帖子[1]是我见过的最完善的讲述磁盘性能的帖子，里面提到了磁盘的构造和各种参数的实际含义，包括转速，平均搜索时间，IOPS等等，有兴趣的人可以看看。
Note: 我看到的其中一个小问题是：RAID5中一个写操作需要后台2次读2次写这个说法只是其中一种可能。严格意义上来说，RAID5会根据现有的数据和需要写入的数据大小来确定如何写入数据，譬如full strip write就不需要这种2次读2次写的操作。摘录wikipedia的原话：“RAID 5 implementations suffer from poor performance when faced with a workload which includes many writes which are smaller than the capacity of a single stripe.” 注意，这里有限定语句：which are smaller than the capacity of a single stripe.
参考文献：
[1] http://www.symantec.com/connect/articles/getting-hang-iops
[2] http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5_performance
[3] http://blogs.oracle.com/bonwick/entry/raid_z
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