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The Network File System

NFS, the network filesystem, is probably the most prominent network services using RPC. It allows to access files on remote hosts in exactly the same way as a user would access any local files. This is made possible by a mixture of kernel functionality on the client side (that uses the remote file system) and an NFS server on the server side (that provides the file data). This file access is completely transparent to the client, and works across a variety of server and host architectures.

NFS offers a number of advantages:

• Data accessed by all users can be kept on a central host, with clients mounting this directory at boot time. For example, you can keep all user accounts on one host, and have all hosts on your network mount /home from that host. If installed alongside with NIS, users can then log into any system, and still work on one set of files.
• Data consuming large amounts of disk space may be kept on a single host. For example, all files and programs relating to LaTeX and METAFONT could be kept and maintained in one place.
• Administrative data may be kept on a single host. No need to use rcp anymore to install the same stupid file on 20 different machines.

NFS is largely the work of Rick Sladkey, who wrote the NFS kernel code and large parts of the NFS server. The latter is derived from the unfsd user-space NFS server originally written by Mark Shand, and the hnfs Harris NFS server written by Donald Becker.

Let's have a look now at how NFS works: A client may request to mount a directory from a remote host on a local directory just the same way it can mount a physical device. However, the syntax used to specify the remote directory is different. For example, to mount /home from host vlager to /users on vale, the administrator would issue the following command on vale:

mount will then try to connect to the mountd mount daemon on vlager via RPC. The server will check if vale is permitted to mount the directory in question, and if so, return it a file handle. This file handle will be used in all subsequent requests to files below /users.

When someone accesses a file over NFS, the kernel places an RPC call to nfsd (the NFS daemon) on the server machine. This call takes the file handle, the name of the file to be accessed, and the user's user and group id as parameters. These are used in determining access rights to the specified file. In order to prevent unauthorized users from reading or modifying files, user and group ids must be the same on both hosts.

On most implementations, the NFS functionality of both client and server are implemented as kernel-level daemons that are started from user space at system boot. These are the NFS daemon (nfsd) on the server host, and the Block I/O Daemon (biod) running on the client host. To improve throughput, biod performs asynchronous I/O using read-ahead and write-behind; also, several nfsd daemons are usually run concurrently.

The NFS implementation of is a little different in that the client code is tightly integrated in the virtual file system (VFS) layer of the kernel and doesn't require additional control through biod. On the other hand, the server code runs entirely in user space, so that running several copies of the server at the same time is almost impossible because of the synchronization issues this would involve. NFS currently also lacks read-ahead and write-behind, but Rick Sladkey plans to add this someday.

The biggest problem with the NFS code is that the kernel as of version 1.0 is not able to allocate memory in chunks bigger than 4K; as a consequence, the networking code cannot handle datagrams bigger than roughly 3500 bytes after subtracting header sizes etc. This means that transfers to and from NFS daemons running on systems that use large UDP datagrams by default (e.g. 8K on SunOS) need to be downsized artificially. This hurts performance badly under some circumstances. This limit is gone in late -1.1 kernels, and the client code has been modified to take advantage of this.

• Preparing NFS
• Mounting an NFS Volume
• The NFS Daemons
• The exports File
• The Automounter