Deviceid-Scoped Layout Recall for NFSv4.2

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Table of Contents

1. Introduction

In the Network File System version 4 (NFSv4) with a Parallel NFS (pNFS) metadata server ([RFC8881]), there is no mechanism for the metadata server to recall layouts from a client based on the deviceid (see Section 3.3.14 of [RFC8881]) those layouts reference. The Flex Files layout type ([RFC8435]) is the primary motivating use case: a Flex Files layout describes data spread across multiple data servers, each identified by a distinct deviceid, often deliberately deployed across separate power or fault domains so that a single domain failure does not lose data.¶

The absence of a deviceid-scoped recall has two distinct operational consequences. This section describes each, shows why the existing recall and notification mechanisms are the wrong shape to address it, and motivates the extension specified in the rest of the document.¶

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.¶

3. Client Capability Advertisement

Before the server may send a CB_LAYOUTRECALL with LAYOUTRECALL4_DEVICEID, it MUST know that the client supports the new union arm. Per [RFC8178] Section 6, a server MUST NOT send a new callback operation or new union arm to a client that has not indicated support for it.¶

A client that supports LAYOUTRECALL4_DEVICEID signals this by setting a new flag in the eia_flags field of the EXCHANGE_ID operation (see Section 18.35 of [RFC8881]):¶

///    const EXCHGID4_FLAG_SUPP_RECALL_DEVICEID = 0x02000000;
///

A client that sets EXCHGID4_FLAG_SUPP_RECALL_DEVICEID in its EXCHANGE_ID request advertises that it supports handling CB_LAYOUTRECALL with a LAYOUTRECALL4_DEVICEID recall type.¶

A server MUST NOT send a CB_LAYOUTRECALL with LAYOUTRECALL4_DEVICEID to a client that did not set EXCHGID4_FLAG_SUPP_RECALL_DEVICEID in its EXCHANGE_ID request. A server that does not wish to support this capability MAY ignore the flag.¶

If the client does not set EXCHGID4_FLAG_SUPP_RECALL_DEVICEID, the server MAY fall back to recalling individual layouts via LAYOUTRECALL4_FILE (one CB_LAYOUTRECALL per layout file that references the unavailable deviceid), as described in Section 20.3 of [RFC8881]. This is less efficient but correct, and preserves interoperability with clients that predate this extension.¶

4. Extension to CB_LAYOUTRECALL - Recall Layout from Client

The original union layoutrecall4 (see Section 20.3.1 of [RFC8881]) is:¶

enum layoutrecall_type4 {
        LAYOUTRECALL4_FILE = LAYOUT4_RET_REC_FILE,
        LAYOUTRECALL4_FSID = LAYOUT4_RET_REC_FSID,
        LAYOUTRECALL4_ALL  = LAYOUT4_RET_REC_ALL
};

union layoutrecall4 switch(layoutrecall_type4 lor_recalltype) {
   case LAYOUTRECALL4_FILE:
           layoutrecall_file4 lor_layout;
   case LAYOUTRECALL4_FSID:
           fsid4              lor_fsid;
   case LAYOUTRECALL4_ALL:
           void;
   };

The proposed extension is:¶

///    const LAYOUT4_RET_REC_DEVICEID  = 4;
///
///    enum layoutrecall_type4 {
///           LAYOUTRECALL4_FILE     = LAYOUT4_RET_REC_FILE,
///           LAYOUTRECALL4_FSID     = LAYOUT4_RET_REC_FSID,
///           LAYOUTRECALL4_ALL      = LAYOUT4_RET_REC_ALL,
///           LAYOUTRECALL4_DEVICEID = LAYOUT4_RET_REC_DEVICEID
///   };
///
/// union layoutrecall4 switch(layoutrecall_type4 lor_recalltype) {
///   case LAYOUTRECALL4_FILE:
///           layoutrecall_file4 lor_layout;
///   case LAYOUTRECALL4_FSID:
///           fsid4              lor_fsid;
///   case LAYOUTRECALL4_DEVICEID:
///           deviceid4          lor_deviceid;
///   case LAYOUTRECALL4_ALL:
///           void;
///   };

