Smart Array 5i Plus Controller and Battery Backed Write Cache Enabler User Guide

Drive Arrays and Fault Tolerance
Compaq Smart Array 5i Plus Controller and Battery Backed Write Cache Enabler User Guide C-9
COMPAQ CONFIDENTIAL
Writer: Amy L. Laffitte File Name: l-appc Drive Arrays and Fault Tolerance
Codename: Executor Part Number: 266328-001 Last Saved On: 4/18/02 2:51 PM
Advantages
The advantages of RAID 5 are that:
• It has a high read performance.
• No loss of data occurs if one physical drive fails.
• More drive capacity is usable than with RAID 1+0 (parity information requires
only the storage space equivalent to one physical drive).
Disadvantages
The disadvantages of RAID 5 are that:
• It has a relatively low write performance.
• Loss of data occurs if a second drive fails before data from the first failed drive is
rebuilt.
Table C-1 summarizes the important features of the different kinds of RAID
described. The decision chart in Table C-2 may help you to determine which option
is best for your situation.
Table C-1: Summary of RAID Methods
RAID 0 RAID 1+0 RAID 5
Alternative
name
Striping (no
fault tolerance)
Mirroring Distributed Data
Guarding
Usable drive
space*
100% 50% 67% to 93%
Usable drive
space formula
n n/2 (n-1)/n
Minimum
number of
physical drives
123
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