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SLED

(Single Large Expensive Disk) The traditional hard disk drive used in minicomputers and mainframes. Such drives were widely used starting in the mid-1960s through the late 1980s. Today, all hard disks are small and inexpensive by comparison. See RAID.



RAID

(Redundant Array of Independent Disks) A disk subsystem that increases performance or provides fault tolerance or both. RAID uses two or more physical disk drives and a RAID controller, which is plugged into motherboards that do not have RAID circuits. Today, most motherboards have built-in RAID but not necessarily every RAID configuration (see below). 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. See RAID 0.





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

RAID 10 combines RAID 1 mirroring with RAID 0 striping for both safety and performance. See RAID 1 and RAID 10.





RAID 3 - Speed and Fault Tolerance
Data are striped across three or more drives for performance, and parity is computed for safety. Similar to RAID 3, RAID 4 uses block level striping but is not as popular. See RAID 3 and RAID parity.





RAID 5 - Speed and Fault Tolerance (Popular)
Data are striped across three or more drives for performance, and parity is computed for safety. RAID 5 is similar to RAID 3, except that the parity is distributed to all drives. RAID 6 offers more reliability than RAID 5 by performing more parity computations. For more details, see RAID 5.









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 first to provide RAID 1 on IDE drives rather than SCSI. This two-drive unit connected to the motherboard with one 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)






USB RAID
Super Talent's USB 3.0 RAID drives provide RAID 0 storage that is faster than the internal hard disk. (Image courtesy of Super Talent Technology Corporation, www.supertalent.com)






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