Smart Array 5300 Controller User Guide

Drive Arrays and Fault Tolerance
D-10 Compaq Smart Array 5300 Controller User Guide
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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
nn/2 (n-1)/n (n-2)/n
Minimum number of
physical drives
12 34
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.