Origin of Raid-Redundant Arrays of Inexpensive Disks; New, used and refurbished

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II. Origins of RAID
The concept of RAID was first defined in 1988, when a group of computer scientists at the University of
California Berkeley, (David Patterson, Garth Gibson, and Randy Katz) published a paper entitled “A
Case for Redundant Arrays of Inexpensive Disks (RAID).”
The group observed that computer CPU speed and memory size was growing exponentially, while I/O performance was increasing at a much slower rate. Unless I/O performance could be significantly improved, computer systems would not be able to take full advantage of the rapidly increasing CPU and memory performance.
At the time, hard drive manufacturers addressed this issue by designing and building Single Large Expensive Disks (SLED). While storage capacities of these disk drives were sufficient for the times, I/O performance was still not keeping up as the inherent mechanical limitations of the hard drives were significantly slower when compared to electronic circuitry.
To overcome these limitations, the UC Berkley scientists proposed that instead of storing all data on one disk drive (with only one spindle), why not combine several small inexpensive disks (with many spindles) and stripe the data (split the data across multiple drives), such that reads or writes could be done in parallel. To simplify the I/O management, a dedicated controller would be used to facilitate the striping and present these multiple drives to the host computer as one large logical drive. They estimated the performance improvements would be an order of magnitude greater than using SLEDs.
The problem with this approach was that the small inexpensive PC disk drives of the time were less reliable than the SLED’s. An artifact of striping data over multiple drives is that if one drive fails, all data on the other drives is rendered unusable. It would be analogous to deleting every 3rd or 4th sentence out of a book, then not knowing what sequence the sentences were written in. To compound this problem, by combining several drives together, the probability of one drive failing increases dramatically.
To overcome this pitfall, the scientists proposed adding extra drives to the RAID group to store redundant information. The thought was; if one drive failed, another drive within the group would contain the missing information, which could then be used to regenerate the lost information. Since all the information was still available, the end user would never be impacted with down time and the rebuild could be done in the background. If users requested information that had not already been rebuilt, the data could be reconstructed on the fly and the end user still would not know about it.