Drive Arrays and Fault Tolerance

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-1summarizes the important features of the different kinds of RAID methods described here. The decision chart in Table D-2may 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

Mirroring

Distributed

Advanced Data

 

fault tolerance)

 

Data Guarding

Guarding

 

 

 

 

 

Usable drive space**

100%

50%

67% to 93%

50% to 96%

 

 

 

 

 

Usable drive space

n

n/2

(n-1)/n

(n-2)/n

formula

 

 

 

 

 

 

 

 

 

Minimum number of

1

2

3

4

physical drives

 

 

 

 

 

 

 

 

 

Tolerates failure of

No

Yes

Yes

Yes

one physical drive?

 

 

 

 

 

 

 

 

 

Tolerates

No

Only if no two

No

Yes

simultaneous failure

 

failed drives are

 

 

of more than one

 

in the same

 

 

physical drive?

 

mirrored pair

 

 

 

 

 

 

 

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.

D-10

HP Smart Array 641/642 Controller User Guide