How is fault-tolerant different from redundant?
Although fault tolerance and redundancy both improve overall system reliability, the latter accomplishes that by adding circuit components. Ironically, adding components reduces MTBF (mean time between failures), since the more components a circuit contains the greater the failure rate must be. Think of this way: A power boat with two outboard engines will necessarily have a greater failure rate than a boat with just one engine, because it is twice as likely that an engine will fail. But ask any skipper whether he feels less comfortable about have two engines! Clearly, if a motor fails, you don't have to call Sea-Tow. Fault tolerance is different and a bit harder to define. Circuits can be overspeced and designed to within certain tolerances and accuracies. In a strict sense, when components drift out of those tolerances, the circuit is not performing to spec. You could say that a fault has occurred. But what if the overall system still performs its intended function with no apparent degradation to the end-user? We say those systems are fault tolerant.
The word redundant is reduntantly used in this redundant sentence to redundantly provide a redundant example sentence for the word reduntant, which he redundantly repeated redundantly. . Or, with fewer steroids: The step-by-step assembly instructions were redundant because the high-level diagram provided sufficient details to assemble the furniture.
The use of multiple adjectives that mean the same thing is redundant, repetitive, superfluous, and unnecessary. When the brewery company sold its last carthorse, its blacksmith became redundant, so he retired to greener pastures. All systems in an airliner are designed with redundant backup systems in case of failure during a flight.
Redundant hardware is typically a second piece of hardware that shares in the workload and can take over if another piece of hardware goes bad. A classic example is redundant power supplies; a computer that has two power supplies can continue to work if one of them fails, by drawing all its power from the other.