Installation guide

Chapter 13.
85
Redundant Array of Independent Disks
(RAID)
The basic idea behind RAID is to combine multiple small, inexpensive disk drives into an array to
accomplish performance or redundancy goals not attainable with one large and expensive drive. This
array of drives appears to the computer as a single logical storage unit or drive.
13.1. What is RAID?
RAID allows information to be spread across several disks. RAID uses techniques such as disk
striping (RAID Level 0), disk mirroring (RAID Level 1), and disk striping with parity (RAID Level 5) to
achieve redundancy, lower latency, increased bandwidth, and maximized ability to recover from hard
disk crashes.
RAID distributes data across each drive in the array by breaking it down into consistently-sized chunks
(commonly 256K or 512k, although other values are acceptable). Each chunk is then written to a hard
drive in the RAID array according to the RAID level employed. When the data is read, the process is
reversed, giving the illusion that the multiple drives in the array are actually one large drive.
13.2. Who Should Use RAID?
System Administrators and others who manage large amounts of data would benefit from using RAID
technology. Primary reasons to deploy RAID include:
• Enhances speed
• Increases storage capacity using a single virtual disk
• Minimizes disk failure
13.3. RAID Types
There are three possible RAID approaches: Firmware RAID, Hardware RAID and Software RAID.
Firmware RAID
Firmware RAID (also known as ATARAID) is a type of software RAID where the RAID sets can be
configured using a firmware-based menu. The firmware used by this type of RAID also hooks into
the BIOS, allowing you to boot from its RAID sets. Different vendors use different on-disk metadata
formats to mark the RAID set members. The Intel Matrix RAID is a good example of a firmware RAID
system.
Hardware RAID
The hardware-based array manages the RAID subsystem independently from the host. It presents a
single disk per RAID array to the host.
A Hardware RAID device may be internal or external to the system, with internal devices commonly
consisting of a specialized controller card that handles the RAID tasks tranparently to the operating
system and with external devices commonly connecting to the system via SCSI, fiber channel, iSCSI,
InfiniBand, or other high speed network interconnect and presenting logical volumes to the system.