Chapter 4 Redundant Array Of Independent Disks (Raid); What Is Raid; Who Should Use Raid; Hardware Raid Versus Software Raid - Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Manual

Section 4.3:Hardware RAID versus Software RAID
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 administrator)
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.
71