Installation guide

Chapter 4.
Redundant Array of Independent Disks (RAID)
4.1. What is 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 will appear to the computer as a single logical storage
unit or drive.
RAID is a method in which information is spread across several disks, using 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 and/or increase bandwidth for reading
or writing to disks, and maximize the ability to recover from hard disk crashes.
The underlying concept of RAID is that data may be distributed across each drive in the
array in a consistent manner. To do this, the data must first be broken into consistently-sized
"chunks" (often 32K or 64K in size, although different sizes can be used). Each chunk is then
written to a hard drive in RAID according to the RAID level used. When the data is to be
read, the process is reversed, giving the illusion that multiple drives are actually one large
drive.
4.2. Who Should Use RAID?
Anyone who needs to keep large quantities of data on hand (such as an average system ad-
ministrator) would benefit by using RAID technology. Primary reasons to use RAID include:
• Enhanced speed
• Increased storage capacity using a single virtual disk
• Lessening the impact of a 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 system manages the RAID subsystem independently from the host and
presents to the host only a single disk per RAID array.
An example of a Hardware RAID device would be one that connects to a 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 controllers also come in the form of cards that act like a SCSI controller to the operating
system but handle all of the actual drive communications themselves. In these cases, you
plug the drives into the RAID controller just like you would a SCSI controller, but then you