HP Array Configuration Utility User Guide (416146-001, January 2006)

Drive arrays and fault-tolerance methods 66
and Qx,y in the figure), allowing data to still be preserved if two drives fail. Each set of parity data uses a
capacity equivalent to that of one of the constituent drives.
This method is most useful when data loss is unacceptable but cost is also an important factor. The
probability that data loss will occur when an array is configured with RAID 6 (ADG) is less than it would
be if it was configured with RAID 5.
Advantages:
• This method has a high read performance.
• This method allows high data availability—Any two drives can fail without loss of critical data.
• More drive capacity is usable than with RAID 1+0—Parity information requires only the storage
space equivalent to two physical drives.
Disadvantages:
The main disadvantage of RAID 6 (ADG) is a relatively low write performance (lower than RAID 5)
because of the need for two sets of parity data.
Comparing the hardware-based RAID methods
NOTE: Not all controllers support RAID 6 (ADG).
Item RAID 0 RAID 1+0 RAID 5 RAID 6 (ADG)
Alternative name
Striping (no
fault
tolerance)
Mirroring
Distributed
Data
Guarding
Advanced
Data
Guarding
Formula for number of drives
usable for data (n = total number
of drives in array)
n n/2 n-1 n-2
Fraction of drive space usable* 100% 50% 67% to 93% 50% to 96%
Minimum number of physical
drives
1 2 3 4
Tolerates failure of one physical
drive
No Yes Yes Yes