Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Fault tolerance and high availability is about keeping systems up and running 24 hours a day, 7 days a week, or at least keeping systems up and running with a reasonable amount of performance. Downed systems can cost an organization thousands of dollars per hour, as outlined in the following table:

A fault-tolerant system is designed to keep running even after a fault has occurred. Fault-tolerant features in early network operating systems included mirrored disks, with both disks reading and writing the same information. If one disk failed, the other kept running in what is called "failover" mode. This fault tolerance was expanded to disk duplexing, in which the disks and disk controllers were duplicated. These redundant components not only provided fault tolerance, but also improved performance since disk reads could come from either disk (writes still had to be performed by both disks). Of course, fault-tolerant systems must provide more than just disk failover. Some other examples of redundant systems include the following:

• RAID disk systems combine multiple hard drives into fault-protected arrays.


• Redundant components (power supplies, I/O boards, and so on).


• Multiple servers are clustered to minimize problems if any of the servers should fail.


• Alternate pathing and load balancing improve throughput and provide redundant links.


• Multiple data centers to protect against local disasters.