Reliability, Availability and Serviceability

Reliability, Availability and Serviceability are computer hardware engineering terms. It originated from IBM to advertise the robustness of their mainframe computers. The concept is often known by the acronym "RAS".

Mainframe computers have a multitude of features that help them "stay up" (stay operational) for long periods of time without"crashing" (stop working) due to system faults. These "long uptimes" (periods between system crashes or "downtime") are a key selling feature of mainframes and even more so for fault-tolerant systems. Such computer vendors pride themselves in uptimes on the order of years without a system crash.

* "Reliability" means features that help avoid and detect such faults. A reliable system does not silently continue and deliver results that include uncorrected corrupted data, instead it corrects the corruption when possible or else stops and reports the corruption.
* "Availability" is the amount of time a device is actually operating as the percentage of total time it should be operating. Availability may be shown as 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 unoperational.
* "Serviceability" takes the form of various methods of easily diagnosing the system when problems arise. Early detection of faults can decrease or avoid system downtime. For example, some of IBM's systems could automatically call an IBM service center (without human intervention) when the system experiences a system fault. Traditional focus has been on making the correct repairs with as little disruption to normal operations as possible.

"RAS" features might include:
* Parity or ECC protection of memory components as well as buses.
* Cyclic redundancy check checksums for data transmission and data storage.
* RAID configurations for magnetic disk storage.
* Journaling file systems for file repair after crashes.
* Over-designing the system for the specified operating ranges of clock frequency, temperature, voltage, vibration.
* Duplication of computing components running in lock-step to perform master-checker or voting schemes.
* Duplication of components to avoid single point of failures (for example power-supplies).
* Hot swapping of components.
* Partitioning/domaining of computer components to allow one large system to act as several smaller systems.
* Computer clustering capability.
* Virtual machines to decrease the severity of operating system software faults.
* Temperature sensors to throttle operating frequency when temperature goes out of specification.
* Surge protector, uninterruptible power supply, auxiliary power.
* Failover capability.

Fault-tolerant designs from vendors like Tandem Computers extended the idea by making "RAS" to be the definingfeature 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, havebecome less popular due to their high cost. High availability systems, using distributed computing techniques like computer clusters, are often used as cheaper alternatives.