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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 Levels | Technical Factors Differentiating RAID Levels ]
Hard Disk Requirements
Different RAID levels have varying requirements for the hard disks used in the array. The most important difference between levels is related to the minimum number of drives in the array, which depends entirely on how the RAID level implements mirroring, striping, and parity. Simple striping (RAID 0) requires two or more drives; mirroring (RAID 1) requires two drives; and striping with parity requires at least three drives (two or more for data stripes and one for parity, whether it is dedicated to a single drive or distributed). Striping with double parity (RAID 6) requires at least four drives. Multiple RAID levels generally require a number of drives equal to the product of the minimum number of drives of the single levels that comprise them. For example, RAID 10 requires a minimum of four drives (and must consist of an even number of drives) and RAID 50 requires at least six!
The maximum number of drives is typically limited by the RAID controller, not by anything inherent in the definition of the RAID level. The exception to this rule is RAID 1, which works with two drives only. This limitation is one reason why RAID 1 is rarely used in larger arrays (RAID 0+1 or 1+0 are used where the advantages of RAID 1 are required in combination with larger capacity.)
Finally, all RAID levels work best when fitted with identical drives of identical capacity. Some RAID levels can tolerate differences in performance between drives in the array better than others (typically the simpler levels). All RAID arrays make best use of the space on the disk when the drives are the same size; see here for more details.
For more on hard disk requirements for RAID and selecting hard disks, see this section.