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Defining RAID Levels

RAID systems are defined by the manner in which they store data, the level and type of data protection they supply, and the tolerance the system can provide in the event of hardware component failure. Any level of RAID may be used to create a single logical volume. Many controllers support the creation of more than one logical volume - on a separate bus or even the same one - and will allow each logical volume to built from a different RAID level. Each logical volume will be accessible simultaneously. The objectives of an array may be to offer either high performance or a protected storage system with many RAID implementations offering both.

There are basically a series of RAID Levels - from Level 0 (zero) to Level 5 with each one offering different advantages and disadvantages. No single RAID level is ideal for every situation, nor should the common error that presumes the higher the number, the higher the RAID Level, be used in determining the level to implement. Each different level needs to be understood by anyone wishing to use a RAID or Array system, or the incorrect or less suitable level may be selected. There are also other certain conditions to meet before you create an array, and any third-party tools you may wish to use should be approached with caution. For more information on these topics please read the following sections:

Comparisons
It is important to be able to draw up a proper comparison between RAID Level implementations in order to find the most suitable level for any particular environment. A RAID implementation that provides very high data security but reduces the overall performance of data throughput reduces access by users and could slow down an organisation or network.

With the exception of Level 0 (zero), all RAID levels and arrays provide data security plus potential improvements in data throughput. This potential improvement in performance however, may be either during the write cycle, during the read cycle, or even both. It is therefore vital that your RAID implementation offers the best data integrity/performance ratio for your environment.

There is a separate Level 6 that provides the same data parity pattern as Level 5, but can provide tolerance against more than one simultaneous drive failure. This level is therefore only a better implementation of RAID Level 5 incorporating more intelligent or flexible controllers rather than a new RAID level in itself.

The three main RAID Level Categories

The summaries of each level, together with each RAID Level that is encapsulated by the category, should be the first read before you move on to selecting the actual RAID level itself. Many engineers themselves who install RAID systems do not appreciate the subtle differences between the RAID Levels and do not necessarily take data types, working environments, and user expectations into account and always seem to automatically select one of three levels: Level 0 Striped, Level 1 Mirrored, or Level 5 Striped with Parity. There are very good reasons for the existence of the other levels so make sure you understand their operating advantages and disadvantages before dismissing them.

It is also important that you know the RAID Level or Levels you will use before you purchase any RAID Controller as not all will offer the option of every level. Most controllers offer RAID Level 0 (some offer variations on it such as Level 1/0), Level 1 to create a simple mirrored pair, and Level 5 as part of their basic support.

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