Hitachi Virtual Storage Platform, User And Reference Manual

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Hitachi VSP User and Reference Guide
2–4
Functional and Operational Characteristics
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 either eight logical tracks (mainframe) or 768 logical blocks
(open). The enhanced RAID 5+ implementation in the Virtual Storage
Platform 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.
Figure 2-3 Sample RAID 5 3D + 1P Layout (Data Plus Parity Stripe) on page
2-4 illustrates RAID 5 data stripes mapped over four physical drives. Data
and parity are striped across each of the data drives in the array group
(hence the term “parity group”). The logical devices (LDEVs) are evenly
dispersed in the array group, so that the performance of each LDEV within
the array group is the same. This figure also shows the parity chunks that
are the Exclusive OR (EOR) of the data chunks. The parity and data chunks
rotate after each stripe. The total data in each stripe is either 24 logical
tracks (eight tracks per chunk) for mainframe data, or 2304 blocks (768
blocks per chunk) for open-systems data. Each of these array groups can
be configured as either 3390-x or OPEN-x logical devices. All LDEVs in the
array group must be the same format (3390-x or OPEN-x). For open
systems, each LDEV is mapped to a SCSI address, so that it has a TID and
logical unit number (LUN).
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 either eight logical tracks
(mainframe) or 768 logical blocks (open).
In the case of 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.
Figure 2-3 Sample RAID 5 3D + 1P Layout (Data Plus Parity Stripe)