Hitachi Universal Storage Platform VM - Manual (Page 47)

Functional and Operational Characteristics 3-3
Hitachi Universal Storage Platform V/VM User and Reference Guide
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RAID-10 using 2D + 2D and 3390-x LDEVs
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Figure 3-1 Sample RAID-10 2D + 2D Layout
RAID-5. A RAID-5 array group consists of four (3D+1P) or eight (7D+1P)
data drives. 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 USP V/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.
Figure 3-2 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. Figure 3-2 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).