Who Should Use Raid; Hardware Raid Versus Software Raid - Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual

Chapter 4.
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.
4.1. What is RAID?
RAID allows information to access 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 consistently distributes data across each drive in the array. RAID then breaks down the data into
consistently-sized chunks (commonly 32K or 64k, 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.

4.2. 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

4.3. Hardware RAID versus Software RAID

There are two possible RAID approaches: Hardware RAID and Software RAID.
4.3.1. 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 connects to the SCSI controller and presents the RAID arrays as a single
SCSI drive. An external RAID system moves all RAID handling "intelligence" into a controller located
in the external disk subsystem. The whole subsystem is connected to the host via a normal SCSI
controller and appears to the host as a single disk.
RAID controller cards function like a SCSI controller to the operating system, and handle all the actual
drive communications. The user plugs the drives into the RAID controller (just like a normal SCSI
controller) and then adds them to the RAID controllers configuration, and the operating system won't
know the difference.
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