HP 166207-B21 Smart Array 5300 Controller User Guide - Page 115

Disadvantage, Table D-1, Summary of RAID Methods

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Drive Arrays and Fault Tolerance 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 Mirroring fault tolerance) Distributed Advanced Data 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 physical drives 3 4 Tolerates failure of No Yes one physical drive? Yes Yes Tolerates No Only if no two No Yes simultaneous failure failed drives are of more than one in a mirrored pair physical drive? 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. D-10 Compaq Smart Array 5300 Controller User Guide

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Drive Arrays and Fault Tolerance
D-10
Compaq Smart Array 5300 Controller User Guide
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.