ABCs of z/OS System Programming Volume 3 - IBM Redbooks

8.2 Redundant array of independent disks (RAID)
Raid- 3
Raid-5
Raid-1
Data+ Parity Data+ Parity
Figure 8-2 Redundant array of independent disks (RAID)
RAID architecture
Redundant array of independent disks (RAID) is a direct access storage architecture where
data is recorded across multiple physical disks with parity separately recorded, so that no loss
of access to data results from the loss of any one disk in the array.
RAID breaks the one-to-one association of volumes with devices. A logical volume is now the
addressable entity presented by the controller to the attached systems. The RAID unit maps
the logical volume across multiple physical devices. Similarly, blocks of storage on a single
physical device may be associated with multiple logical volumes. Because a logical volume is
mapped by the RAID unit across multiple physical devices, it is now possible to overlap
processing for multiple cache misses to the same logical volume because cache misses can
be satisfied by separate physical devices.
The RAID concept involves many small computer system interface (SCSI) disks replacing a
big one. The major RAID advantages are:
► Performance (due to parallelism)
► Cost (SCSI are commodities)
► zSeries compatibility
► Environment (space and energy)
However, RAID increased the chances of malfunction due to media and disk failures and the
fact that the logical device is now residing on many physical disks. The solution was
448 ABCs of z/OS System Programming Volume 3
RAID Disks
Primary Alternate
Record X
ABCDEF
Record X
ABCDEF
Data Data Data Parity
1/3 Record X
AB
Record X
ABCDEF
Record W
TRSVAB
1/3 Record X
CD
Record Y
#IJKLM
Parity
PPPPPP
1/3 Record X
EF
Parity bits
PP
Data+ Parity Data+ Parity
Record B
PQRSTU
Record V
CDERST
Parity
PPPPPP
Record T
QRUBXA