Extending the Opening of Files in NFSv4.2

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Table of Contents

1. Introduction

In the Network File System version4 (NFSv4) a client may be granted a delegation for a file. This allows the client to act as the authority of the file's metadata and data. In this document, we introduce some new semantics to both the open and the delegation process which allows the client to:¶

• detect an offline file, which may be located off premise.¶
• determine the extension of OPEN (see Section 18.16 of [RFC5661]) flags.¶
• during the OPEN procedure, get either the open or delegation stateids, but not both.¶
• cache both the access and modify times, reducing the number of times the client needs to go to the server to get that information.¶
• for clients using Parallel NFS (pNFS) (see Section 12 of [RFC5661]), periodically report the attributes of the data files to the metadata server.¶

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. Offline Files

If a file is offline, then the server locally has the file's attributes, but not the file's content. It has to be able to present to the client enough information to describe the file, but the content is not readily available. The cost of retrieving the data content is expensive, to the extent that the content should only be retrieved if it is going to be used. A graphical file manager (such as OSX's Finder) may want to access the beginning of the file to preview it for an user who is hovering his pointer over the file name. If the file is retrieved, it will most likely either be immediately thrown away or returned.¶

A compound with a GETATTR or READDIR can report the file's attributes without bringing the file online. However, either an OPEN or a LAYOUTGET might cause the file server to retrieve the archived data contents, bringing the file online. For non-pNFS systems, the OPEN operation requires a filehandle to the data content. For pNFS systems, the filehandle retrieved from an OPEN need not cause the data content to be retrieved. But when the LAYOUTGET operation is processed, a layout type specific mapping will cause the data content to be retrieved from offline storage.¶

If an operating system is not aware that the file is offline, it might inadvertently open the file to determine what type of file it is accessing. By adding the new attribute FATTR4_OFFLINE, a client can predetermine the availability of the file, avoiding the need to open it at all. Being offline might also mean that the file is archived in the cloud, i.e., there can be an expense in both retrieving the file to bring online and in sending the file back to offline status.¶

<CODE BEGINS>¶

///
/// typedef bool            fattr4_offline;
///

///
/// const FATTR4_OFFLINE            = 83;
///

<CODE ENDS>¶

3. Determining the Arguments to OPEN

The OPEN (See Section 18.16 of [RFC5661]) procedure returns an open stateid to the client to reference the state of the file. The client could also request a delegation stateid in the OPEN arguements. The file is said to be "open" to the client as long as the count of open and delegated stateids is greater than 0. Either type of stateid is suffucient to keep the file open, which allows READ (See Section 18.22 of [RFC5661]), WRITE (See Section 18.2 of [RFC5661]), LOCK (See Section 18.10 of [RFC5661]), and LAYOUTGET (see Section 18.43 of [RFC5661]) operations to proceed. If the client gets both a open and a delegation stateid as part of the OPEN, then it has to return them both. And during each operation, the client can send a costly GETATTR (See Section 18.7 of [RFC5661]).¶

If the client knows that the server supports the OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION flag (as determined by an earlier GETATTR operation which queried for the FATTR4_OPEN_ARGUMENTS attribute), then the client can supply that flag during the OPEN and only get either an open or delegation stateid.¶

The client is already prepared to not get a delegation stateid even if requested. In order to not send an open stateid, the server can indicate that fact with the result flag of OPEN4_RESULT_NO_OPEN_STATEID. The open stateid field, OPEN4resok.stateid (see Section 18.16.2 of [RFC5661]), should also be set to the special all zero stateid.¶

///
/// struct open_arguments4 {
///   bitmap4  oa_share_access;
///   bitmap4  oa_share_deny;
///   bitmap4  oa_share_access_want;
///   bitmap4  oa_open_claim;
///   bitmap4  oa_create_mode;
/// };
///

///
/// enum open_args_share_access4 {
///    OPEN_ARGS_SHARE_ACCESS_READ  = 1,
///    OPEN_ARGS_SHARE_ACCESS_WRITE = 2,
///    OPEN_ARGS_SHARE_ACCESS_BOTH  = 3
/// };
///

