HP Surestore Disk Array 12h - And FC60 Service Manual page 53

RAID 3 works well for single-task applications using large block I/Os. It is not a good
choice for transaction processing systems because the dedicated parity drive is a
performance bottleneck. Whenever data is written to a data disk, a write must also be
performed to the parity drive. On write operations, the parity disk can be written to four
times as often as any other disk module in the group.
RAID 5
RAID 5 uses parity to achieve data redundancy and disk striping to enhance performance.
Data and parity information is distributed across all the disks in the RAID 5 volume group.
A RAID 5 volume group consists of three or more disks. For highest availability, the disks in
a RAID 5 volume group must be in different enclosures.
If a disk fails or becomes inaccessible, the disk array can dynamically reconstruct all user
data from the data and parity information on the remaining disks. When a failed disk is
replaced, the disk array automatically rebuilds the contents of the failed disk on the new
disk. The rebuilt volume group contains an exact replica of the information it would have
contained had the disk not failed.
Until a failed disk is replaced (or a rebuild on a global hot spare is completed), the volume
group operates in degraded mode. The volume group must now use the data and parity on
the remaining disks to recreate the content of the failed disk, which reduces performance.
In addition, while in degraded mode, the volume group is susceptible to the failure of the
second disk. If a second disk in the volume group fails while in degraded mode, parity can
no longer be used and all data on the volume group becomes inaccessible.
Figure 22 illustrates the distribution of user and parity data in a five-disk RAID 5 volume
group. The the stripe segment size is 8 blocks, and the stripe size is 40 blocks (8 blocks
times 5 disks). The disk block addresses in the stripe proceed sequentially from the first
disk to the second, third, fourth, and fifth, then back to the first, and so on.
Disk Array High Availability Features
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