Redundant Array Of Inexpensive Disks

Redundant Array Of Inexpensive Disks (RAID) - Technical Paper

Arvind Kumar

May 2008

Version 1.0

1. Introduction

1.1 Background:

The storage capacity and data retrieval speeds of Hard Disks have increased multiple folds in last few years. However for large

business organizations, which not only need to store terabytes of invaluable data but access them frequently as well. These

organizations cannot afford to let their systems go offline even for a short duration of time. Moreover they cannot even think of

loosing even small amount of data due to disk failure or for that matter any other reason.

1.2 Issues:

A single hard disk cannot fulfill all these requirements no matter how large is the storage capacity or how good the performance and

quality of disk is. The need here arises of a system that can store large volumes of data, provides fault tolerance, scalable in terms of

increasing storage capacity and above all that can be reliable.

1.3 Solution:

RAID - Redundant Array of Inexpensive Disks, system solves most of the above stated problems. The idea behind RAID is usage of

multiple independent hard disks, such that they appear to be a single large disk to the user of system, but at the same time provide

faster access and data protection. RAID can be implemented in a number of ways depending on the type of application and

environment where they are intended to be used. However every innovation that eases our problems comes with an added cost and

complexity. But as stated, no one wants to loose their data which is invaluable as compared to the cost spend for implementing RAID.

1.4 Types of RAID:

RAID can be implemented as Hardware or Software. A hardware RAID system requires a dedicated RAID Controller, on the other

hand a software RAID doesn't require any additional hardware and is bundled as a feature in the operating system. Since hardware

RAID employs dedicated controller setup for its implementation, it has performance benefits over the software RAID.

Due to decreasing costs of the hardware, sooner or later the RAID subsystem would be a part of basic PC system configuration.

1.5 RAID - How useful it is?

We saw that RAID offers a lot of advantages over usage of a single disk system. However it is not a magic wand that can blow off all

the issues and problems. If we simply say that some RAID level will always provide fault tolerance with high availability and improved

performance we MIGHT be wrong. It all depends upon the specific usage, which RAID level we intend to use and how do we

implement and manage it. We'll discuss these issues in detail.

2. RAID Level Implementations and concepts

Different RAID levels are implemented differently and have different behaviors. Depending on the implementations the performance

of each RAID level is different. One has to understand the key concept of how the disks are arranged in an array, how fault tolerance

is to be provided - i.e. using mirroring or parity. Read and Write operations can be affected in few cases. Not to forget there's cost

and complexity issues involved as well.

Following are the terms one should understand before using the RAID system:

2.1 Drive Array:

A drive array is a collection of hard drives that are managed in a special way. All the drives are formatted in same file system and

most implementations (RAID levels) want the drive size to be same.