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.
Next: Array Capacity and Storage Efficiency
