A local-area network consists of independent computers confined to small geographical area. Since this area is small, the communication network can be fast (3-100 Mbps) and reliable.
A local-area network offers, in addition to the services offered by long-haul networks, sharing of expensive devices such as printers and large secondary store. This sharing is important when the nodes are workstations -- powerful personal computers that come with high-quality displays and small or no local disks. Typically, a network of workstations has one or more special machines connected to large disks that run disk server or file server processes. These processes allow processes on other machines to use the disks for storing files. Thus LANs are built on top of server-based operating systems.
A disk server provides each client with a small virtual disk since the only service it normally provides is sharable secondary storage. A disk server provides each client with a small virtual disk and primitives to read and write blocks on the virtual disk. The client can use these primitives to build its own file system on top of the virtual disk. A file server, on the other hand, allows access at the file-level, and provides primitives to open, read, and write files. (Often a workstation that provides sharable secondary store is called a disk server machine or a file server machine, since the only service it normally provides is sharable secondary storage.)
Each approach to shared secondary storage has its advantages. A disk server is simple and supports customized file systems. A file server gives higher-level primitives and allows sharing of files among its different clients.
A network of computers will often have several machines connected to sharable secondary storage. Therefore it is useful if files could be shared among these machines also. (Note this feature is different from providing special programs that can transfer files from one computer to another.) There are two complementary schemes for naming files in a system that supports such sharing. The simpler scheme incorporates the machine name in the file. It is not convenient to move files around in this system. The alternative scheme makes the file location transparent to the user. Under this approach the operating system maintains tables and uses algorithms which allow it to find a file anywhere in the network.
A popular example of a transparent file system is the Network File System (NFS) developed by Sun. It allows a directory belonging to a remote disk to be mounted on a local directory, much in the way the root directory of a logical disk (also called file system) can be mounted on some other directory. For instance the command: mount A:/a/b l
mounts the directory `/a/b' on machine `A' to the local directory `l'. Now the file `/a/b/f' on `A' can be accessed as `l/f'.
Like a distributed shared file system, it may be useful to provide a distributed shared memory system, for similar reasons: Cooperating programs running on different computers may be wish to share memory - for example distributed processes of a chess program may wish to share the chessboard state and the set of moves searched so far. Moreover, a program may wish to use the large memory of a memory server as backing store. Networks are now fast enough that fetching a page from a remote server is as fast as fetching it from a local disk.