Internet DRAFT - draft-haynes-nfsv4-layrec
draft-haynes-nfsv4-layrec
Network File System Version 4 T. Haynes
Internet-Draft T. Myklebust
Updates: 8435 (if approved) Hammerspace
Intended status: Standards Track 12 March 2023
Expires: 13 September 2023
Reporting of Errors via LAYOUTRETURN in NFSv4.2
draft-haynes-nfsv4-layrec-00
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). A problem with servers
which do client side mirroring there is no means by which the client
can report errors to the metadata server. As such, the metadata
server has to assume that file needs resilvering. This document
presents a refinement to RFC8435 to allow the client to update the
metadata
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 13 September 2023.
Copyright Notice
Copyright (c) 2023 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. File Recovery . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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].
1.1. Definitions
See Section 1.1 of [RFC8435] for a more complete set of definitions.
(file) data: that part of the file system object that contains the
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data to be read or written. It is the contents of the object
rather than the attributes of the object.
data server (DS): a pNFS server that provides the file's data when
the file system object is accessed over a file-based protocol.
(file) metadata: the part of the file system object that contains
various descriptive data relevant to the file object, as opposed
to the file data itself. This could include the time of last
modification, access time, EOF position, etc.
metadata server (MDS): the pNFS server that provides metadata
information for a file system object.
1.2. 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.
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
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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]).
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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.
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>.
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Appendix A. Acknowledgments
None yet...
Authors' Addresses
Thomas Haynes
Hammerspace
Email: loghyr@hammerspace.com
Trond Myklebust
Hammerspace
Email: trondmy@hammerspace.com
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