Reliability, Availability and Serviceability (RAS) is a set of related attributes that must be considered when designing, manufacturing, purchasing or using a computer product or component. The term was first used by IBM to define specifications for their mainframe s and originally applied only to hardware . Today RAS is relevant to software as well and can be applied to network s, application program s, operating systems ( OS s), personal computers ( PC s), server s and supercomputer s.
The term reliability refers to the ability of a computer-related hardware or software component to consistently perform according to its specifications. In theory, a reliable product is totally free of technical errors. In practice, vendors commonly express product reliability as a percentage. The Institute of Electrical and Electronics Engineers ( IEEE ) sponsors an organization devoted to reliability in engineering known as the IEEE Reliability Society (IEEE RS).
Availability is the ratio of time a system or component is functional to the total time it is required or expected to function. This can be expressed as a direct proportion (for example, 9/10 or 0.9) or as a percentage (for example, 90%). It can also be expressed in terms of average downtime per week, month or year or as total downtime for a given week, month or year. Sometimes availability is expressed in qualitative terms, indicating the extent to which a system can continue to work when a significant component or set of components goes down.
Serviceability is an expression of the ease with which a component, device or system can be maintained and repaired. Early detection of potential problems is critical in this respect. Some systems have the ability to correct problems automatically before serious trouble occurs; examples include built-in features of OSs such as Microsoft Windows XP and auto-protect-enabled anti-virus software and spyware detection and removal programs. Ideally, maintenance and repair operations should cause as little downtime or disruption as possible.
Some of the key elements of RAS are:
- Over-engineering, which is designing systems to specifications better than minimum requirements.
- Duplication, which is extensive use of redundant systems and components.
- Recoverability, which is the use of fault-tolerant engineering methods.
- Automatic updating, which is keeps OSs and applications current without user intervention.
- Data backup , which prevents catastrophic loss of critical information.
- Data archiving , which keeps extensive records of data in case of audits or other recovery needs.
- Power-on replacement, which is the ability to hot swap components or peripherals.
- The use of virtual machine s, which minimizes the impact of OS or software faults.
- Use of surge suppressor s, which minimizes risk of component damage resulting from power-line anomalies.
- Continuous power, which is the use of an uninterruptible power supply keeps systems operational while switching from commercial power to backup or auxiliary power.
- Backup power sources, which includes batteries and generators to keep systems operational during extended interruptions in commercial power.