RAID Technology Overview - September 2007

1 Introduction to RAID Technology
This chapter provides an overview of RAID technology and descriptions of the different RAID
levels that are supported by HP Smart Array Controllers. This chapter addresses the following
topics:
“What is RAID?” (page 11)
“Performance and Data Redundancy” (page 13)
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 disks 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 offline to replace the failed disk.
The RAID Concept
The RAID study proposed a multilevel concept for improved data input/output performance
(by combining multiple physical disks) and improved data availability (by avoiding the impact
of disk drive failures). Five original RAID configurations, or “levels” (RAID 1 through RAID 5),
were defined to meet the needs of various computing environments.
As the five original RAID configurations progress from RAID 1 through RAID 5, data redundancy
increases.
What is RAID? 11