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可靠性、可用性和可服務性Reliability, Availability and Serviceability, RAS)是電腦硬體工程上的術語,最初來自IBM公司為其大型主機所做的宣傳廣告,強調大型主機系統的堅韌強固:穩定性、可靠性和服務性。如今這個概念已廣為人知,並被簡稱為RAS。


Reliability can be defined as the probability that it will produce correct outputs up to some given time t. 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, e.g., 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 may failover to redundant replacement hardware, etc.), or else by halting the affected program or the entire system and reporting the corruption. Reliability is often characterized in terms of mean time between failures (MTBF), with reliability = exp(-t/MTBF).