1.7 Changes from the NFS Version 2 Protocol

The ROOT and WRITECACHE procedures have been removed. A MKNOD procedure has been defined to allow the creation of special files, eliminating the overloading of CREATE. Caching on the client is not defined nor dictated by the NFS version 3 protocol, but additional information and hints have been added to the protocol to allow clients that implement caching to manage their caches more effectively. Procedures that affect the attributes of a file or directory may now return the new attributes after the operation has completed to optimize out a subsequent GETATTR used in validating attribute caches. In addition, operations that modify the directory in which the target object resides return the old and new attributes of the directory to allow clients to implement more intelligent cache invalidation procedures. The ACCESS procedure provides access permission checking on the server, the FSSTAT procedure returns dynamic information about a file system, the FSINFO procedure returns static information about a file system and server, the READDIRPLUS procedure returns file handles and attributes in addition to directory entries, and the PATHCONF procedure returns POSIX pathconf information about a file.

Below is a list of the important changes between the NFS version 2 protocol and the NFS version 3 protocol.

File handle size

The file handle has been increased to a variable-length array of 64 bytes maximum from a fixed array of 32 bytes. This addresses some known requirements for a slightly larger file handle size. The file handle was converted from fixed length to variable length to reduce local storage and network bandwidth requirements for systems which do not utilize the full 64 bytes of length.

Maximum data sizes

The maximum size of a data transfer used in the READ and WRITE procedures is now set by values in the FSINFO return structure. In addition, preferred transfer sizes are returned by FSINFO. The protocol does not place any artificial limits on the maximum transfer sizes. Filenames and pathnames are now specified as strings of variable length. The actual length restrictions are determined by the client and server implementations as appropriate. The protocol does not place any artificial limits on the length. The error, NFS3ERR_NAMETOOLONG, is provided to allow the server to return an indication to the client that it received a pathname that was too long for it to handle.

Error return

Error returns in some instances now return data (for example, attributes). nfsstat3 now defines the full set of errors that can be returned by a server. No other values are allowed.

File type

The file type now includes NF3CHR and NF3BLK for special files. Attributes for these types include subfields for UNIX major and minor devices numbers. NF3SOCK and NF3FIFO are now defined for sockets and fifos in the file system.

File attributes

The blocksize (the size in bytes of a block in the file) field has been removed. The mode field no longer contains file type information. The size and fileid fields have been widened to eight-byte unsigned integers from four-byte integers. Major and minor device information is now presented in a distinct structure. The blocks field name has been changed to used and now contains the total number of bytes used by the file. It is also an eight-byte unsigned integer.

Set file attributes

In the NFS version 2 protocol, the settable attributes were represented by a subset of the file attributes structure; the client indicated those attributes which were not to be modified by setting the corresponding field to -1, overloading some unsigned fields. The set file attributes structure now uses a discriminated union for each field to tell whether or how to set that field. The atime and mtime fields can be set to either the server's current time or a time supplied by the client.

LOOKUP

The LOOKUP return structure now includes the attributes for the directory searched.

ACCESS

An ACCESS procedure has been added to allow an explicit over-the-wire permissions check. This addresses known problems with the superuser ID mapping feature in many server implementations (where, due to mapping of root user, unexpected permission denied errors could occur while reading from or writing to a file). This also removes the assumption which was made in the NFS version 2 protocol that access to files was based solely on UNIX style mode bits.

READ

The reply structure includes a Boolean that is TRUE if the end-of-file was encountered during the READ. This allows the client to correctly detect end-of-file.

WRITE

The beginoffset and totalcount fields were removed from the WRITE arguments. The reply now includes a count so that the server can write less than the requested amount of data, if required. An indicator was added to the arguments to instruct the server as to the level of cache synchronization that is required by the client.

CREATE

An exclusive flag and a create verifier was added for the exclusive creation of regular files.

MKNOD

This procedure was added to support the creation of special files. This avoids overloading fields of CREATE as was done in some NFS version 2 protocol implementations.

READDIR

The READDIR arguments now include a verifier to allow the server to validate the cookie. The cookie is now a 64 bit unsigned integer instead of the 4 byte array which was used in the NFS version 2 protocol. This will help to reduce interoperability problems.

READDIRPLUS

This procedure was added to return file handles and attributes in an extended directory list.

FSINFO

FSINFO was added to provide nonvolatile information about a file system. The reply includes preferred and maximum read transfer size, preferred and maximum write transfer size, and flags stating whether links or symbolic links are supported. Also returned are preferred transfer size for READDIR procedure replies, server time granularity, and whether times can be set in a SETATTR request.

FSSTAT

FSSTAT was added to provide volatile information about a file system, for use by utilities such as the Unix system df command. The reply includes the total size and free space in the file system specified in bytes, the total number of files and number of free file slots in the file system, and an estimate of time between file system modifications (for use in cache consistency checking algorithms).

COMMIT

The COMMIT procedure provides the synchronization mechanism to be used with asynchronous WRITE operations.