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Chapter 1: Features of the CHALLENGE RAID Storage System
RAID-5: Individual Access Array
This configuration usually consists of five disk modules (but can have three
to sixteen) bound as a RAID-5 group. Because there are five internal SCSI-2
buses in the CHALLENGE RAID system, an array of five disk modules (or
fewer) provides the greatest level of data redundancy.
A RAID-5 group maintains parity data that lets the disk group survive a disk
module failure without losing data. In addition, in CHALLENGE RAID
storage systems, the group can survive a single SCSI-2 internal bus failure if
each disk module in the group was bound on an independent SCSI-2
internal bus. For highest data availability for a RAID-5 group, the disk
modules making up the group should be on different SCSI internal buses (A,
B, C, and so on).
With RAID-5 technology, the hardware writes parity information to each
module in the array. If a module fails, the SP can reconstruct all user data
from the user data and parity information on the other disk modules. After
you replace a failed disk module, the SP automatically rebuilds the disk
array using the information stored on the remaining modules. The rebuilt
disk array contains a replica of the information it would have contained had
the disk module never failed.
A RAID-5 group uses disk striping; see “Enhanced Performance: Disk
Striping,” earlier in this chapter for an explanation of this feature.
Figure 1-12 illustrates user and parity data with the default stripe element
size of 128 sectors (65,536 bytes) in a five-module RAID-5 group. The stripe
size comprises all stripe elements. Notice that the disk block addresses in the
stripe proceed sequentially from the first module to the second, third, and
fourth, fifth, then back to the first, and so on.
