Compaq 166207-B21 - Smart Array 5302/32 RAID Controller Reference Manual page 10

1-2 Advanced Data Guarding for the Smart Array 5300 Controller Reference Guide
Alternative name
Usable drive space*
Usable drive space
formula
Minimum number
of hard drives
Tolerant of single
hard drive failure?
Tolerant of multiple
simultaneous hard
drive failure?
Read performance
Write performance
Relative cost
*Note: The value for usable drive space is calculated assuming a maximum of 14 hard drives of the same
capacity (or a maximum of 56 for RAID ADG) with no online spares. Compaq recommends that these
maximum figures (excluding any allowable online spares) are not exceeded when configuring a drive array,
due to the increased likelihood of logical drive failure with more hard drives.
The main advantages of RAID ADG are:
I
High read performance
I
High data availability – any two drives can fail without loss of critical
data
The only significant disadvantage is a relatively low write performance (lower
than RAID 5), due to the need for two sets of parity data.
The following table summarizes the most important features of the different
varieties of RAID available. The decision chart in Figure 1-2 may help you
determine which RAID option is best for your situation.
Summary of RAID Features
RAID 0
Striping (no
fault tolerance)
100%
n
1
No
No
High
High
Low
Table 1-1
RAID 1 / RAID 0+1
Mirroring Distributed
Data Guarding
50% 67% to 93%
n/2 (n-1)/n
2 3
Yes Yes
In RAID 0+1, only No
when the failed
drives are not
mirrored to each
other
High High
Medium Low
High Medium
RAID 5 RAID ADG
Advanced Data
Guarding
50% to 96%
(n-2)/n
4
Yes
Yes
High
Low
Medium