///
/// enum open_args_share_deny4 {
///    OPEN_ARGS_SHARE_DENY_NONE  = 0,
///    OPEN_ARGS_SHARE_DENY_READ  = 1,
///    OPEN_ARGS_SHARE_DENY_WRITE = 2,
///    OPEN_ARGS_SHARE_DENY_BOTH  = 3
/// };
///

///
/// enum open_args_share_access_want4 {
///    OPEN_ARGS_SHARE_ACCESS_WANT_ANY_DELEG           = 3,
///    OPEN_ARGS_SHARE_ACCESS_WANT_NO_DELEG            = 4,
///    OPEN_ARGS_SHARE_ACCESS_WANT_CANCEL              = 5,
///    OPEN_ARGS_SHARE_ACCESS_WANT_SIGNAL_DELEG_WHEN_RESRC_AVAIL
///                                                    = 17,
///    OPEN_ARGS_SHARE_ACCESS_WANT_PUSH_DELEG_WHEN_UNCONTENDED
///                                                    = 18,
///    OPEN_ARGS_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS    = 20,
///    OPEN_ARGS_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION = 21
/// };
///

///
/// enum open_args_open_claim4 {
///    OPEN_ARGS_OPEN_CLAIM_NULL          = 0,
///    OPEN_ARGS_OPEN_CLAIM_PREVIOUS      = 1,
///    OPEN_ARGS_OPEN_CLAIM_DELEGATE_CUR  = 2,
///    OPEN_ARGS_OPEN_CLAIM_DELEGATE_PREV = 3,
///    OPEN_ARGS_OPEN_CLAIM_FH            = 4,
///    OPEN_ARGS_OPEN_CLAIM_DELEG_CUR_FH  = 5,
///    OPEN_ARGS_OPEN_CLAIM_DELEG_PREV_FH = 6
/// };
///

///
/// enum open_args_createmode4 {
///    OPEN_ARGS_CREATEMODE_UNCHECKED4     = 0,
///    OPEN_ARGS_CREATE_MODE_GUARDED       = 1,
///    OPEN_ARGS_CREATEMODE_EXCLUSIVE4     = 2,
///    OPEN_ARGS_CREATE_MODE_EXCLUSIVE4_1  = 3
/// };
///

///
/// typedef open_arguments4 fattr4_open_arguments;
///

///
/// %/*
/// % * Determine what OPEN4 supports.
/// % */
/// const FATTR4_OPEN_ARGUMENTS     = 86;
///

///
/// const OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION = 0x200000;
///

///
/// const OPEN4_RESULT_NO_OPEN_STATEID = 0x00000010;
///

4. Proxying of Times

When a client is granted a write delegation on a file, it is the authority for the file. If the server queries the client as to the state of the file via a CB_GETATTR (see Section 20.1 of [RFC5661]), then it can only determine the size of the file. Likewise, if the client holding the delegation wants to know either of the access, modify, or change times, it has to send a GETATTR to the server. While it is the authority for these values, it has no way to guarantee these values after the delegation has been returned. And as such, it can not pass these times up to an application expecting posix compliance.¶

With the addition of the new flag: OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS, the client and server can negiotiate that the client will be the authority for these values and upon return of the delegation stateid via a DELEGRETURN (see section 18.6 of [RFC5661]), the times will be passed back to the server. If the server is queried by another client for either the size or the times, it will need to use a CB_GETATTR to query the client which holds the delegation (see Section 20.1 of [RFC5661]).¶

If a server informs the client via the FATTR4_OPEN_ARGUMENTS attribute that it supports OPEN_ARGS_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS and it returns a valid delegation stateid for an OPEN operation which sets the OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS flag, then it MUST be able to query the client via a CB_GETATTR for the FATTR4_TIME_DELEG_ACCESS attribute and FATTR4_TIME_DELEG_MODIFY attribute. (The change time can be derived from the modify time.) Further, when it gets a SETATTR (see Section 18.30 of [RFC5661]) in the same compound as the DELEGRETURN, then it MUST accept those FATTR4_TIME_DELEG_ACCESS attribute and FATTR4_TIME_DELEG_MODIFY attribute changes and derive the change time or reject the changes with NFS4ERR_DELAY.¶

