Reliability, availability and serviceability, also known as reliability, availability, and maintainability, is a computer hardware engineering term involving reliability engineering, high availability, and serviceability design. The phrase was originally used by International Business Machines as a term to describe the robustness of their mainframe computers. Computers designed with higher levels of RAS have many features that protect data integrity and help them stay available for long periods of time without failure This data integrity and uptime is a particular selling point for mainframes and fault-tolerant systems.
Definitions
While RAS originated as a hardware-oriented term, systems thinking has extended the concept of reliability-availability-serviceability to systems in general, including software.
Reliability can be defined as the probability that a system 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, for example: by retrying an operation for transient or intermittent errors, or else, for uncorrectable errors, isolating the fault and reporting it to higher-level recovery mechanisms, or else by halting the affected program or the entire system and reporting the corruption. Reliability can be characterized in terms of mean time between failures, with reliability = exp.
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.
Serviceability or maintainability 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 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.
Failure types
Physical faults can be temporary or permanent.
Permanent faults lead to a continuing error and are typically due to some physical failure such as metal electromigration or dielectric breakdown.
Temporary faults include transient and intermittent faults.
* Transient 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.
Failure responses
Transient and intermittent faults can typically be handled by detection and correction by e.g., ECC codes or instruction replay. 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 the operating system to provide information for predictive failure analysis.
Hardware features
Example hardware features for improving RAS include the following, listed by subsystem:
Fault-tolerant designs extended the idea by making RAS to be the defining feature of their computers for applications like stock market exchanges or air traffic control, where system crashes would be catastrophic. Fault-tolerant computers, which tend to have duplicate components running in lock-step for reliability, have become less popular, due to their high cost. High availability systems, using distributed computing techniques like computer clusters, are often used as cheaper alternatives.
