9 Hardware Reliability

In the IT field the term “fault tolerance” is often widely used as “reliability improvement”. The question to be clarified is the relationship between reliability and fault tolerance. In a general sense reliability will be understood as ability of a component/system to function correctly over a specified period of time, mostly under predefined conditions. Fault tolerance is defined as the ability of the system to continue operation in the event of a failure. Fault tolerance means that a computer system or component is designed such that, in case a component fails, a backup component or backup procedure can immediately take its place with no loss of functionality. Reliability can be improved through fault tolerance. Metrics of “classical” reliability theory are well known and numerous. Metrics of fault tolerance are less common, e.g. number of tolerated faults, number of checkpoints, reconfiguration time, etc. The most important method supporting fault tolerance/reliability is redundancy. Redundancy is duplication of components or repetition of operations to provide alternative functional channels in case of failure. Redundancy can be implemented in different ways: structural (hot and standby redundancy), temporal, functional, etc. Application of redundancy is always connected with an increase in cost and/or complexity as well as sometimes with synchronisation problems. Predicting the system reliability by modelling during the design phase, and measuring the parameters of a real system are two completely different approaches. This chapter is sub-divided into five sections depending on the primary goal of the readers. The sections of this chapter are presented as set of references structured according to the various reliability metrics (RM). An index is provided at the end of the book so that specific issues can be referenced directly. The chapter is organised as follows: