User guide

D-8 Compaq Smart Array 431 Controller User Guide
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.
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Figure D-6. Drive Mirroring stores identical copy of the data