from: http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html
Amazon CTO对伸缩性的解释
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What is it that we really mean by scalability? A service is said to be scalable if when we increase the resources in a system, it results in increased performance in a manner proportional to resources added. Increasing performance in general means serving more units of work, but it can also be to handle larger units of work, such as when datasets grow.
— 简单说，系统的吞吐和性能能随着投入的服务器资源数量的增加而成正比的增加
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In distributed systems there are other reasons for adding resources to a system; for example to improve the reliability of the offered service. Introducing redundancy is an important first line of defense against failures.An always-on service is said to be scalable if adding resources to facilitate redundancy does not result in a loss of performance.
— 随着系统资源的增加，为了支持容错的冗余机器的比例会下降，但是同时不能降低服务的质量
— 简单说就是规模效应
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