The number of physical drives must be exactly divisible by the number of parity groups. Therefore, the
number of parity groups that you can specify is restricted by the number of physical drives. The maximum
number of parity groups possible for a particular number of physical drives is the total number of drives
divided by the minimum number of drives necessary for that RAID level (three for RAID 50, 4 for RAID
60).
All data is lost if a third drive in a parity group fails before one of the other failed drives in the parity group
has finished rebuilding. A greater percentage of array capacity is used to store redundant or parity data
than with non-nested RAID methods.
This method has the following benefits:
• Higher performance than for RAID 6, especially during writes.
• Better fault tolerance than RAID 0, 5, 50, or 6.
• Up to 2n physical drives can fail (where n is the number of parity groups) without loss of data, as long
as no more than two failed drives are in the same parity group.
Comparing the hardware-based RAID methods
Not all controllers support all RAID levels.
Item
RAID 0
RAID 1+0
RAID 5
RAID 6 (ADG) RAID 1(0)
(ADM)
Alternative name
Striping (no
fault tolerance)
Mirroring
Distributed Data
Guarding
Advanced
Data Guarding
Advanced
Data Mirroring
Formula for number of
drives usable for data
(n = total number of
drives in array)
n
n/2
n-1
n-2
n/3
Percentage of drive
space usable
100%
50%
67% to 93%
50% to 96%
33%
Minimum number of
physical drives
1
2
3
4
3
Tolerates failure of
one physical drive
No
Yes
Yes
Yes
Yes
Tolerates
simultaneous failure
of more than one
physical drive
No
Only if no
two failed
drives are in
the same
mirrored pair
No
Yes
Only if no
three drives
are in the
same mirror
group
Read performance
High
High
High
High
High
Write performance
High
Medium
Low
Low
Medium
Relative cost
Low
High
Medium
Medium
Very high
Values for the percentage of drive space usable are calculated with these assumptions: (1) all physical drives in the
array have the same capacity; (2) online spares are not used; (3) no more than 14 physical drives are used per array
for RAID 5; and (4) no more than 56 drives are used with RAID 6 (ADG).
Mirror groups include the physical drives in each mirror.
92
Hardware issues
