RAID Primer: What's in a number?

Introduction

The majority of home users have experienced the agony of at least one hard drive failure in their lives. Power users often experience bottlenecks caused by their hard drives when they try and accomplish I/O-intensive tasks. Every IT person who has been in industry for any length of time has dealt with multiple hard drive failures. In short, hard drives have long caused the majority of support headaches in standard desktop or server configurations today, with little hope of improvement in the near term.

With the increased use of computers in the daily lives of people worldwide, the dollar value of data stored on the average computer has steadily increased. Even as MTBF figures have moved from 8000 hours in the 1980s (example: MiniScribe M2006) to the current levels of over 750,000 hours (Seagate 7200.11 series drives), this increase in data value has offset the relative decrease of hard drive failures. The increase in the value of data, and the general unwillingness of most casual users to back up their hard drive contents on a regular basis, has put increasing focus on technologies which can help users to survive a hard drive failure. RAID (Redundant Array of Inexpensive Disks) is one of these technologies.

Drawing on whitepapers produced in the late 1970s, the term RAID was coined in 1987 by researchers at the University of California, Berkley in an effort to put in practice theoretical gains in performance and redundancy which could be made by teaming multiple hard drives in a single configuration. While their paper proposed certain levels of RAID, the practical needs of the IT industry have brought several slightly differing approaches. Most common now are:

RAID 0 - Data Striping
RAID 1 - Data Mirroring
RAID 5 - Data Striping with Parity
RAID 6 - Data Striping with Redundant Parity
RAID 0+1 - Data Striping with a Mirrored Copy

Each of these RAID configurations has its own benefits and drawbacks, and is targeted for specific applications. In this article we'll go over each and discuss in which situations RAID can potentially help - or harm - you as a user.