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General Network File System Version 4 Internet-Draft Network File System version 4.2 (NFSv4.2) clients may cache the file attributes returned by READDIR alongside each directory entry. Such a cache is not invalidated by the directory's change attribute, which reflects changes to the directory and its entries but not writes to the files those entries name, so it can become stale when another client changes one of those files. In some deployments this produces incorrect size and timestamp values often enough to be a problem. This document introduces an uncacheable dirent metadata attribute for NFSv4.2 that allows a server to identify a directory for which an honoring client goes to the server for each enumeration, and does not report an entry's attributes from a value it held before that READDIR. Note to Readers Note to RFC Editor: please remove this section prior to publication. 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 may cache the file attributes returned by READDIR alongside each directory entry, to reduce the volume of follow-on GETATTR traffic for entries the client has already seen. This caching is inherently best-effort -- writes to the underlying files can change those attributes at any time, and the directory's change attribute does not track such writes. In some deployments the cost of that staleness is high enough to be a problem; the conditions are described in . In this document, the term directory is used to describe the context in which directory entries are retrieved. The uncacheable dirent metadata attribute applies to dirent metadata -- the file object attributes, such as size and timestamps, returned alongside each entry -- and to what an honoring client reports of it. It does not prohibit caching of the directory object itself, nor does it affect caching of file data. When this best-effort caching returns stale size and timestamp information for concurrently modified files, it also undermines the effectiveness of uncacheable file data semantics () in the same deployment: applications can observe inconsistent metadata and data views even when file data caching is disabled. This document introduces the uncacheable dirent metadata attribute to NFSv4.2 to allow servers to identify the directories for which reporting dirent metadata a client held beforehand is unsuitable. Using the process detailed in , the revisions in this document become an extension of NFSv4.2 . They are built on top of the external data representation (XDR) generated from .

Deployment Motivation A class of deployment uses NFSv4.2 to serve a shared directory to many concurrent NFSv4.2 client writers, each writing files within the directory. Workloads of this kind are typical of High-Performance Computing (HPC) environments, where a single output directory may receive results from hundreds or thousands of compute nodes simultaneously, and of large-scale data-ingest pipelines where many producers append to a common landing directory. The files within such a directory have their attributes -- size and timestamps in particular -- modified at a high rate by clients other than the one performing READDIR. Section 10.6 permits a client to cache the file attributes returned by READDIR on the same basis as attributes obtained by GETATTR: cached per file, bounded by an upper time boundary, and revalidated against that file's change attribute. In a directory receiving writes from thousands of compute nodes, any nonzero cache lifetime yields stale size and time_modify for most entries most of the time, and revalidating each entry individually costs one GETATTR per entry -- the very traffic that requesting attributes in READDIR exists to avoid. NFSv4.2 gives a server no in-band way to tell a client that the file attributes it holds for the children of a particular directory should be refreshed each time the directory is enumerated; mount options shorten attribute cache lifetimes out of band and per client, not per directory. The staleness has correctness consequences, not merely cosmetic ones. An incremental backup or a directory-tree synchronization pass that decides what to copy from the size and time_modify reported for each entry will silently skip a file whose cached metadata predates a concurrent write, leaving data uncopied. This attribute lets a server mark the directories where that outcome is likely, so that an honoring client fetches current metadata on each enumeration. The fattr4_uncacheable_dirent_metadata attribute is the server's mechanism to identify a directory for which this risk is high enough that client-side caching is not safe. When the server sets the attribute on a directory, an honoring client retrieves dirent metadata from the server on each READDIR rather than from a local cache.

Definitions

readdir

A directory-read request made by an application, however the client's interface batches entries. Written in lower case throughout this document to distinguish it from READDIR, the NFSv4.2 operation ( Section 18.23).

enumeration

One pass over a directory: the readdirs of that pass and the READDIRs the client issues to satisfy them. A READDIR ordinarily supplies enough entries for many readdirs, so a requirement scoped to an enumeration does not imply a READDIR per readdir.

dirent

A directory entry -- the (name, fileid) pair that names a file or subdirectory within a directory. This is what a client maintains for an entry, whatever a given READDIR response carries on the wire; it is the pair POSIX exposes as d_name and d_ino. A dirent itself does not include the file attributes returned alongside it.

