RAID 2, which is rarely used in practice, stripes data at the bit (rather than block) level, and uses a Hamming code for error correction. The disks are synchronized by the controller to spin at the same angular orientation.
Characteristics & Advantages
Extremely high data transfer rate
Disk failure has an insignificant impact on throughput
Low ratio of ECC (Parity) disks to data disks, that means high efficiency
Cannot handle simultaneous requests
Internal error correction complexity of the Hamming Code, offers little advantage over parity.
RAID 2 is rarely implemented for commercial purpose