Reporting of Errors via LAYOUTRETURN in NFSv4.2

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Table of Contents

1. Introduction

In the Network File System version4 (NFSv4) with a Parallel NFS (pNFS) Flexible File Layout ([RFC8435]) server, during file recovery after a restart, there is no mechanism for the client to inform the metadata servers for when an error occurred during a WRITE operation to the data servers.¶

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. File 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 and the ff_ioerr4 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 as there is no control protocol between the metadata server and the data servers, the metadata server has to assume that the client could have written data whilst it held a layout of iomode LAYOUTIOMODE4_RW.¶

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.¶

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.¶

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 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. If the metadata server is tracking write intents (the number of outstanding layouts with iomode of LAYOUTIOMODE4_RW), then it can relax this constraint and start a resilvering once all write intents have been recovered for that file.¶

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 MAY assume that any files which are neither explicitly recovered with a CLAIM_PREVIOUS nor have a reported error via a LAYOUTRETURN, do not need to be resilvered. The client is most likely using the forgetful model of returning layouts (see Section 12.5.5.1 of [RFC8881]).¶

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