Abstract
HDDs are like the bread in a peanut butter and jelly sandwich—sort of an unexciting piece of hardware necessary to hold the “software.” They are simply a means to an end. HDD reliability, however, has always been a significant weak link, perhaps the weak link, in data storage. In the late 1980s people recognized that HDD reliability was inadequate for large data storage systems so redundancy was added at the system level with some brilliant software algorithms, and RAID (redundant array of inexpensive disks) became a reality. RAID moved the reliability requirements from the HDD itself to the system of data disks. Commercial implementations of RAID range from n+1 configurations (mirroring) to the more common RAID-4 and RAID-5, and recently to RAID-6, the n+2 configuration that increases storage system reliability using two redundant disks (dual parity). Additionally, reliability at the RAID group level has been favorably enhanced because HDD reliability has been improving as well.
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