wdiff rfc9737.original rfc9737.txt

Network File System Version 4

Internet Engineering Task Force (IETF) T. Haynes
Internet-Draft
Request for Comments: 9737 T. Myklebust
Intended status:
Category: Standards Track Hammerspace
Expires: 25 May
ISSN: 2070-1721 February 2025 21 November 2024

Reporting of Errors via LAYOUTRETURN in NFSv4.2
draft-ietf-nfsv4-layrec-04

Abstract

The Parallel Network File System (pNFS) allows for a file's metadata
(MDS) and data (DS) to be on different servers. When the metadata
server is restarted, the client can still modify the data file
component. During the recovery phase of startup, the metadata server
and the data servers work together to recover state (which files are
open, last modification time, size, etc.). If the client has not
encountered errors with the data files, then the state can be
recovered, avoiding
recovered and the resilvering of the data files. files can be avoided. With
any errors, there is no means by which the client can report errors
to the metadata server. As such, the metadata server has to assume
that a file needs resilvering. This document presents an extension
to
RFC8435 RFC 8435 to allow the client to update the metadata and avoid the
resilvering.

Note

This note is to be removed before publishing as an RFC.

Discussion of this draft takes place on the NFSv4 working group
mailing list (nfsv4@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/nfsv4/. Working Group
information can be found at https://datatracker.ietf.org/wg/nfsv4/
about/.

Status of This Memo

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This Internet-Draft will expire on 25 May 2025.
https://www.rfc-editor.org/info/rfc9737.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Layout State Recovery . . . . . . . . . . . . . . . . . . . . 3
2.1. When to Resilver . . . . . . . . . . . . . . . . . . . . 4
2.2. Version Mismatch Considerations . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
Appendix A.
Acknowledgments . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7

1. Introduction

In the Network File System version4 version 4 (NFSv4) with a Parallel NFS
(pNFS) Flexible File Layout ([RFC8435]) [RFC8435] server, during recovery after a
restart, there is no mechanism for the client to inform the metadata
server about an error which that occurred during a WRITE operation (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 External Data Representation (XDR) [RFC4506] generated
from [RFC7863].

1.1. Definitions

See Section 1.1 of [RFC8435] for a set of definitions.

1.2. Requirements Language

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
'OPTIONAL'
"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.

2. Layout State Recovery

When a metadata server restarts, clients are provided a grace
recovery period where they are allowed to recover any state that they
had established. With open files, the client can send an OPEN
operation (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 operation (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 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 if one or more of the mirrors is are
corrupt and then repair the mirrors via resilvering (see Section 1.1
of [RFC8435]). The client can use LAYOUTRETURN (see Section 18.44 of
[RFC8881]) and the ff_ioerr4 structure (see Section 9.1.1 of
[RFC8435])
structure to inform the metadata server of I/O errors.

A problem is that arises when the metadata server restarts and the client has
errors it needs to report, it can not report but cannot do so. Section 12.7.4 of
[RFC8881] requires that the client MUST stop using layouts. While
the intent there is that the client MUST stop doing I/O to the
storage devices, it is also true that the layout stateids are no
longer valid. The LAYOUTRETURN needs a layout stateid to proceed proceed,
and the client can not cannot get a layout during grace recovery (see
Section 12.7.4 of [RFC8881]) 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.

To fix this issue, the metadata server MUST accept for the
lrf_stateid in LAYOUTRETURN (see Section 18.44.1 of [RFC8881]) the anonymous
stateid of all zeros (see Section 8.2.3 of [RFC8881]) for the
lrf_stateid in LAYOUTRETURN (see Section 18.44.1 of [RFC8881]). The
client can use this anonymous stateid to inform the metadata server
of errors encountered. The metadata server can then accurately
resilver the file by picking the mirror(s) that do does not have any
associated errors.

During the grace period, if the client sends a an lrf_stateid in the
LAYOUTRETURN with any value other than the anonymous stateid of all
zeros, then the metadata server MUST now respond with an error of
NFS4ERR_GRACE (see Section of 15.1.9.2 of [RFC8881]). After the grace
period, if the client sends a an lrf_stateid in the LAYOUTRETURN with a
value of the anonymous stateid of all zeros, then the metadata server
MUST now 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
when a an lrf_stateid with the value of the anonymous stateid of all
zeros it MUST NOT bump the seqid of the lorr_stateid.

