Raid implementation, Array groups and raid levels, Raid implementation -2 – Hitachi Universal Storage Platform VM User Manual
Page 46: Array groups and raid levels -2
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Functional and Operational Characteristics
Hitachi Universal Storage Platform V/VM User and Reference Guide
RAID Implementation
This section provides an overview of the implementation of RAID technology
on the Universal Storage Platform V/VM:
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LDEV Striping Across Array Groups
Array Groups and RAID Levels
The array group (also called parity group) is the basic unit of storage capacity
for the USP V/VM. Each array group is attached to both boards of a BED pair
over 16 fibre paths, which enables all data drives in the array group to be
accessed simultaneously by the BED pair. Each array frame has two canister
mounts, and each canister mount can have up to 128 physical data drives.
The USP V/VM supports the following RAID levels: RAID-10, RAID-5, RAID-6.
RAID-0 is not supported on the USP V/VM. When configured in four-drive
RAID-5 parity groups (3D+1P), ¾ of the raw capacity is available to store user
data, and ¼ of the raw capacity is used for parity data.
RAID-10.
illustrates a sample RAID-10 (2D+2D) layout. A RAID-10
(2D+2D) array group consists of two pairs of data drives in a mirrored
configuration, regardless of data drive capacity. A RAID-10 (4D+4D)* group
combines two RAID-10 (2D+2D) groups. Data is striped to two drives and
mirrored to the other two drives. The stripe consists of two data chunks. The
primary and secondary stripes are toggled back and forth across the physical
data drives for high performance. Each data chunk consists of either eight
logical tracks (mainframe) or 768 logical blocks (open systems). A failure in a
drive causes the corresponding mirrored drive to take over for the failed drive.
Although the RAID-5 implementation is appropriate for many applications, the
RAID-10 option can be ideal for workloads with low cache-hit ratios.
*Note for RAID-10(4D+4D): It is recommended that both RAID-10
(2D+2D) groups within a RAID-10 (4D+4D) group be configured under the
same BED pair.