Reporting of Errors via LAYOUTRETURN in NFSv4.2

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 22 September 2024.¶

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

In the Network File System version4 (NFSv4) with a Parallel NFS (pNFS) Flexible File Layout ([RFC8435]) server, during recovery after a restart, there is no mechanism for the client to inform the metadata server about an error which occurred during a WRITE (see Section 18.32 of [RFC8881]) operation to the data servers in the period of the outage.¶

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. Layout State Recovery

When a metadata server restarts, clients are provided a grace period where they are allowed to recover any state that they had established. With open files, the client can send an OPEN (see Section 18.16 of [RFC8881]) operation with a claim type of CLAIM_PREVIOUS (see Section 9.11 of [RFC8881]). The client uses the RECLAIM_COMPLETE (see Section 18.51 of [RFC8881]) operation to notify the metadata server that it is done reclaiming state.¶

The NFSv4 Flexible File Layout Type allows for the client to mirror files (see Section 8 of [RFC8435]). With client side mirroring, it is important for the client to inform the metadata server of any I/O errors encountered with one of the mirrors. This is the only way for the metadata server to determine one or more of the mirrors is corrupt and then repair the mirrors via resilvering. The client can use LAYOUTRETURN (see Section 18.44 of [RFC8881]) and the ff_ioerr4 (see Section 9.1.1 of [RFC8435]) structure to inform the metadata server of I/O errors.¶

A problem is that if the metadata server restarts and the client has errors it needs to report, it can not do so. The LAYOUTRETURN needs a layout stateid to proceed and there is no way for the client to recover layout state. As such, clients have no choice but to not recover files with I/O errors. In turn, the metadata server MUST assume that the mirrors are inconsistent and pick one for resilvering. It is a MUST because even if the metadata server can determine that the client did modify data during the outage, it MUST NOT assume those modifications were consistent.¶

If the server were to allow the client to use the anonymous stateid of all zeros (see Section 8.2.3 of [RFC8881]) for lrf_stateid in LAYOUTRETURN (see Section 18.44.1 of [RFC8881]), then the client could inform the metadata server of errors encountered. That in turn would allow the metadata server to accurately resilver the file by picking the correct mirror(s).¶

There are two error scenarios that can occur:¶

During the grace period:

If the client were to send any lrf_stateid in the LAYOUTRETURN other than the anonymous stateid of all zeros, then the metadata server would respond with an error of NFS4ERR_GRACE (see Section of 15.1.9.2 [RFC8881]).¶

After the grace period:

If the client were to send any lrf_stateid in the LAYOUTRETURN with the anonymous stateid of all zeros, then the metadata server would respond with an error of NFS4ERR_NO_GRACE (see Section 15.1.9.3 of [RFC8881]).¶

Also, when the metadata server builds the reply to the LAYOUTRETURN, it MUST NOT bump the seqid of the lorr_stateid.¶

The metadata server MUST NOT resilver a file if there are clients with outstanding layouts with iomode of LAYOUTIOMODE4_RW. Whether the metadata server prevents all I/O to the file until the resilvering is done or forces all I/O to go through the metadata server or allows a proxy server to update the new data file as it is being reslivered is all an implementation choice. The constraint is that the metadata server is responsible for the reconstruction of the data file and for the consistency of the mirrors. The metadata server MUST NOT have been resilvering the file such that it has a different layout (set of mirror instances) than the client before the restart of the metadata server. Further, the metadata server MUST NOT start a new resilvering of the file during the grace period unless there are no outstanding layouts with iomode of LAYOUTIOMODE4_RW.¶

If the metadata server detects that the layout being returned in the LAYOUTRETURN does not match the current mirror instances found for the file, then it should ignore the LAYOUTRETURN and resilver the file in question.¶

Finally, the metadata server MUST determine that any files which are neither explicitly recovered with a CLAIM_PREVIOUS nor have a reported error via a LAYOUTRETURN, have to be resilvered. The client has most likely restarted and lost any state.¶

3. Security Considerations

There are no new security considerations beyond those in [RFC7862].¶

4. IANA Considerations

IANA should use the current document (RFC-TBD) as the reference for the new entries.¶

Authors' Addresses