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 MUST ignore the LAYOUTRETURN and resilver the file
in question.

The metadata server MUST resilver any files which that are neither
explicitly recovered with a CLAIM_PREVIOUS nor have a reported error
via a LAYOUTRETURN. The client has most likely restarted and lost
any state.

2.1. When to Resilver

A write intent occurs when a client opens a file and gets a
LAYOUTIOMODE4_RW from the metadata server. The metadata server MUST
track outstanding write intents intents, and when it restarts, it MUST track
recovery of those write intents. The method that the metadata server
uses to track write intents is implementation specific, i.e., outside
of
the scope of this document.

The decision to resilver a file depends on how the client recovers
the file before the grace period ends. If the client reclaims the
file and reports no errors, the metadata server MUST NOT resilver the
file. If the client reports an error on the file, then the file MUST
be resilvered. If the client does not reclaim or report an error
before the grace period ends, then under the old behavior, the
metadata server MUST resilver the file.

The resilvering process is broadly to:

1. fence the file (see Section 2.2 of [RFC8435]),

2. record the need to resilver,

3. release the write intent, and

4. once there are no write intents on the file, start the
resilvering process.

The metadata server MUST NOT resilver a file if there are clients
with outstanding write intents. I.e., intents, i.e., multiple clients might have the
file open with write intents. As it the metadata server MUST track
write intents, it MUST also track the need to resilver. I.e., resilver, i.e., if the
metadata server restarts during the grace period, it MUST restart the
file recovery if it replays the write intent intent, or else it MUST start
the resilvering if it replays the resilvering intent.

Whether the metadata server prevents all I/O to the file until the
resilvering is done or done, forces all I/O to go through the metadata
server
server, or allows a proxy server to update the new data file as it is
being reslivered resilvered 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.

If the metadata server does allow the client access to the file
during the resilvering, then the client MUST have the same layout
(set of mirror instances) after the metadata server as before. One
way that such a resilvering can occur is for a proxy server to be
inserted into the layout. That server will be copying a good mirror
instance to a new instance. As it gets I/O via the layout, it will
be responsible for updating the copy it is performing. This
requirement is that the proxy server MUST stay in the layout until
the grace period is finished.

2.2. Version Mismatch Considerations

The metadata server has no expectations for the client to use this
new functionality. Therefore, if the client does not use it, the
metadata server will function normally.

If the client does use the new functionality and the metadata server
does not support it, then the metadata server MUST reply with a
NFS4ERR_BAD_STATEID to the LAYOUTRETURN. If the client detects a
NFS4ERR_BAD_STATEID error in this scenario, it should fall back to
the old behavior of not reporting errors.

3. Security Considerations

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

4. IANA Considerations

There are

This document has no IANA considerations for this document. actions.

5. References

5.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.

[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.

[RFC7863] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 External Data Representation Standard (XDR)
Description", RFC 7863, DOI 10.17487/RFC7863, November
2016, <https://www.rfc-editor.org/info/rfc7863>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.

[RFC8435] Halevy, B. and T. Haynes, "Parallel NFS (pNFS) Flexible
File Layout", RFC 8435, DOI 10.17487/RFC8435, August 2018,
<https://www.rfc-editor.org/info/rfc8435>.

[RFC8881] Noveck, D., Ed. and C. Lever, "Network File System (NFS)
Version 4 Minor Version 1 Protocol", RFC 8881,
DOI 10.17487/RFC8881, August 2020,
<https://www.rfc-editor.org/info/rfc8881>.

Appendix A.

Acknowledgments

Tigran Mkrtchyan, Jeff Layton, and Rick Macklem provided reviews of
the document.

Authors' Addresses

Thomas Haynes
Hammerspace
Email: loghyr@gmail.com

Trond Myklebust
Hammerspace
Email: trondmy@hammerspace.com