Q. Compare and contrast reliability and availability?

Ans. Reliability: The probability of failure-free system operation over a specified time in a given environment for a given purpose is called reliability Availability:The probability that a system at a point in time will be operational and able to deliver the requested services is known as availability.

Both of these attributes can be expressed quantitatively. It is sometimes possible to subsume system availability under system reliability. Clearly if a system is unavailable it is not delivering the specified system services. Nevertheless it is possible to have systems with low reliability that must be available. Consequently long as system failures can be repaired quickly and don't damage data low reliability mayn't be a problem. Availability acquires repair time into account.