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In the past 15 years, hard disk capacities have grown by a factor of almost a thousand. 15 years ago, a typical hard disk had a capacity of between 300 and 500 Mbytes, and they were just as expensive as today’s 300 to 500 Gbyte disks. Because PCs with two or more disks are now quite common, home users can easily afford the data redundancy and higher performance that used to be the reserve of enterprise-level servers. The technology that makes this possible is known as RAID. RAID – an Introduction RAID [1] was designed some 20 years ago by Berkeley post-graduate students David Patterson, Garth Gibson, and Randy Katz. At the time, RAID was the answer to a difficult problem: if you needed a lot of storage capacity, you had to choose either a single, large disk that was reliable but expensive or a lot of small, fairly unreliable, inexpensive disks.

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