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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
Red Hat Enterprise Linux 4: System Administration Guide
Prev Chapter 9. Redundant Array of Independent Disks (RAID) Next

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.

9.3.2. 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 [1] 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 hardware independent. The performance of a software-based array is dependent on the server CPU performance and load.

For information on configuring Software RAID during installation, refer to the Chapter 10 Software RAID Configuration.

For those interested in learning more about what Software RAID has to offer, here are the most important features:

  • Threaded rebuild process

  • 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

Notes

[1]

A hot-swap chassis allows you to remove a hard drive without having to power-down your system.
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Who Should Use RAID? Up RAID Levels and Linear Support

 
 
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