The Network File System

You can log onto any PC or Sun workstation, and your filestore looks the same. You can also access tens of GigaBytes of files belonging to the department, much more than can fit on the disk inside each machine. How is this done?

The computers are all connected by a Local Area Network (LAN), using FDDI and Ethernet technology, which allows information to be transferred between them at high speed. Also on the LAN, but hidden away in another room, is a fileserver computer known as jeeves. This is similar to the computers you can use, but bigger and faster, with over 100GB of disks and 200GB of magnetic tapes attached. The computers you sit at are referred to as clients of the fileserver.

Some software called the Network File System (NFS) coordinates filestore across the machines. It allows part of the directory tree which is physically stored on one disc to be "grafted" onto the tree for another disc. This grafting process is called mounting. Every machine can mount the filestore from every disc, with the result that the whole filestore looks like one big tree, wherever you are. For instance, suppose we have a disc for students whose filestore looks like this:

and a disc for staff with a tree like this:

then if we mount both the staff directories and the student directories below /home, the complete directory tree looks like this:

Again, this is a slightly unrealistic example - we actually keep the teaching and research filestore and machines pretty well separated, with different parts of the filestore available in the two areas. We try to arrange it so that staff can see everything, but the students can't see most staff areas. However, the permissions still control access to each individual file, so we can't actually see any file or directory you don't want us to.