Raid 3; Raid 5 - HP 2000fc Reference Manual

A mirrored virtual disk is also highly reliable, because both disk drives in a pair
must fail for the virtual disk to fail. In an virtual disk with five pairs of mirrored
disk drives, for example, the virtual disk can maintain its integrity even if five disks
fail—as long as each pair is left with one good disk. The main disadvantage of a
mirrored virtual disk is its cost. Because all disk drives must have a twin, you must
use twice the number of disk drives that actually contribute to the virtual disk
capacity. In an eight-disk virtual disk, for example, you have only four disks of
usable capacity.

RAID 3

RAID 3 virtual disks contain redundant information in the form of parity data,
which is calculated block-by-block for all user data. The user data is distributed
across all but one of the disk drives in the virtual disk. The parity data is written
exclusively to the parity disk (also known as the check disk). In the event of a disk
drive failure, the data can be reconstructed from corresponding data stripes on the
remaining disk drives in the virtual disk.
RAID 3 provides excellent I/O performance for applications that require high data
transfer rates such as image processing, video processing, scientific data collection,
batch data processing, or sequential reads and writes.
RAID 3 is not well suited for transaction processing or other applications that
require simultaneous reads and writes.

RAID 5

RAID 5 virtual disks contain redundant information in the form of parity data,
which is calculated block-by-block for all user data. The parity information is
distributed across the disk drives in the virtual disk and occupies the equivalent
capacity of approximately one disk drive. Data is interspersed with the parity
information. If one disk drive in the virtual disk fails, the data on the failed disk
drive can be reconstructed from the parity data and user data on the remaining disk
drives. Two disk drives must fail before the entire virtual disk fails.
The read performance of a RAID 5 virtual disk is excellent—comparable to that of
a RAID 0 virtual disk. Write performance is lower than that of a RAID 0 virtual
disk, because write operations involve calculating and writing new parity data as
well as writing the new user data.
250 HP StorageWorks 2000 Family Modular Smart Array reference guide • August 2008