Note that LAYOUT4RET_REC* constants are shared between layoutrecall_type4 (used in CB_LAYOUTRECALL) and layoutreturn_type4 (used in LAYOUTRETURN, see Section 18.44.1 of [RFC8881]). Adding LAYOUT4_RET_REC_DEVICEID = 4 therefore also extends layoutreturn_type4 with a LAYOUTRETURN4_DEVICEID value. A client that receives a LAYOUTRETURN with lor_recalltype set to LAYOUTRETURN4_DEVICEID and does not recognise it MUST return NFS4ERR_UNION_NOTSUPP per [RFC8178].¶

The server MUST NOT send CB_LAYOUTRECALL with LAYOUTRECALL4_DEVICEID to a client that has not set EXCHGID4_FLAG_SUPP_RECALL_DEVICEID (see Section 3). This satisfies the requirement in Section 6 of [RFC8178] that a server establish client awareness before sending new callback extensions.¶

The existing clora_iomode field in CB_LAYOUTRECALL4args (see Section 20.3.1 of [RFC8881]) applies normally: the client MUST return all layouts matching both the given deviceid and the given iomode. The server can determine that the client no longer has any layouts with the given deviceid and iomode once the client replies with NFS4ERR_NOMATCHING_LAYOUT.¶

5. Extraction of XDR

This document contains the external data representation (XDR) [RFC4506] description of the extension to CB_LAYOUTRECALL. 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:¶

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

For example, if the script is named 'extract.sh' and this document is named 'spec.txt', execute the following command:¶

<CODE BEGINS>
sh extract.sh < spec.txt > recalldevice.x
<CODE ENDS>

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 [RFC7863]). 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 extracted XDR extends types that are already defined in [RFC7863] rather than introducing new types. An implementer MUST integrate the extracted block into the base XDR by replacing the existing layoutrecall_type4 enum declaration and layoutrecall4 union declaration with the extended versions in this document, and by adding the LAYOUT4RET_REC_DEVICEID constant in the same file area as the existing LAYOUT4_RET_REC* constants. Appending the extracted block verbatim to the RFC 7863 XDR produces duplicate- type errors at compile time.¶

The EXCHGID4_FLAG_SUPP_RECALL_DEVICEID constant is purely additive and is added to the EXCHGID4 flag constant block without replacing anything.¶

6. Security Considerations

The extension introduced by this document inherits the Security Considerations of CB_LAYOUTRECALL in Section 20.3 of [RFC8881] and of NFSv4.2 in [RFC7862]. Three aspects of the deviceid-scoped recall warrant explicit mention.¶

Blast radius:

A single CB_LAYOUTRECALL with LAYOUTRECALL4_DEVICEID can cause a client to return many layouts in one callback exchange. This is not a new risk category -- LAYOUTRECALL4_FSID and LAYOUTRECALL4_ALL already allow wider-scoped recalls -- but administrators sizing callback-channel timeouts and client-side recall-handler resources SHOULD account for the possibility that a deviceid-scoped recall triggers per-layout cleanup work proportional to the number of layouts that reference the deviceid.¶

Cross-client scope:

In deployments where a single deviceid is shared across clients serving different tenants or applications, a deviceid-scoped recall is cross-client by construction: every client that holds a layout referencing the deviceid is recalled. This is the same property as LAYOUTRECALL4_FSID and does not introduce a new information-exposure surface. Deployments that prefer per-tenant failure-domain isolation MAY treat this extension as an operational benefit: by assigning distinct deviceids per tenant, deviceid-scoped recall becomes a per-tenant operation with no cross-tenant effect.¶

Capability negotiation as defence in depth:

The capability flag defined in Section 3 ensures that a server cannot silently change the callback wire format for a client that did not opt in. A client that does not advertise EXCHGID4_FLAG_SUPP_RECALL_DEVICEID continues to receive only the layoutrecall_type4 values defined in [RFC8881]. A server that sends LAYOUTRECALL4_DEVICEID to a client that did not advertise support is in violation of Section 6 of [RFC8178]; clients that encounter such a violation MUST return NFS4ERR_UNION_NOTSUPP and SHOULD log the server for operator attention.¶

7. IANA Considerations

This document has no IANA actions.¶

8. References

Acknowledgments

Trond Myklebust and Paul Saab were involved in the initial requirements for this functionality.¶

Brian Pawlowski and Gorry Fairhurst helped guide this process.¶

Author's Address