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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Concepts and Issues | RAID Reliability Issues ]
When most people talk about the "enhanced reliability of RAID", what they really are referring to is the fault tolerance of most RAID implementations. Reliability and fault tolerance are not the same thing at all, as described in the discussion of RAID array reliability. Fault tolerance refers to the ability of a RAID array to withstand the loss of some of its hardware without the loss of data or availability. When a fault occurs, the array enters a degraded state and the failed drive must be replaced and rebuilt.
The capability of an array to tolerate hard disk faults depends entirely on the RAID level implemented. RAID 0, which has no redundant information, has no fault tolerance for that reason; if any drive fails the array goes down. RAID levels 1, 2, 3, 4, 5 and 7 can tolerate the loss of one hard disk; RAID 6 can tolerate the loss of two. The multiple RAID levels can often tolerate the loss of multiple hard disks depending on which ones they are and how the array is configured. For example, if you have a RAID 10 array consisting of two pairs of mirrored drives striped together, the array can tolerate the simultaneous loss of two of the four drives, as long as they are not in the same pair.
Ultimately, fault tolerance of a RAID-equipped PC depends not only on the reliability of the drives, but also the other hardware in the system. For example, most RAID implementations are dependent upon the RAID controller not failing; if it goes down then your data is probably fine, but availability is not--this is one reason why duplexing is sometimes used, since it can tolerate the loss of a drive and a controller. Since items such as power supplies have a high failure rate and ultimately can bring down any array, fault tolerance in a PC equipped with RAID is also often used to describe features implemented on the system as a whole to prevent faults in this support equipment; see here for more details.