RFC 9766: Extensions for Weak Cache Consistency in NFSv4.2's Flexible File Layout

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 https://www.rfc-editor.org/info/rfc9766.¶

Copyright Notice

Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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1. Introduction

In the Parallel NFS (pNFS) flexible file layout (see [RFC8435]), there is no mechanism for the data servers to update the metadata servers when the data portion of the file is modified. The metadata server needs this knowledge to correspondingly update the metadata portion of the file. If the client is using NFSv3 as the protocol with the data server, it can leverage Weak Cache Consistency (WCC) to update the metadata server of the attribute changes. In this document, we introduce a new operation called LAYOUT_WCC to NFSv4.2, which allows the client to periodically report the attributes of the data files to the metadata server.¶

Using the process detailed in [RFC8178], the revisions in this document become an extension of NFSv4.2 [RFC7862]. They are built on top of the External Data Representation (XDR) [RFC4506] generated from [RFC7863].¶

2. Weak Cache Consistency (WCC)

A pNFS layout type enables the metadata server to inform the client of both the storage protocol and the locations of the data that the client should use when communicating with the storage devices. The flexible file layout type, as specified in [RFC8435], describes how data servers using NFSv3 can be accessed. The client is restricted to performing the following NFSv3 operations on the filehandles provided in the layout: READ, WRITE, and COMMIT (see Sections 3.3.6, 3.3.7, and 3.3.21 of [RFC1813], respectively). In other words, the client may only use NFSv3 operations that act directly on the data portion of the file.¶

Because there is no control protocol (see [RFC8434]) possible with all data servers, NFSv3 is used as the control protocol. As such, the following NFSv3 operations are commonly used by the metadata server: CREATE, GETATTR, and SETATTR (see Sections 3.3.8, 3.3.1, and 3.3.2 of [RFC1813], respectively). That is, the metadata server is only allowed to use NFSv3 operations that directly act on the metadata portion of the data file. GETATTR allows the metadata server to mainly retrieve the mtime (modify time), ctime (change time), and atime (access time). The metadata server can use this information to determine if the client modified the file whilst it held an iomode of LAYOUTIOMODE4_RW (see Section 3.3.20 of [RFC8881]). Then it can determine the following for the metadata file: time_modify, time_metadata, and time_access (see Sections 5.8.2.43, 5.8.2.42, and 5.8.2.37 of [RFC8881], respectively). That is, it can determine the information to return to clients in an NFSv4.2 GETATTR response.¶

For example, the metadata server might issue an NFSv3 GETATTR operation to the data server, which is typically triggered by a client's NFSv4 GETATTR request to the metadata server. In addition to the cost of each individual GETATTR operation, the data server can be overwhelmed by a large volume of such requests. NFSv3 addressed a similar challenge by including a post-operation attribute in the READ and WRITE operations to report WCC data (see Section 2.6 of [RFC1813]).¶

Each NFSv3 operation entails a single round trip between the client and server. Consequently, issuing a WRITE followed by a GETATTR would require two round trips. In that situation, the retrieved attribute information is regarded as having strict server-client consistency. By contrast, NFSv4 enables a WRITE and GETATTR to be combined within a compound operation, which requires only one round trip. This combined approach is likewise considered to have strict server-client consistency. Essentially, NFSv4 READ and WRITE operations omit post-operation attributes, allowing the client to determine whether it requires that information.¶

Whilst NFSv4 got rid of the requirement for WCC information to be supplied by the WRITE or READ operations, the introduction of pNFS reintroduces the same problem. The metadata server has to communicate with the data server in order to get the data that could be provided by a WCC model.¶

With the flexible file layout type, the client can leverage the NFSv3 WCC to service the proxying of times (see Section 5 of [RFC9754]), but the granularity of this data is limited. With client-side mirroring (see Section 8 of [RFC8435]), the client has to aggregate the N mirrored files in order to send one piece of information instead of N pieces of information. Also, the client is limited to sending that information only when it returns the delegation.¶

This document introduces a new NFSv4.2 operation, LAYOUT_WCC, which enables the client to provide the metadata server with information obtained from the data server. The client is responsible for gathering the NFSv3 WCC data, returned by the three permissible NFSv3 operations, and conveying it back to the metadata server as part of NFSv4.2 attributes. The metadata server MAY therefore avoid issuing costly NFSv3 GETATTR calls to the data servers. Because this approach relies on a weak model, the metadata server MAY still perform these calls if it chooses to strengthen the model.¶

3. Operation 77: LAYOUT_WCC - Layout Weak Cache Consistency

<CODE BEGINS>
/// struct LAYOUT_WCC4args {
///         stateid4        lowa_stateid;
///         layouttype4     lowa_type;
///         opaque          lowa_body<>;
/// };

<CODE ENDS>

<CODE BEGINS>
/// struct LAYOUT_WCC4res {
///         nfsstat4                lowr_status;
/// };

<CODE ENDS>

<CODE BEGINS>
/// struct ff_data_server_wcc4 {
///             deviceid4            ffdsw_deviceid;
///             stateid4             ffdsw_stateid;
///             nfs_fh4              ffdsw_fh_vers<>;
///             fattr4               ffdsw_attributes;
/// };
///
/// struct ff_mirror_wcc4 {
///             ff_data_server_wcc4  ffmw_data_servers<>;
/// };
///
/// struct ff_layout_wcc4 {
///             ff_mirror_wcc4       fflw_mirrors<>;
/// };

<CODE ENDS>

4. Extraction of XDR

This document contains the XDR [RFC4506] description of the new NFSv4.2 operation LAYOUT_WCC. 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 new NFSv4.2 operation LAYOUT_WCC.¶

<CODE BEGINS>
#!/bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'

<CODE ENDS>

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:¶

<CODE BEGINS>
sh extract.sh < spec.txt > layout_wcc.x

<CODE ENDS>

The effect of the script is to remove leading blank space from each line, plus a sentinel sequence of '///'. 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 [RFC7863]). This includes both nfs types that end with a 4 (such as offset4 and length4) as well as more generic types (such as uint32_t and uint64_t).¶

While the XDR can be appended to that from [RFC7863], the various code snippets belong in their respective areas of that XDR.¶

5. Security Considerations

There are no new security considerations beyond those in [RFC8435].¶

6. IANA Considerations

This document has no IANA actions.¶

Authors' Addresses