Add LAYOUT_WCC to NFSv4.2

1.1. Definitions

(file) data:

that part of the file system object that contains the 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.¶

weak cache consistency (WCC):

In NFSv3, WCC allows the client to check for file attribute changes before and after an operation. (See Section 2.6 of [RFC1813])¶

2.1. ARGUMENT

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

<CODE ENDS>

2.2. RESULT

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

<CODE ENDS>

2.3. DESCRIPTION

When using pNFS (See Section 12 of [RFC8881]), the client is most likely to be performing file operations to the storage device and not the metadata server. With some layout types (most notably the flexible files layout type in [RFC8435]) there is no control protocol ([RFC8434]) between the metadata server and the storage device. In order to update the metadata state of the file, the metadata server will need to track the metadata state of the data file - once the layout is issued, it is not able to see the NFSv3 file operations from the client to the storage device. Thus the metadata server will be required to query the storage device for the data file attributes.¶

For example, with a flexible files layout type, the metadata server would issue a NFSv3 GETATTR to the storage device. These queries are most likely triggered in response to a NFSv4 GETATTR to the metadata server. Not only are these GETATTRs to the storage device individually expensive, the storage device can become inundated by a storm of such requests. NFSv3 solved a similar issue by having the READ and WRITE operations employ a post-operation attribute to report the weak cache consistency (WCC) data (See Section 2.6 of [RFC1813]).¶

Each NFSv3 operation corresponds to one round trip between the client and server. So a WRITE followed by a GETATTR would require two round trips. In that scenario, the attribute information retrieved is considered to be strict server-client consistency. For NFSv4, the WRITE and GETATTR can be issued together inside a compound, which only requires one round trip between the client and server. And this is also considered to be a strict server-client consistency. In essence, the NFSv4 READ and WRITE operations drop the post-operation attributes, allowing the client to decide if it needs that information.¶

With the flexible files layout type, the client can leverage the NFSv3 WCC to service the proxying of times (See Section 4 of [delstid]). But the granularity of this data is limited. With client side mirroring (See Section 8 of [RFC8435]), the client has to aggregate the N mirrored files in order to send one piece of information instead of N pieces of information. Also, the client is limited to sending that information only when it returns the delegation.¶

The current filehandle and the lowa_stateid identifies the particular layout for the LAYOUT_WCC operation. The lowa_type indicates how to unpack the layout type specific payload inside the lowa_body field. The lowa_type is defined to be a value from the IANA registry for "pNFS Layout Types Registry".¶

The lowa_body will contain the data file attributes. The client will be responsible for mapping the NFSv3 post-operation attributes to those in a fattr4. Just as the post-operation attributes may be ignored by the client, the server may ignore the attributes inside the LAYOUT_WCC. But the server can also use those attributes to avoid querying the storage device for the data file attributes. Note that as these attributes are optional and there is nothing the client can do if the server ignores one, there is no need to return a bitmap4 of which attributes were accepted in the result of the LAYOUT_WCC.¶

2.4. Allowed Errors

The LAYOUT_WCC operation can raise the errors in Table 1. When an error is encountered, the metadata server can decide to ignore the entire operation or depending on the layout type specific payload, it could decide to apply a portion of the payload.¶

Valid Error Returns for LAYOUT_WCC¶

2.5. Extension of Existing Implementations

The new LAYOUT_WCC operation is OPTIONAL for both NFSv4.2 ([RFC7863]) and the flexible files layout type ([RFC8435]).¶

2.6. Flex Files Layout Type

<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>

The flex file layout type specific results MUST correspond to the ff_layout4 data structure as defined in Section 5.1 of [RFC8435]. There MUST be a one-to-one correspondence between:¶

• ff_data_server4 -> ff_data_server_wcc4¶
• ff_mirror4 -> ff_mirror_wcc4¶
• ff_layout4 -> ff_layout_wcc4¶

Each ff_layout4 has an array of ff_mirror4, which have an array of ff_data_server4. Based on the current filehandle and the lowa_stateid, the server can match the reported attributes.¶

But the positional correspondence between the elements is not sufficient to determine the attributes to update. Consider the case where a layout had three mirrors and two of them had updated attributes, but the third did not. A client could decide to present all three mirrors, with one mirror having an attribute mask with no attributes present. Or it could decide to present only the two mirrors which had been changed.¶

In either case, the combination of ffdsw_deviceid, ffdsw_stateid, and ffdsw_fh_vers will uniquely identify the attributes to be updated. All three arguments are required. A layout might have multiple data files on the same storage device, in which case the ffdsw_deviceid and ffdsw_stateid would match, but the ffdsw_fh_vers would not.¶

The ffdsw_attributes are processed similar to the obj_attributes in the SETATTR arguments (See Section 18.34 of [RFC8881]).¶

3.1. Code Components Licensing Notice

Both the XDR description and the scripts used for extracting the XDR description are Code Components as described in Section 4 of "Legal Provisions Relating to IETF Documents" [LEGAL]. These Code Components are licensed according to the terms of that document.¶