相关资料搜了许久也没有找到专门的描述，这个描述比较清楚一点，但没能找到资料源头。

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Reliability, Availability, Serviceability (RAS)

2011-07-04 16:57:04|  分类： Terms |  标签：ras   |字号 订阅

经常听到Reliability, Availability, 和Serviceability，但是对这几个概念一直都是一知半解。
今天查了一下，终于搞明白它们的含义了。
可靠性描述系统是否能够正确工作。
可使用性描述系统能够正确工作的时间。
可服务性描述系统的容错性，即出现错误之后恢复的难易程度。

Reliability refers to the ability of a computer-related hardware or software component to consistently perform according to its specifications. In theory, a reliable product is totally free of technical errors. In practice, vendors commonly express product reliability as a percentage.可靠性指软、硬件是否按照specification所说的那样提供功能，通常通过百分比来指明其可靠性。

Availability is the ratio of time a system or component is functional to the total time it is required or expected to function. This can be expressed as a direct proportion (for example, 9/10 or 0.9) or as a percentage (for example, 90%). It can also be expressed in terms of average downtime per week, month or year or as total downtime for a given week, month or year. Sometimes availability is expressed in qualitative terms, indicating the extent to which a system can continue to work when a significant component or set of components goes down. 可使用性和我们通常说的宕机时间相关，即正常系统工作的时间和总的时间的比例。

Serviceability is an expression of the ease with which a component, device or system can be maintained and repaired. Early detection of potential problems is critical in this respect. Some systems have the ability to correct problems automatically before serious trouble occurs; examples include built-in features of OSs such as Microsoft Windows XP and auto-protect-enabled anti-virus software and spyware detection and removal programs. Ideally, maintenance and repair operations should cause as little downtime or disruption as possible.