HP A7143A RAID160 SA Controller Support Guide, February 2007

RAID Technology Overview

What is RAID?

Chapter 1

“Summary of RAID Methods” on page 20

“Choosing a RAID Method” on page 21

NOTE If you are ready to install the Smart Array Controller and you are familiar with RAID concepts

and the RAID levels supported by the Smart Array Controllers, proceed to Chapter 4,

“Installing the RAID160 SA Controller,” on page 33.

What is RAID?

The RAID concept was proposed in 1987 when “A Case for Redundant Arrays of Inexpensive Disks (RAID)”

was published by David Patterson, Garth Gibson, and Randy Katz at the University of California, Berkeley.

The idea was to combine multiple small, inexpensive physical disk drives into an array that would function as

a single logical drive, but provide better performance and higher data availability than a single large

expensive disk drive (SLED).

The study defined five different disk array configurations, or RAID levels. All of the RAID levels provided

fault tolerance, and each RAID level offered different feature sets and performance to accommodate different

system administration priorities and computing environments.

NOTE RAID now stands for “Redundant Array of Independent Disks”, because disks have become

inexpensive.

Small disk drives are lower in performance and have less capacity compared to large disk drives. Small drives

also have lower storage density than large drives. However, small disk drives are equal to or better than large

disk drives in four areas:

• I/O per actuator (multiple I/O capability)

• Cost per megabyte

• Mean time between failures (MTBF)

• SCSI controller per disk drive (better cost/performance ratio)

Grouping small disk drives into an array provides the following additional advantages:

• High transfer rates

• Increased disk capacity

•High I/O rates

The RAID study pointed out that as the number of disk drives in an array (also called a stripe set) increases,

the MTBF of the array decreases. At the time the RAID study was published, if a disk drive crashed data

restoration was typically dependent on backup from a tape drive. In addition, the system was taken off-line to

replace the failed disk.