Installation guide

64 Chapter 4. Redundant Array of Independent Disks (RAID)

add them to the RAID controller’s conﬁguration, and the operating system never knows the

difference.

4.3.2. Software RAID

Software RAID implements the various RAID levels in the kernel disk (block device) code. It

offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis

are not required. Software RAID also works with cheaper IDE disks as well as SCSI disks.

With today’s fast CPUs, Software RAID performance can excel against Hardware RAID.

The MD driver in the Linux kernel is an example of a RAID solution that is completely hard-

ware independent. The performance of a software-based array is dependent on the server

CPU performance and load.

For information on conﬁguring Software RAID in the Red Hat Linux installation program,

refer to the Chapter 5.

For those interested in learning more about what Software RAID has to offer, here is a brief

list of the most important features:

• Threaded rebuild process

• Fully kernel-based conﬁguration

• Portability of arrays between Linux machines without reconstruction

• Backgrounded array reconstruction using idle system resources

• Hot-swappable drive support

• Automatic CPU detection to take advantage of certain CPU optimizations

4.4. RAID Levels and Linear Support

RAID supports various conﬁgurations, including levels 0, 1, 4, 5, and linear. These RAID

types are deﬁned as follows:

• Level 0 — RAID level 0, often called "striping," is a performance-oriented striped data

mapping technique. This means the data being written to the array is broken down into

strips and written across the member disks of the array, allowing high I/O performance

at low inherent cost but provides no redundancy. The storage capacity of a level 0 array is

equal to the total capacity of the member disks in a Hardware RAID or the total capacity

of member partitions in a Software RAID.

• Level 1 — RAID level 1, or "mirroring," has been used longer than any other form of RAID.

Level 1 provides redundancy by writing identical data to each member disk of the array,

leaving a "mirrored" copy on each disk. Mirroring remains popular due to its simplic-

ity and high level of data availability. Level 1 operates with two or more disks that may

use parallel access for high data-transfer rates when reading but more commonly oper-

ate independently to provide high I/O transaction rates. Level 1 provides very good data

reliability and improves performance for read-intensive applications but at a relatively

1. A hot-swap chassis allows you to remove a hard drive without having to power-down your

system.