Add LAYOUT_WCC to NFSv4.2's Flex File Layout Type

Introduction In the Network File System version4 (NFSv4) with a Parallel NFS (pNFS) Flexible File Layout () server, there is no mechanism for the data servers to update the metadata servers for when the data portion of the file is modified. The metadata server needs this knowledge to correspondingly update the metadata portion of the file. If the client is using NFSv3 as the protocol with the data server, it can leverage weak cache consistency (WCC) to update the metadata server of the attribute changes. In this document, we introduce a new operation called LAYOUT_WCC which allows the client to periodically report the attributes of the data files to the metadata server. Using the process detailed in , the revisions in this document become an extension of NFSv4.2 . They are built on top of the external data representation (XDR) generated from .

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 )

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 when, and only when, they appear in all capitals, as shown here.

Operation 77: LAYOUT_WCC - Layout Weak Cache Consistency

ARGUMENT ; /// }; ]]>

RESULT

DESCRIPTION When using pNFS (See Section 12 of ), the client is most likely to be performing file operations to the storage device and not the metadata server. With a NFSv3 data server in the flexible files layout type (in ) there is no control protocol () 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, 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 NFSv3 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 ). 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 ). But the granularity of this data is limited. With client side mirroring (See Section 8 of ), 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.

Implementation

Examples of when to use LAYOUT_WCC The only way for the metadata server to detect modifications to the data file is to probe the data servers via a GETATTR. It can compare the mtime results across multiple calls to detect a NFSv3 WRITE operation by the client. Likewise, the atime results indicate the client having issued a NFSv3 READ operation. As such, the client should leverage the LAYOUT_WCC operation whenever it has the belief that the metadata server would need to refresh the attributes of the data files. While The client can send a LAYOUT_WCC at any time, there are times it will want to do this operation in order to avoid having the metadata server issue NFSv3 GETATTR requests to the data servers:

Whenever it sends a GETATTR for any of the following attributes: size, space_used, change, time_access, time_metadata, and time_modify.
Whenever it sends an NFS4ERR_ACCESS error via LAYOUTRETURN or LAYOUTERROR - it could have already gotten the NFSv3 uid and gid values back in the WCC of the WRITE, READ, or COMMIT operation which got the error.
Whenever it sends a SETATTR to refresh the proxied times ((See Section 4 of ) - the metadata server is going to want to correlate these times in order to detect later modification to the data file.

Examples of what to send in the LAYOUT_WCC The NFSv3 attributes returned in the WCC of WRITE, READ, and COMMIT are a smaller subset of what can be transmitted as a NFSv4 attribute. The mapping of NFSv3 to NFSv4 attributes shown in also details which attributes the LAYOUT_WCC SHOULD be providing to the metadata server, Both the uid and gid are stringified into their respective attributes of owner and owner_group. The reason to provide these two attributes is in case of NFS4ERR_ACCESS, the metadata server can compare what it expects the values of the uid and gid of the data file to be versus the actual values. It can then repair the permissions as needed or modify the expected values it has cached.

NFSv3 Attribute	NFSv4 Attribute
size	size
used	space_used
uid	owner
gid	owner_group
atime	time_access
mtime	time_modify
ctime	time_metadata

Allowed Errors The LAYOUT_WCC operation can raise the errors in . 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

Errors
NFS4ERR_ADMIN_REVOKED, NFS4ERR_BADXDR, NFS4ERR_BAD_STATEID, NFS4ERR_DEADSESSION, NFS4ERR_DELAY, NFS4ERR_DELEG_REVOKED, NFS4ERR_EXPIRED, NFS4ERR_FHEXPIRED, NFS4ERR_GRACE, NFS4ERR_INVAL, NFS4ERR_ISDIR, NFS4ERR_MOVED, NFS4ERR_NOFILEHANDLE, NFS4ERR_NOTSUPP, NFS4ERR_NO_GRACE, NFS4ERR_OLD_STATEID, NFS4ERR_OP_NOT_IN_SESSION, NFS4ERR_REP_TOO_BIG, NFS4ERR_REP_TOO_BIG_TO_CACHE, NFS4ERR_REQ_TOO_BIG, NFS4ERR_RETRY_UNCACHED_REP, NFS4ERR_SERVERFAULT, NFS4ERR_STALE, NFS4ERR_TOO_MANY_OPS, NFS4ERR_UNKNOWN_LAYOUTTYPE, NFS4ERR_WRONG_CRED, NFS4ERR_WRONG_TYPE

Errors

NFS4ERR_ADMIN_REVOKED, NFS4ERR_BADXDR, NFS4ERR_BAD_STATEID, NFS4ERR_DEADSESSION, NFS4ERR_DELAY, NFS4ERR_DELEG_REVOKED, NFS4ERR_EXPIRED, NFS4ERR_FHEXPIRED, NFS4ERR_GRACE, NFS4ERR_INVAL, NFS4ERR_ISDIR, NFS4ERR_MOVED, NFS4ERR_NOFILEHANDLE, NFS4ERR_NOTSUPP, NFS4ERR_NO_GRACE, NFS4ERR_OLD_STATEID, NFS4ERR_OP_NOT_IN_SESSION, NFS4ERR_REP_TOO_BIG, NFS4ERR_REP_TOO_BIG_TO_CACHE, NFS4ERR_REQ_TOO_BIG, NFS4ERR_RETRY_UNCACHED_REP, NFS4ERR_SERVERFAULT, NFS4ERR_STALE, NFS4ERR_TOO_MANY_OPS, NFS4ERR_UNKNOWN_LAYOUTTYPE, NFS4ERR_WRONG_CRED, NFS4ERR_WRONG_TYPE

Extension of Existing Implementations The new LAYOUT_WCC operation is OPTIONAL for both NFSv4.2 () and the flexible files layout type ().

Flex Files Layout Type ; /// fattr4 ffdsw_attributes; /// }; /// /// struct ff_mirror_wcc4 { /// ff_data_server_wcc4 ffmw_data_servers<>; /// }; /// /// struct ff_layout_wcc4 { /// ff_mirror_wcc4 fflw_mirrors<>; /// }; ]]> The flex file layout type specific results MUST correspond to the ff_layout4 data structure as defined in Section 5.1 of . 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 ).