User guide

2–8 Hardware architecture
Hitachi Unified Storage VM Block Module Hardware User Guide
RAID 6
A RAID 6 array group consists of eight data drives (6D+2P). The data is
written across the eight drives in a stripe that has six data chunks and two
parity chunks. Each chunk contains 768 logical blocks..
In RAID 6, data can be assured when up to two drives in an array group fail.
Therefore, RAID 6 is the most reliable of the RAID levels.
The following figure illustrates the RAID 5 configuration. The table following
the figure describes it.
Sequential data striping
The enhanced RAID 5+ implementation on the HUS VM attempts to keep
write data in cache until parity can be generated without referencing old
parity or data. This capability to write entire data stripes, which is usually
achieved only in sequential processing environments, minimizes the write
penalty incurred by standard RAID 5 implementations. The device data and
Note: There are two configurations of RAID 6: 6D+2P configuration
(eight disk drives) and 14D+2P configuration (sixteen disk drives). The
following diagram shows the 6D+2P configuration.
Item Description
Description Data blocks are scattered to multiple disks in the same way as RAID
5 and two parity disks, P and Q, are set in each row. Therefore, data
can be assured even when failures occur in up to two disk drives in a
parity group.
Advantage RAID 6 is far more reliable than RAID 1 and RAID 5 because it
canrestore data even when failures occur in up to two disks in a parity
group.
Disadvantage Because the parity data P and Q must be updated when data is
updated, RAID 6 is imposed write penalty heavier than that on RAID
5, performance of the random writing is lower than that of RAID 5 in
the case where the number of drives makes a bottleneck.