Compaq Smart Array 221 User Manual page 110

C-8 Understanding Drive Arrays
Drive Mirroring (RAID 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 1+0 or RAID 10.
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 DATA
1 4 1 4
2 3 2 3
WAR2-051.AI, 9-1.EPS
Figure C-6. Drive Mirroring stores identical copy of the data
Writer: Karen Hale Project: Compaq Smart Array 221 Controller User Guide Comments: 388541-001
File Name: K-APPC.DOC Last Saved On: 11/5/98 1:40 PM
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