Redundant Array Of Independent Disks (Raid); What Is Raid; Who Should Use Raid; Hardware Raid Versus Software Raid - Red Hat ENTERPRISE LINUX 4 System Administration Manual

Chapter 9.
Redundant Array of Independent Disks
(RAID)

9.1. 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 appears to the computer as a single logical storage unit or drive.
RAID is a method in which information is 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 and/or to increase bandwidth for reading or writing to disks, and
to 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 the
RAID array according to the RAID level used. When the data is to be read, the process is reversed,
giving the illusion that the multiple drives in the array are actually one large drive.

9.2. Who Should Use RAID?

Those who need to keep large quantities of data on hand (such as system administrators) would
benefit by using RAID technology. Primary reasons to use RAID include:
• Enhanced speed
• Increased storage capacity using a single virtual disk
• Lessened impact of a disk failure

9.3. Hardware RAID versus Software RAID

There are two possible RAID approaches: Hardware RAID and Software RAID.

9.3.1. Hardware RAID

The hardware-based array 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 add them to the RAID
controller's configuration, and the operating system never knows the difference.
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