The widespread use of computers has brought about revolutionary changes in society. Computers are becoming vital in all aspects of human life, whether employed in life-critical systems such as air traffic control and autopilot navigation control systems, or in point-of-sales management systems and cinema ticket purchasing systems. Data stored in computer systems is often a company's most valuable asset, one that must be protected at all costs. Businesses also must be prepared to provide continued service in case of a disaster.Fault-tolerance techniques have been employed to increase computer system availability, and to reduce the damage caused by component failure. Vital data is stored on stable storage, which survives failures such as electrical outages or system crashes. Also, redundant copies of data can be placed on multiple stable storage devices. This approach protects data if failures in storage media are independent, but may be ineffective if disaster strikes. Recall the 1906 earthquake in San Francisco, which destroyed more than half the city. When the U.S. Federal Building in Oklahoma City was bombed, data as well as on-site backups were destroyed. Since data losses and system unavailability resulting from a disaster cripple the operation of an organization, federal legislation now requires the development of recovery plans . Extensive backup procedures have been developed to protect against data losses during disasters, such as the grandfather-father-son backup procedure, the incremental logging technique, and the data image dumping method. In addition to guarding against data losses, a system must also provide its normal services after a disaster strikes. Thus, as with data, computer hardware must also be replicated.