Fault tolerance:
Machine failures, Communication faults, storage device crashes, and decays of storage media must be tolerated by the system to some extent. A fault tolerant system must continue to function perhaps with degraded form when these failures are encountered. The degradation is able to be either in performance or functionality or both. Mainly commercial systems provide only limited fault tolerance. Though, fault tolerant systems are hard to execute. In the DEC VAX cluster multiple computers are permissible to share a set of disks. If a system crashes, users are able to still their information from another system. However, if a disk fails after that all the systems will lose access.