Drive arrays and fault-tolerance methods 114
RAID 6 (ADG), like RAID 5, generates and stores parity information to protect against data loss caused by
drive failure. With RAID 6 (ADG), however, two different sets of parity data are used (denoted by P
x,y
and
Q
x,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.
