Fault Tolerance
As far as computers are concerned, fault tolerance refers to the capability of the computer system or network to provide continued data availability in the event of hardware failure. Every component within a server, from CPU fan to power supply, has a chance of failure. Some components such as processors rarely fail, whereas hard disk failures are well documented.
Almost every component has fault-tolerant measures. These measures typically require redundant hardware components that can easily or automatically take over when there is a hardware failure.
Of all the components inside computer systems, the one that requires the most redundancy are the hard disks. Not only are hard disk failures more common than any other component but they also maintain the data, without which there would be little need for a network.