDR description, use the following shell script: #!/bin/sh grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??' ]]> For example, if the script is named 'extract.sh' and this document is named 'spec.txt', execute the following command: sh extract.sh < spec.txt > uncacheable_prot.x ]]> This script removes leading blank spaces and the sentinel sequence '///' from each line. 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 NFSv4.2 nfs4_prot.x file (generated from ). 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. While the XDR can be appended to that from , the code snippets should be placed in their appropriate sections within the existing XDR.

Security Considerations The uncacheable file data attribute does not introduce new authentication or authorization mechanisms and does not alter existing NFSv4.2 access control semantics. All operations that set or clear the attribute are subject to existing access control and server policy. In particular, a server MUST enforce appropriate authorization checks for SETATTR operations that modify the fattr4_uncacheable_file_data attribute. The ability to set or clear the attribute may be restricted based on administrative configuration, export policy, or other server-defined criteria. Because the attribute is visible to and may affect the behavior of multiple clients, servers SHOULD consider the implications of allowing unprivileged users to modify it. Inappropriate use of the attribute could impact performance or data access patterns for other clients accessing the same file. The uncacheable file data attribute is advisory and does not provide a security boundary. Clients MUST NOT rely on the presence or absence of this attribute to make access control decisions. Use of this attribute does not replace or modify existing cache consistency mechanisms or data integrity protections provided by NFSv4.2.

IANA Considerations This document has no IANA actions.

References Normative References This document describes the External Data Representation Standard (XDR) protocol as it is currently deployed and accepted. This document obsoletes RFC 1832. [STANDARDS-TRACK] This document describes NFS version 4 minor version 2; it describes the protocol extensions made from NFS version 4 minor version 1. Major extensions introduced in NFS version 4 minor version 2 include the following: Server-Side Copy, Application Input/Output (I/O) Advise, Space Reservations, Sparse Files, Application Data Blocks, and Labeled NFS. This document provides the External Data Representation (XDR) description for NFS version 4 minor version 2. This document describes the rules relating to the extension of the NFSv4 family of protocols. It covers the creation of minor versions, the addition of optional features to existing minor versions, and the correction of flaws in features already published as Proposed Standards. The rules relating to the construction of minor versions and the interaction of minor version implementations that appear in this document supersede the minor versioning rules in RFC 5661 and other RFCs defining minor versions. This document describes the Network File System (NFS) version 4 minor version 1, including features retained from the base protocol (NFS version 4 minor version 0, which is specified in RFC 7530) and protocol extensions made subsequently. The later minor version has no dependencies on NFS version 4 minor version 0, and is considered a separate protocol. This document obsoletes RFC 5661. It substantially revises the treatment of features relating to multi-server namespace, superseding the description of those features appearing in RFC 5661. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Linux man-pages project Linux man-pages project Oracle Solaris Documentation

Acknowledgments Trond Myklebust, Mike Snitzer, Jon Flynn, Keith Mannthey, and Thomas Haynes all worked on the prototype at Hammerspace. Rick Macklem, Chuck Lever, Dave Noveck, Barry Leiba, Vijay Gurbani, Claudio Allocchio, and Jamie Koehl reviewed the document. Chris Inacio, Chuck Lever, Brian Pawlowski, and Gorry Fairhurst helped guide this process.