A key prerequisite of this approach is that the server and client are in time synchronization with each other. Note that while the base NFSv4.2 does not require such synchronization, the use of RPCSEC_GSS typically makes such a requirement. When the client presents either FATTR4_TIME_DELEG_ACCESS or FATTR4_TIME_DELEG_MODIFY attributes to the server, the server MUST decide whether the times presented are before the old times or past the current time. If the time presented is before the original time, then the update is ignored. If the time presented is in the future, the server can either clamp the new time to the current time, or it may return NFS4ERR_DELAY to the client, allowing it to retry. Note that if the clock skew is large, this policy will result in access to the file being denied until such time that the clock skew is exceeded.¶

A change in the access time MUST not advance the change time, also known as the time_metadata attribute (see Section 5.8.2.42 of [RFC5661]), but a change in the modify time might advance the change time (and in turn the change attribute (See Section 5.8.1.4 of [RFC5661]). If the modify time is greater than the change time and before the current time, then the change time is adjusted to the modify time and not the current time (as is most likely done on most SETATTR calls that change the metadata). If the modify time is in the future, it will be clamped to the current time.¶

Note that each of the possible times, access, modify, and change, are compared to the current time. They should all be compared against the same time value for the current time. I.e., do not retrieve a different value of the current time for each calculation.¶

If the client sets the OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS flag in an OPEN operation, then it MUST support the FATTR4_TIME_DELEG_ACCESS and FATTR4_TIME_DELEG_MODIFY attributes both in the CB_GETATTR and SETATTR operations.¶

///
/// /*
///  * attributes for the delegation times being
///  * cached and served by the "client"
///  */
/// typedef nfstime4        fattr4_time_deleg_access;
/// typedef nfstime4        fattr4_time_deleg_modify;
///

///
/// %/*
/// % * New RECOMMENDED Attribute for
/// % * delegation caching of times
/// % */
/// const FATTR4_TIME_DELEG_ACCESS  = 84;
/// const FATTR4_TIME_DELEG_MODIFY  = 85;
///

///
/// const OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS = 0x100000;
///

5. Operation 77: LAYOUT_WCC - Layout Weak Cache Consistency

<CODE BEGINS>
/// struct LAYOUT_WCC4args {
///         stateid4        lowa_stateid;
///         layouttype4     lowa_type;
///         opaque          lowa_body<>;
/// };

<CODE ENDS>

<CODE BEGINS>
/// struct LAYOUT_WCC4res {
///         nfsstat4                lowr_status;
/// };

<CODE ENDS>

<CODE BEGINS>
/// struct ff_data_server_wcc4 {
///             deviceid4            ffdsw_deviceid;
///             stateid4             ffdsw_stateid;
///             nfs_fh4              ffdsw_fh_vers<>;
///             fattr4               ffdsw_attributes;
/// };
///
/// struct ff_mirror_wcc4 {
///             ff_data_server_wcc4  ffmw_data_servers<>;
/// };
///
/// struct ff_layout_wcc4 {
///             ff_mirror_wcc4       fflw_mirrors<>;
/// };

<CODE ENDS>

6. Extraction of XDR

This document contains the external data representation (XDR) [RFC4506] description of the new open flags for delegating the file to the client. The XDR description is embedded in this document in a way that makes it simple for the reader to extract into a ready-to-compile form. The reader can feed this document into the following shell script to produce the machine readable XDR description of the new flags:¶

<CODE BEGINS>¶

#!/bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'

<CODE ENDS>¶

That is, if the above script is stored in a file called "extract.sh", and this document is in a file called "spec.txt", then the reader can do:¶

sh extract.sh < spec.txt > delstid_prot.x

The effect of the script is to remove leading white space from each line, plus a sentinel sequence of "///". 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.¶

While the XDR can be appeneded to that from [RFC7863], the various code snippets belong in their respective areas of the that XDR.¶

7. Security Considerations

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

8. IANA Considerations

There are no IANA considerations.¶

Authors' Addresses