5.2.1 Asynchronous Transfer Mode

		(DFS) included as part of the Distributed Computing Environment (DCE). AFS effectively became a proprietary technology before Linux was developed, so AFS never played much of a role in the design of Beowulf systems. Recently, however, AFS-based file systems have become available for Linux, and a new interest in this network file system has emerged in the Beowulf community. The inability of NFS to effectively scale to systems containing on the order of 100 processors has motivated this experimentation with more scalable file system architectures. However, improvements have been made in the Linux NFS code which may obviate the need to explore alternative network file systems.

		5.6.3 Autofs: The Automounter

		As you add more nodes to a Beowulf, the startup time of the system can increase dramatically because of contention for the NFS server on the worldly node that exports home directories to the rest of the system. NFS is implemented using ONC RPC, which only supports synchronous RPC calls. Therefore the NFS server becomes a single bottleneck through which the other systems must pass, one at a time. This phenomenon was a problem on local area networks until Sun Microsystems developed an automounting mechanism for NFS. The Linux version of this mechanism is the autofs service. Autofs only mounts NFS partitions when they are first accessed, rather than automatically at startup. If the NFS partition is not accessed for a configurable period of time (typically 5 minutes), autofs will unmount it. By using autofs, you can reduce system startup times as well as reduce overall system load.

		5.7 Remote Command Execution

		5.7.1 BSD R Commands

		The BSD R commands are a set of programs that first appeared in 4.2BSD to execute commands and copy files on remote machines. The major commands are:

		rsh The remote shell allows you to execute shell commands on remote nodes and also initiate interactive login sessions. Interactive login sessions are initiated by invoking rlogin.

		rlogin The remote login command allows you to start a terminal session by logging in to a remote node.

		rcp Allows the copying of files from one node to the other.