每个用例应指定一个有意义的功能块，说明系统如何为参与者提供有价值的东西。

Each use case should specify a significant chunk of functionality and show how the system provides value to an actor.

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vmstat是很多Linux发行版的标准配置，它提供了三种形式的CPU负载，一般磁盘块出入以及监视很多其他系统功能的能力。

Standard on many Linux distributions, vmstat provides three forms of CPU load, generalized disk blocks in and out, and the capability for monitoring many other system functions.

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由于这个原因，Linux现在支持文件系统将丢弃块传递到SSD的功能(自内核版本2.6.29开始)。

For this reason, Linux now supports the ability of the file system to communicate discarded blocks to the SSD (as of kernel version 2.6.29).

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