User guide

Hardware architecture 2–3
Hitachi Unified Storage VM Block Module Hardware User Guide
Array groups and RAID levels
The array group (also called parity group) is the basic unit of storage
capacity for the HUS VM storage system. It is a set of four physical drives
installed into any disk trays (in any “roaming” order on HUS VM). When a
set of one or two such Array Groups (four or eight drives) is formatted using
a RAID level, the resulting RAID formatted entity is called a Parity Group.
Although technically the term Array Group refers to a group of bare physical
drives, and the term Parity Group refers to something that has been
formatted as a RAID level and therefore actually has initial parity data (here
we consider a RAID-1 mirror copy as parity data), be aware that this
technical distinction is often lost. You will see the terms Parity Group and
Array Group used interchangeably in the field.
The HUS VM supports the following RAID levels: RAID 1, RAID 5, and RAID
6. RAID 0 is not supported on the HUS VM. When configured in four-drive
RAID 5 parity groups (3D+1P), three-fourths of the raw capacity is available
to store user data, and one fourth of the raw capacity is used for parity data.
RAID 1
The following two figures illustrate the RAID 1configurations. The tables
following the figures describes each configuration.
Item Description
Description Mirror disks (duplicated writing) Two disk drives, primary and
secondary disk drives, compose a RAID pair (mirroring pair) and the
identical data is written to the primary and secondary disk drives.
Further, data is scattered on the two RAID pairs.
Advantage RAID 1 is highly usable and reliable because of the duplicated data. It
has higher performance than ordinary RAID 1 (when it consists of two
disk drives) because it consists of the two RAID pairs.