Drive Arrays and Fault Tolerance
D-10
Compaq Smart Array 5300 Controller User Guide
COMPAQ CONFIDENTIAL
Writer: John Turner File Name: n-appd drive arrays and fault tolerance.doc
Codename: SilverHammer Part Number: 135606-004 Last Saved On: 7/22/02 11:02 AM
Disadvantage
The only significant disadvantage of RAID ADG is a relatively low write
performance (lower than RAID 5), due to the need for two sets of parity data.
Table D-1 summarizes the important features of the different kinds of RAID methods
described here. The decision chart in Table D-2 may help you to determine which
option is best for your situation.
Table D-1: Summary of RAID Methods
RAID 0
RAID 1+0
RAID 5
RAID ADG
Alternative name
Striping (no
fault tolerance)
Mirroring
Distributed
Data Guarding
Advanced Data
Guarding
Usable drive space*
100%
50%
67% to 93%
50% to 96%
Usable drive space
formula
n
n
/2
(
n
-1)/
n
(
n
-2)/
n
Minimum number of
physical drives
1
2
3
4
Tolerates failure of
one physical drive?
No
Yes
Yes
Yes
Tolerates
simultaneous failure
of more than one
physical drive?
No
Only if no two
failed drives are
in a mirrored pair
No
Yes
Read performance
High
High
High
High
Write performance
High
Medium
Low
Low
Relative cost
Low
High
Medium
Medium
*Values for usable drive space 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; (4) No more than 56 drives are used with RAID ADG.