Computers designed with higher levels of RAS have many features that protectdata integrity and help them stayavailable for long periods of time withoutfailure.[3] This data integrity anduptime is a particular selling point for mainframes andfault-tolerant systems.
While RAS originated as a hardware-oriented[citation needed] term,systems thinking has extended the concept of reliability-availability-serviceability to systems in general, includingsoftware:[4]
Reliability can be defined as the probability that a system will produce correct outputs up to some given timet.[5] Reliability is enhanced by features that help to avoid, detect and repair hardware faults. A reliable system does not silently continue and deliver results that include uncorrected corrupted data. Instead, it detects and, if possible, corrects the corruption, for example: by retrying an operation for transient (soft) or intermittent errors, or else, for uncorrectable errors, isolating the fault and reporting it to higher-level recovery mechanisms (which mayfailover to redundant replacement hardware, etc.), or else by halting the affected program or the entire system and reporting the corruption. Reliability can be characterized in terms ofmean time between failures (MTBF), with reliability = exp(−t/MTBF).[5]
Availability means the probability that a system is operational at a given time, i.e. the amount of time a device is actually operating as the percentage of total time it should be operating. High-availability systems may report availability in terms of minutes or hours of downtime per year. Availability features allow the system to stay operational even when faults do occur. A highly available system would disable the malfunctioning portion and continue operating at a reduced capacity. In contrast, a less capable system might crash and become totally nonoperational. Availability is typically given as a percentage of the time a system is expected to be available, e.g., 99.999 percent ("five nines").
Serviceability ormaintainability is the simplicity and speed with which a system can be repaired or maintained; if the time to repair a failed system increases, then availability will decrease. Serviceability includes various methods of easily diagnosing the system when problems arise. Early detection of faults can decrease or avoid system downtime. For example, some enterprise systems can automatically call a service center (without human intervention) when the system experiences a system fault. The traditional focus has been on making the correct repairs with as little disruption to normal operations as possible.
Note the distinction between reliability and availability: reliability measures the ability of a system to function correctly, including avoiding data corruption, whereas availability measures how often the system is available for use, even though it may not be functioning correctly. For example, a server may run forever and so have ideal availability, but may be unreliable, with frequent data corruption.[6]
Transient (a.k.a.soft) faults lead to independent one-time errors and are not due to permanent hardware faults: examples include alpha particles flipping a memory bit, electromagnetic noise, or power-supply fluctuations.
Intermittent faults occur due to a weak system component, e.g. circuit parameters degrading, leading to errors that are likely to recur.[5]
Transient and intermittent faults can typically be handled by detection and correction by e.g., ECC codes or instruction replay (see below). Permanent faults will lead to uncorrectable errors which can be handled by replacement by duplicate hardware, e.g., processor sparing, or by the passing of the uncorrectable error to high level recovery mechanisms. A successfully corrected intermittent fault can also be reported to theoperating system (OS) to provide information forpredictive failure analysis.
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Example hardware features for improving RAS include the following, listed by subsystem:
Processor instruction error detection (e.g. residue checking of results[7]) with instruction retry e.g.alternative processor recovery in IBM mainframes,[8] or "Instruction replay technology" inItanium systems.[9]
^Data Processing Division, International Business Machines Corp., 1970 (1970). "Data processor, Issues 13-17".{{cite journal}}:|author= has generic name (help);Cite journal requires|journal= (help)CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)- "The dependability [...] experienced by other System/370 users is the result of a strategy based on RAS (Reliability-Availability-Serviceability)"
^For example:Laros III, James H. (4 September 2012).Energy-Efficient High Performance Computing: Measurement and Tuning. SpringerBriefs in Computer Science. et al. Springer Science & Business Media (published 2012). p. 8.ISBN9781447144922. Retrieved2014-07-08.Historically, Reliability Availability and Serviceability (RAS) systems were commonly provided by vendors on mainframe class systems. [...] The RAS system shall be a systematic union of software and hardware for the purpose of managing and monitoring all hardware and software components of the system to their individual potential.
^abcE.J. McClusky & S. Mitra (2004)."Fault Tolerance" in Computer Science Handbook 2ed. ed. A.B. Tucker. CRC Press.
^Spencer, Richard H.; Floyd, Raymond E. (11 July 2011).Perspectives on Engineering. Bloomington, Indiana: AuthorHouse (published 2011). p. 33.ISBN9781463410919. Retrieved2014-05-05.[...] a system server may have excellent availability (runs forever), but continues to have frequent data corruption (not very reliable).
