HP Compaq Integrated Smart Array Controller User Manual
Page 114
B-8 Compaq Integrated Smart Array Controller User Guide
Compaq Confidential – Need to Know Required
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Part Number: 153236-001 File Name: i-appb Understanding Drive Arrays.doc Last Saved On: 8/27/99 11:35 AM
Drive Mirroring and Striping
(RAID 1 and RAID 0+1)
Drive mirroring, also called RAID 1, is the highest performance and highest
fault-tolerance RAID method. RAID 1 is the only option offering
fault-tolerance protection if only two drives are installed or selected for an
array. Drive mirroring creates fault tolerance by storing two sets of duplicate
data on a pair of disk drives. Therefore, RAID 1 is the most expensive
fault-tolerance method because 50 percent of the drive capacity is used to store
the redundant data. RAID 1 always requires an even number of drives. To
improve performance in configurations with four or more drives, the data is
striped across the drives and then mirrored. This is also referred to as
RAID 0+1.
If a drive fails, the mirror drive provides a backup copy of the files and normal
system operations are not interrupted. The mirroring feature requires a
minimum of two drives and, in a multiple drive configuration (four or more
drives), mirroring can withstand multiple simultaneous drive failures as long
as the failed drives are not mirrored to each other. If two drives mirrored to
each other fail, the volume is failed and data loss may occur.
DATA
1
4
3
2
DATA
1
4
3
2
Figure B-6. Drive mirroring stores an identical copy of the data