APC iSCSI SATA II - Manual (Page 12)

[12]
Dual-level RAID achieves a balance between the increased data availability inherent
in RAID 1 and RAID 5 and the increased read performance inherent in disk striping
(RAID 0). These arrays are sometimes referred to as RAID 0+1 or RAID 10 and RAID
0+5 or RAID 50.
RAID 6 is similar to RAID 5 in that data protection is achieved by writing parity
information to the physical drives in the array. With RAID 6, however, two sets of
parity data are used. These two sets are different, and each set occupies a capacity
equivalent to that of one of the constituent drives. The main advantage of RAID 6 is
High data availability – any two drives can fail without loss of critical data.
In summary:
Š RAID 0 is the fastest and most efficient array type but offers no fault-tolerance.
RAID 0 requires a minimum of two drives.
Š RAID 1 is the best choice for performance-critical, fault-tolerant environments.
RAID 1 is the only choice for fault-tolerance if no more than two drives are used.
Š RAID 3 can be used to speed up data transfer and provide fault-tolerance in single-
user environments that access long sequential records. However, RAID 3 does not
allow overlapping of multiple I/O operations and requires synchronized-spindle
drives to avoid performance degradation with short records. RAID 5 with a small
stripe size offers similar performance.
Š RAID 5 combines efficient, fault-tolerant data storage with good performance
characteristics. However, write performance and performance during drive failure
is slower than with RAID 1. Rebuild operations also require more time than with
RAID 1 because parity information is also reconstructed. At least three drives are
required for RAID 5 arrays.
Š RAID 6 is essentially an extension of RAID level 5 which allows for additional fault
tolerance by using a second independent distributed parity scheme (two-
dimensional parity). Data is striped on a block level across a set of drives, just like
in RAID 5, and a second set of parity is calculated and written across all the drives;
RAID 6 provides for an extremely high data fault tolerance and can sustain
multiple simultaneous drive failures. It is a perfect solution for mission critical
applications.