The RAID Levels: An Overview
Patterson, Gibson, and Katz realized that different types of systems would inevitably have different performance and redundancy requirements. The Berkeley Papers provided specifications for five levels of RAID, offering various compromises between performance and data redundancy. After the publication of the Berkeley Papers, however, the computer industry quickly realized that some of the original levels failed to provide a good balance between cost and performance, and therefore weren’t really worth using.
RAID-2 and RAID-3, for example, quickly became useless. RAID-2 implemented a read/write level error correction code (ECC) that later became a standard firmware feature on hard drives. This development left RAID-2 without any advantage in redundancy over other RAID levels. The ECC implementation now required unnecessary overhead that hurt performance. RAID-3 required that all disks operate in lockstep (all disk spindles are synchronized). This added additional design considerations and did not provide any significant advantage over other RAID levels.
RAID has changed a great deal since the Berkeley Papers were written. While some of the original levels are no longer used, the storage industry quickly made additions to the original specification. This book will cover all of the RAID levels available to Linux users, but will not cover obsolete implementations like RAID-2 and RAID-3. Below you will find a concise overview of each RAID level. Chapter 2 covers each of the RAID levels in more detail, including hybrid arrays that are built by combining multiple RAID levels.
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