dirent metadata

The file attributes (size, time_modify, time_metadata, time_access, mode, owner, etc.) that a READDIR response can return alongside each dirent, whether or not a particular response carried them. These attributes belong to the underlying file object, not to the directory; they change when the underlying file is written, which is independent of the directory's change attribute. The term "dirent metadata" in this document is a naming convenience for "the file attributes a READDIR response carries alongside an entry"; it names that class of attributes, not the subset a given response happened to return, and it does not assert that those attributes inherit the directory's cache-coherence semantics.

dirent caching

A client-side cache of the dirents themselves -- the (name, fileid) pairs -- used to avoid repeated READDIR traffic. Whether such a cache remains valid is governed by the directory's change attribute: the directory changes when an entry is created, removed, or renamed, and a fileid is stable for as long as its entry names the same object. A fileid is therefore cached with the name rather than with the file attributes: writes to a file change its size and timestamps without touching either the name or the fileid. This document does not change what a client may hold in this cache or how it validates it; for a directory on which the attribute is set, however, each enumeration is satisfied by a READDIR, so for that directory the cache no longer avoids that traffic.

dirent metadata caching

A client-side cache of the dirent metadata returned alongside those entries, used to avoid repeated GETATTR traffic. Because those file attributes are not invalidated by the directory's change attribute (only by writes to the underlying files), this caching is inherently best-effort and subject to staleness whenever the underlying files are modified. This is the caching whose results the attribute defined in this document constrains: for a directory on which the attribute is set, limits what an honoring client may report for an entry, however the client structures the cache the value comes from.

uncacheable dirent metadata attribute

An NFSv4.2 file attribute that advises clients not to report dirent metadata, such as size and timestamps, from a value held before the READDIR that most recently returned the entry it describes.

honoring client

A client that implements this attribute and enforces the always-refetch behavior it defines for a directory on which the attribute is set. The attribute is advisory: a client that does not implement it, or that declines to enforce it, is non-honoring and may continue to report dirent metadata it held beforehand.

This document assumes familiarity with NFSv4.2 operations, attributes, and error handling as defined in and .

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.

Caching of Dirent Metadata The uncacheable dirent metadata attribute constrains what an honoring client may report for the entries of a particular directory. If both the client and the server support this attribute, and the attribute is set on a directory, an honoring client goes to the server for each enumeration and does not report an entry's attributes from a value it held before that READDIR. states the requirement normatively. It adds no constraint on the objects the entries name: an honoring client may continue to hold the dirents themselves, validated by the directory's change attribute as it would be for any other directory. Clients typically hold a single cache of a file object's attributes, populated by whichever operation last returned them; the requirement is on what the client reports, not on how it structures that cache. A server sets it on the directories where it knows the staleness of cached READDIR attributes is particularly likely and particularly damaging. It 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. Because the attribute governs what an honoring client reports for the entries of one directory, rather than the objects those entries name, it makes no claim about those objects. A file reached through a directory on which the attribute is not set is unaffected, including where the same file is linked into both a directory on which it is set and one on which it is not. This document specifies the required observable behavior rather than mandating a particular internal implementation strategy. Clients MAY employ more sophisticated mechanisms, such as time-limited caches that revalidate against the server on each READDIR, provided that the externally visible behavior satisfies . A client can determine whether the uncacheable dirent metadata attribute is supported for a given directory by examining the supported_attrs attribute for that directory's filesystem or by probing support using the procedures described in . A change to the attribute while a directory is in use may not be reflected in client behavior immediately. A client that has cached the directory's attributes MAY continue to behave as it did before the change and is not required to act on a value it has not yet observed. Two ordinary mechanisms bound that delay: the client's cached attributes for the directory expire under the upper time boundary described in Section 10.6, and a client revalidating a cached directory inspects the directory's change attribute ( Section 10.8.2), which moves when a server sets or clears this one. Clients are expected to observe the change through those mechanisms and to apply the rule in to subsequent enumerations. The uncacheable dirent metadata attribute governs what an honoring client may report for a directory's entries. It does NOT govern:

• How long a value remains reportable between enumerations. Attributes a client obtains for an individual entry at other times, by a direct GETATTR following a LOOKUP for example, remain governed between enumerations by the attribute-cache mechanisms already defined by NFSv4.2 and are subject to the same staleness from concurrent writes. The rule in applies at each enumeration and is indifferent to which operation last supplied a value.
• The directory's own attribute cache. The directory object's own attributes (mode, owner, etc.) can be cached normally and revalidated via the directory's change attribute as usual.
• Operations that do not return file attributes in their response (for example, LOOKUP without a following GETATTR, ACCESS). These are unaffected.

