The RAID Levels that Time Forgot

RAID2

This stripes data across multiple drives, but instead of blocks of data, it stripes at the bit level, complete with Hamming code error correction. High data rates are possible, but it’s complex, and requires large arrays of synchronized spinning disks. Now, hard drives have much of the error correction built in, making RAID 2 pretty much redundant.

RAID3

This stripes bits as RAID 2, and writes parity data to a dedicated parity disk. It proved to be not as reliable or cheap as other systems, the single parity disk being a major bottleneck.

RAID4

Similar to RAID 5, this stripes data at block level, but still puts all the parity on one disk. The large capacities of modern drives mean rebuilding the parity disk after a failure is a long, long job. Best to stick to the distributed parity used in RAID 5 and 6.

RAID7

A proprietary level of the now defunct Storage Computer Corporation. It turned out to be a variation on RAID 4 with a cache. After unsuccessful attempts to enforce spurious patents on the makers of other parity RAID systems, it all collapsed around the year 2000.