REDUNDANCY, SPARES, AND REPAIRS This chapter deals with a variety of techniques for improving system reliability and availability. Underlying all these techniques is the basic concept of redundancy, providing alternate paths to allow the system to continue operation even when some components fail. Alternate paths can be provided by parallel components (or systems). The parallel elements can all be continuously operated, in which case all elements are powered up and the term parallel redundancy or hot standby is often used | Reliability of Computer Systems and Networks Fault Tolerance Analysis and Design Martin L. Shooman Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-29342-3 Hardback 0-471-22460-X Electronic 3 REDUNDANCY SPARES AND REPAIRS INTRODUCTION This chapter deals with a variety of techniques for improving system reliability and availability. Underlying all these techniques is the basic concept of redundancy providing alternate paths to allow the system to continue operation even when some components fail. Alternate paths can be provided by parallel components or systems . The parallel elements can all be continuously operated in which case all elements are powered up and the term parallel redundancy or hot standby is often used. It is also possible to provide one element that is powered up on-line along with additional elements that are powered down standby which are powered up and switched into use either automatically or manually when the on-line element fails. This technique is called standby redundancy or cold redundancy. These techniques have all been known for many years however with the advent of modern computer-controlled digital systems a rich variety of ways to implement these approaches is available. Sometimes system engineers use the general term redundancy management to refer to this body of techniques. In a way the ultimate cold redundancy technique is the use of spares or repairs to renew the system. At this level of thinking a spare and a repair are the same thing except the repair takes longer to be effected. In either case for a system with a single element we must be able to tolerate some system downtime to effect the replacement or repair. The situation is somewhat different if we have a system with two hot or cold standby elements combined with spares or repairs. In such a case once one of the redundant elements fails and we detect the failure we can replace or repair the failed element while the system continues to operate as long as the 83 84 .