The uncacheable dirent metadata attribute addresses a different aspect of client-side caching than fattr4_uncacheable_file_data (). The file data attribute governs caching of file contents, while the dirent metadata attribute governs what a client reports for the entries of a directory it enumerates. The attributes are independent and may be used separately. This attribute follows the same pattern as fattr4_uncacheable_file_data () applied at the file-data layer. In both cases:

• The underlying NFSv4.2 protocol permits client-side caching that can become stale.
• Client caching of the relevant data is widely implemented in practice and reduces network traffic for stable objects.
• For specific objects where the deployment knows the caching will produce incorrect results, the server requires a mechanism to instruct an honoring client not to rely on it for those specific objects.
• The attribute does not redefine the legality of caching in the general case. It is a per-object server-side signal that the caching is known to be unsuitable for that object.

The attribute does NOT make dirent metadata caching reliable for directories where it is not set. Clients MUST NOT interpret the absence of fattr4_uncacheable_dirent_metadata, or its value being false, as a guarantee that cached READDIR attributes are authoritative. As stated in Section 10.6, all client-cached attributes are subject to staleness; the attribute defined in this document only identifies directories for which staleness is particularly likely and particularly damaging. The base specification separates the two concerns this attribute is often accused of conflating: Section 10.8.2 governs caching of the directory entries themselves, while Section 10.6 governs caching of the file attributes that arrive alongside them. This attribute leaves what a client may hold under either unchanged; it requires a READDIR for each enumeration and constrains what may be reported for an entry, as states. This attribute does not define behavior for positive or negative name caching, nor for LOOKUP results other than the file attributes it constrains an honoring client from reporting for an entry. A directory delegation ( Section 10.9) lets a client cache a directory's entries and the directory's own attributes until the server recalls the delegation. It is not recalled when the attributes of an entry within the directory change ( Sections 10.9.2 and 10.9.4), so a directory delegation does not, by itself, keep the file attributes returned by READDIR fresh. NOTIFY4_CHANGE_CHILD_ATTRS, requested through GET_DIR_DELEGATION, can deliver changed child attributes to a delegated client, but it is not a substitute for this attribute in the deployments of : GET_DIR_DELEGATION is OPTIONAL and is not implemented by the clients and servers those deployments use; notification cost scales with the number of delegated clients times the number of changes, which a directory written by thousands of clients makes prohibitive (and Section 10.9.4 permits a server that finds a directory is causing too many notifications to decline to delegate it); and the dirent_notif_delay attribute lets a server bound or refuse child-attribute notification, so a client cannot rely on notification for freshness.

