A distributed file system is of little value if the files it contains are not
accessible to users and applications on the network. One approach NFS uses to make sure files are accessible on demand is automatic mounting via the automounter, discussed in the section called "The Automounter" on page 22.

Although the automounter does an excellent job of keeping files accessible when the server is active and healthy, it will do no good if the server becomes unavailable because of a hardware or software failure, or because it has been taken off line for routine maintenance purposes! When this occurs, work associated with the server must stop. And, although interruptions of this nature are infrequent, they are intolerable in environments where mission critical applications are being used. Mission critical applications are by definition critical to the daily operation of business. Some examples include airline reservation systems, bank funds transfer systems and stock quotes systems. In these environments, extra steps must be taken to ensure that resources do not become unavailable.

Highly Available NFS

What is needed is a solution that makes the NFS service highly available. The definition of a highly available data service is a service that "exhibits the appearance of continuous availability despite a single point of failure". This requires duplication of both hardware and software so that when one component fails, the other can take over. However in the case of software, simple duplication is not enough. A decision making entity must also be present that can 1) determine when a server is legitimately failing and needs to be taken over, and 2) transfer control to the other server.

There are a handful of products from NFS licensees that provide highly available NFS implementations. A new product from Sun, encompassed by the recently announced Solstice strategy for enterprise management, includes the components necessary to make NFS highly available. When using this product, clients connected to the highly available NFS server will not experience a significant disruption of service despite a single point of failure. A fault tolerant hardware configuration is required to support the highly available NFS software. The highly available NFS software implementation will be available from SunSoft to qualified OEMs in late 1995.