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(Redundant Array of Independent Disks) A disk subsystem that increases performance or provides fault tolerance or both. RAID uses two or more hard drives and a RAID controller, which is plugged into motherboards that do not have built-in RAID circuits. Today, most motherboards have RAID built in. In the past, RAID was also accomplished by software only, but was much slower. In the late 1980s, the "I" in RAID stood for "inexpensive," but was later changed to "independent."

     In large storage area networks (SANs), floor-standing RAID units are common with terabytes of storage and huge amounts of cache memory. RAID is also used in desktop computers by gamers for speed and by business users for reliability. Following are the various RAID configurations. See NAS and SAN.






RAID 0 - Disk Striping for Performance (Popular)
Widely used for gaming, disk striping interleaves data across multiple drives for performance. However, there are no safeguards against failure.






RAID 1 - Mirroring for Fault Tolerance (Popular)
Widely used, mirroring writes two drives at the same time so that data are duplicated. It provides the highest reliability, but doubles the number of drives needed.

RAID 10 (RAID 1+0)
RAID 10 combines RAID 1 and RAID 0. Drives are mirrored for fault tolerance (RAID 1) and striped for performance (RAID 0). For more speed, RAID 100 combines RAID 10 and 0. It adds a layer of striping (RAID 0) on top of two or more RAID 10 configurations.





RAID 3 - Speed and Fault Tolerance
Data are striped across three or more drives. Used to achieve the highest data transfer because all drives operate in parallel. Using byte level striping, parity bits are stored on separate, dedicated drives. Similar to RAID 3, RAID 4 uses block level striping, but is not as popular. For more on parity computations, see RAID parity.






RAID 5 - Speed and Fault Tolerance (Popular)
Data are striped across three or more drives for performance, and parity bits are used for fault tolerance. The parity bits from two drives are stored on a third drive and are interspersed with user data. RAID 5 is widely used in servers.

RAID 6 - Speed and Fault Tolerance
Similar to RAID 5 but performs two parity computations or the same computation on overlapping subsets of the data. Highest reliability because it can recover from two failed disks, but not widely used.









Big RAID
EMC has been a leader in high-end RAID systems for years with systems storing multiple terabytes of data. (Image courtesy of EMC Corporation.)





Little RAID
Arco was the first to provide RAID 1 mirroring on inexpensive IDE drives rather than SCSI. This unit took up two drive bays and connected to one Parallel ATA (PATA) cable like a single drive. (Image courtesy of Arco Computer Products, Inc., www.arcoide.com)





Early RAID
This RAID prototype was built by University of Berkeley graduate students in 1992. Housing 36 320MB disk drives, total storage was 11GB. (Image courtesy of The Computer History Museum, www.computerhistory.org)