Uncacheable Dirent Metadata The fattr4_uncacheable_dirent_metadata attribute is a read-write boolean attribute that applies to directory objects. Authorization to query or modify this attribute is governed by existing NFSv4.2 authorization mechanisms. The attribute applies only to directory objects. A server that receives a GETATTR requesting fattr4_uncacheable_dirent_metadata on an object that is not a directory 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 an object the attribute does not describe. A server that receives a SETATTR requesting fattr4_uncacheable_dirent_metadata on an object that is not a directory MUST return NFS4ERR_WRONG_TYPE ( Section 15.1.2.9). This attribute is set per directory. This document does not define propagation of the attribute to subdirectories created within a directory on which it is set; any such inheritance is a matter of local server policy. If a directory object has the uncacheable dirent metadata attribute set, an honoring client MUST NOT satisfy a readdir of that directory from READDIR results obtained during a different enumeration, and MUST NOT report, for an entry, a value of a dirent metadata attribute that it received before the READDIR that most recently returned that entry, whatever operation supplied that value. A client holding an OPEN_DELEGATE_WRITE delegation on a file the directory names may hold a size or change value more current than the server's, which the server obtains from it by CB_GETATTR ( Section 10.4.3). The requirement above is directed at values older than what the server would return, and does not require such a client to replace its own values with older ones. An honoring client therefore either names the attributes it will report in that READDIR's attr_request, or obtains them afterwards; a value it held beforehand is not usable for that entry. An honoring client SHOULD name them in attr_request: obtaining them afterwards costs one GETATTR per entry, which is the traffic the deployments of use this attribute to avoid. Entries carried by the READDIRs of a single enumeration MAY be retained until that enumeration completes, and their metadata MAY be retained after it: what bounds the reporting of that metadata is the next enumeration, under the rule above, and the attribute-cache mechanisms of Section 10.6 until then. Section 10.8.2 requires such a cache to be a consistent snapshot of directory contents, validated by the directory's change attribute; because that attribute does not move when a file the directory names is written, it provides no corresponding guarantee for the entries' file attributes, which are as of the READDIR that carried them. The uncacheable dirent metadata attribute does not modify the semantics of the NFSv4.2 change attribute. Clients MUST continue to use the change attribute to detect directory modifications and to determine when directory contents may have changed, even for a directory on which this attribute is set. Constraining what an honoring client may report for an entry does not remove the need for change-based validation. This attribute is advisory, so servers SHOULD NOT rely on it for correctness: a client that does not implement it, or that declines to enforce it, may continue to report dirent metadata from a value it held before the READDIR that most recently returned the entry. A server cannot distinguish those clients from honoring ones. Observing a GETATTR or a SETATTR of the attribute shows only that a client knows the attribute exists, not that it enforces the rule of , so such a request is not a basis for assuming it does. A directory delegation would let a client serve dirent metadata from its cache without refetching, which is incompatible with the always-refetch rule this attribute defines. Accordingly, if a directory has the uncacheable dirent metadata attribute set and an outstanding directory delegation, the server MUST recall the delegation, after which the client follows the always-refetch rule on each subsequent readdir. A server MUST NOT grant a new directory delegation on a directory while the uncacheable dirent metadata attribute is set on that directory.

Example: Directory Enumeration With and Without Dirent Metadata Caching This example illustrates the difference in client-visible behavior when dirent metadata caching is enabled versus when the uncacheable dirent metadata attribute is set on a directory. In both scenarios each readdir("/dir") is a separate enumeration -- the application opens the directory, reads it to end-of-file and closes it, and opens it again for the second. The set of entries does not change between the two; an attribute value of one entry is updated at the server between them. The difference is whether the stat after the second readdir observes the updated value.

Classic Directory Enumeration (Dirent Metadata Cached) In this scenario, the client caches dirent metadata obtained from the server and reuses it after a second readdir.

Dirent Metadata Cached ------------------------> | entries: | a (size=100) | b (size=200) |<--------------------<------------------------ |<-- names a, b (attributes retained per entry, bounded by the attribute cache lifetime) (concurrent writer extends a from size=100 to size=500) readdir("/dir") | (served from the cached | READDIR result; no | network traffic) |<-- names a, b stat("/dir/a") | (served from the retained | READDIR attributes) |<-- size=100 ]]>

In this case, shows a second readdir satisfied from the cached result of the first. No READDIR reaches the server, so nothing refreshes what the client holds for entry a, and the stat that follows reports the size as it was at the time of the first READDIR -- not the update the server took between the two. This behavior maximizes performance and is what Sections 10.6 and 10.8.2 permit, but for the duration of the cache lifetime it can result in applications observing dirent attribute values that do not reflect the current state of the files the entries name.

Directory Enumeration With Uncacheable Dirent Metadata In this scenario, the directory has the uncacheable dirent metadata attribute set. The client retrieves dirent metadata from the server on each READDIR.

Dirent Metadata Not Cached ------------------------> | entries: | a (size=100) | b (size=200) |<--------------------<------------------------ |<-- names a, b (concurrent writer extends a from size=100 to size=500) readdir("/dir") | | READDIR, size and | time_modify requested | (cached result not used) |-------------------->------------------------> | entries: | a (size=500) | b (size=200) |<--------------------<------------------------ |<-- names a, b stat("/dir/a") |<-- size=500 ]]>

In this case, shows the second readdir going to the server, and the stat that follows reporting what that READDIR returned. The set of entries is unchanged between the two calls; only the attribute value differs. The client may still cache other information, provided it reports no value for these entries that it received before the READDIR that most recently returned them.

