High Availability Theoretical Basics - Schneider Electric

High Availability Theoretical Basics
Fault Tolerant System
High Availability Theoretical Basics
This section describes basic high availability terms, concepts and formulas, and
includes examples for typical applications.
A Fault Tolerant System usually refers to a system that can operate even though a
hardware component becomes inoperative. The Redundancy principle is often used
to implement a Fault Tolerant Systems, because an alternate component takes over
the task transparently.
Lifetime and Failure Rate
Considering a given system or device, its Lifetime corresponds to the number of
hours it can function under normal operating conditions. This number is the result of
the individual life expectancy of the components used in its assembly.
Lifetime is generally seen as a sequence of three subsequent phases: the “early life”
(or “infant mortality”), the "useful life,” and the "wear out period.”
Failure Rate (λ) is defined as the average (mean) rate probability at which a system
will become inoperative.
When considering events occurring on a series of systems, for example a group of
light bulbs, the units to be used should normally be “failures-per-unit-of- system-time.”
Examples include failures per machine-hour or failures per system-year. Because the
scope of this document is limited to single repairable entities, we will usually discuss
failures-per-unit-of-time.
Failure Rate Example: For a human being, Failure Rate (λ) measures the
probability of death occurring in the next hour. Stating λ (20 years) = 10 -6 per
hour would mean that the probability for someone age 20 to die in the next
hour is 10 -6 .
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