User's Manual

Drive Arrays and Fault Tolerance
D-10 HP Smart Array 641/642 Controller User Guide
HP CONFIDENTIAL
Writer: John Turner File Name: l-appd drive arrays & fault tolerance
Codename: darkmatter -antimatter Part Number: 309311-001 Last Saved On: 11/6/02 4:34 PM
Disadvantage
The main disadvantage of RAID ADG is a relatively low write performance (lower
than RAID 5), because of the need for two sets of parity data.
Comparison of RAID Methods
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 the same
mirrored pair
No Yes
Read performance High High High High
Write performance High Medium Low Low
Relative cost Low High Medium Medium
*Not all controllers support RAID ADG.
**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.