Reliability, availability and serviceability (computer hardware)
Reliability, availability, and serviceability (RAS) is a computer hardware engineering term. The phrase was originally used by IBM as a term to describe the robustness of their mainframe computers. The concept is often known by the acronym RAS.
Computers designed with higher levels of RAS have a multitude of 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.
- 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).
- Availability is the probability 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. In high availability applications, availability may be reported 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 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 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. It 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 that reliability and availability are distinct concepts: Reliability is a measure of the ability of a system to function correctly, including avoiding data corruption, whereas availability measures how often it 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 mechanical failure such as metal electromigration or dielectric breakdown. Temporary faults include transient and intermittent faults. Transient (aka soft) faults lead to independent one-time errors and are not due to permanent hardware fault: examples include alpha particles flipping a memory bit, or 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 (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 the OS to provide information for Predictive failure analysis.
RAS features 
Example hardware features for improving RAS include the following, listed by subsystem:
- Processor instruction error detection (e.g. residue checking of results) with instruction retry e.g. alternative processor recovery in IBM mainframes, or "Instruction replay technology" in Itanium systems.
- Processors running in lock-step to perform master-checker or voting schemes.
- Machine check architecture to report errors to the OS.
- Extended precision, including 128-bit, floating point to minimize roundoff error in intermediate calculations, and hardware decimal floating point to avoid decimal-binary conversion errors.
- Duplication of components to avoid single point of failures (for example power-supplies).
- Over-designing the system for the specified operating ranges of clock frequency, temperature, voltage, vibration.
- Temperature sensors to throttle operating frequency when temperature goes out of specification.
- Surge protector, uninterruptible power supply, auxiliary power.
- Hot swapping of components.
- Predictive failure analysis to predict which intermittent correctable errors will lead eventually to hard non-correctable errors.
- Partitioning/domaining of computer components to allow one large system to act as several smaller systems.
- Virtual machines to decrease the severity of operating system software faults.
- Computer clustering capability with failover capability, for complete redundancy of hardware and software.
- Dynamic software updating to avoid the need to reboot the system for a kernel software update, for example Ksplice under Linux.
- Independent service processor for serviceability: remote monitoring, alerting and control.
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 (e.g., see Tandem Computers and Stratus Technologies), 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.
See also 
- Daniel P. Siewiorek, Robert S. Swarz (1998). Reliable computer systems: design and evaluation. p. 508.. "The acronym RAS (reliability, accessibility and serviceability) came into widespread acceptance at IBM as the replacement for the subset notion of recovery management."
- Data Processing Division, International Business Machines Corp., 1970 (1970). Data processor, Issues 13-17.- "The dependability [...] experienced by other System/370 users is the result of a strategy based on RAS (Reliability-Availability-Serviceability)"
- Sam Siewert (Mar 2005). "Big iron lessons, Part 2: Reliability and availability: What's the difference?".
- E.J. McClusky and S. Mitra (2004). "Fault Tolerance" in Computer Science Handbook 2ed. ed. A.B. Tucker. CRC Press.
- Richard H. Spencer and Raymond E. Floyd (2001). Perspectives on Engineering. Bloomington.
- Daniel Lipetz and Eric Schwarz (2011). "Self Checking in Current Floating-Point Units. Proceedings of 2011 20th IEEE Symposium on Computer Arithmetic".
- L. Spainhower and T. A. Gregg (September 1999). "IBM S/390 parallel enterprise server G5 fault tolerance: a historical perspective. IBM Journal of Research and Development. Volume 43 Issue 5". CiteSeerX: 10.1.1.85.5994.
- "Intel Instruction Replay Technology Detects and Corrects Errors". Retrieved 2012-12-07.
- "IBM zEnterprise EC12 Technical Introduction". Retrieved 2012-12-07.
- HP. "Memory technology evolution: an overview of system memory technologies Technology brief, 9th edition (page 8)".
- Intel Corp. (2003). "PCI Express Provides Enterprise Reliability, Availability, and Serviceability".
- Itanium Reliability, Availability and Serviceability (RAS) Features Overview of RAS features in general and specific features of the Itanium processor.
- POWER7 System RAS Key Aspects of Power Systems Reliability, Availability, and Serviceability. Daniel Henderson, Jim Mitchell, and George Ahrens. February 10, 2012 Overview of RAS features in POWER processors.
- Intel Corp. Reliability, Availability, and Serviceability for the Always-on Enterprise (appendix B) and Intel Xeon Processor E7 Family: supporting next generation RAS servers. White paper. Overview of RAS features in Xeon processors.
- zEnterprise 196 System Overview. IBM Corp. (Chapter 10) Overview of RAS features of IBM z196 processor and zEnterprise 196 server.