User guide

Hardware architecture 2–5
Hitachi Unified Storage VM Block Module Hardware User Guide
RAID 5
A RAID 5 array group consists of four or eight data drives, (3D+1P) or
(7D+1P. The data is written across the four (or eight) drives in a stripe that
has three (or seven) data chunks and one parity chunk. Each chunk
contains 768 logical blocks, because HUS VM supports only open systems.
The enhanced RAID 5+ implementation in the HUS VM minimizes the write
penalty incurred by standard RAID 5 implementations by keeping write data
in cache until an entire stripe can be built and then writing the entire data
stripe to the drives. The 7D+1P RAID 5 increases usable capacity and
improves performance.
The following two figures illustrate the RAID 5 configurations. The tables
following the figures describes each configuration.
Figure 2-2: Sample RAID 1 (2D + 2D) Layout
Note: There are two configurations of RAID 5: 3D+1P configuration (four
disk drives) and 7D+1P configuration (eight disk drives). The following
diagram shows the 3D+1P configuration. In the 7D+1P configuration, data
is arranged in the same way.