Discussion This example demonstrates that the uncacheable dirent metadata attribute does not mandate a particular client implementation, but it does require the always-refetch behavior specified in . The attribute ensures that an honoring client's report of an entry's attributes reflects the server's state as of the enumeration that returned it, in deployments where staleness of READDIR-returned attributes is known to be a recurring problem.

Implementation Status Note to RFC Editor: please remove this section prior to publication. There is a prototype Hammerspace server which implements the uncacheable dirent metadata attribute and a prototype Linux client which treats the attribute as an indication to retrieve directory- entry metadata from the server on each READDIR rather than from a local cache. In the prototype, directories whose contents change at the server at a rate exceeding typical client cache lifetimes are marked with the fattr4_uncacheable_dirent_metadata attribute. The Linux client decodes the attribute in fs/nfs/nfs4xdr.c into a per-inode flag (nfsi->uncacheable_dirent_metadata, declared in include/linux/nfs_fs.h). The readdir path in fs/nfs/dir.c consults this flag to skip the readdir cache and refetch from the server on each readdir call that is not already at end of directory. It also forces the client onto its attribute-returning READDIR path for such a directory, where the server has been found capable of it: a READDIR that returns only names would refresh the entries but leave their attributes to the client's attribute caches, and the cache-bypassing path above never accrues the cache usage that would otherwise select that path for the continuation READDIRs of a large directory. Clients may employ more sophisticated mechanisms, such as time-limited caches that revalidate against the server on each READDIR, provided that the externally observable behavior satisfies . The Linux client implementation encodes this attribute as a flag distinct from the companion file-data attribute defined in ; the two attributes are separated as the two documents specify. That implementation is posted to linux-nfs (patches 4-6 of https://lore.kernel.org/linux-nfs/cover.1785140181.git.snitzer@kernel.org/). Experience with the prototype indicates that the attribute enables servers to identify directories whose contents change faster than typical NFSv4.2 client cache lifetimes can track, while remaining compatible with existing NFSv4.2 semantics.

XDR for the Uncacheable Dirent Metadata Attribute

Extraction of XDR This document contains the external data representation (XDR) description of the uncacheable dirent metadata 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: For example, if the script is named 'extract.sh' and this document is named 'spec.txt', execute the following command: 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 This attribute is not a security mechanism. It addresses correctness of client-side caching when client-cached dirent metadata can become stale relative to the current state of the directory at the server. It does not change NFSv4.2 authentication or authorization semantics, and it does not impose access controls on the entries it describes. Authorization to set or modify the fattr4_uncacheable_dirent_metadata attribute is governed by existing NFSv4.2 authorization mechanisms. Servers MAY restrict modification of this attribute based on local policy, file ownership, or access control rules. This document does not define a new authorization model. Because the attribute is visible to and affects the caching behavior of all honoring clients, servers should consider the implications of allowing unprivileged users to set or clear it. Setting the attribute on a directory forces honoring clients to abandon READDIR caching and refetch dirent metadata on every enumeration, which can increase load on the server and on other clients. A server MAY restrict modification of the attribute based on administrative configuration, export policy, or ownership. This attribute does not change the semantics of sec_label or the enforcement of MAC security policies. A client's obligations under Labeled NFS (see for background, and Section 9 for the NFSv4.2 mechanism) are the same whether dirent metadata is refetched or served from a cache. The uncacheable dirent metadata attribute allows servers to indicate that dirent metadata should not be assumed to remain valid across enumerations of the directory.

IANA Considerations This document has no IANA actions. NFSv4.2 attribute numbers are assigned by working group coordination rather than through an IANA registry. This document uses attribute number 88, chosen alongside attribute number 87 in .

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 Hammerspace 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. This memo outlines high-level requirements for the integration of flexible Mandatory Access Control (MAC) functionality into the Network File System (NFS) version 4.2 (NFSv4.2). It describes the level of protections that should be provided over protocol components and the basic structure of the proposed system. The intent here is not to present the protocol changes but to describe the environment in which they reside.

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, Sorin Faibish, Christoph Hellwig, Jeff Layton, and Jamie Koehl reviewed the document. Chris Inacio, Brian Pawlowski, Chuck Lever, Zahed Sarker, and Gorry Fairhurst helped guide this process.