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RAID stands for Redundant Array of Independent Disks (the ‘I’ used to be inexpensive but this has been superseded) and is a system designed to increase the performance and resilience of computer storage.

There are several systems that can be implemented to achieve slightly different results; These systems are knows as ‘Levels’ and differ in the system they implement, the resultant fault tolerance and performance.

RAID Levels

[expand title=”Level 0″]

This is a system the simply uses 2 drives to write data to, and writes to both drives at once in a striped pattern. it is mainly used to increase system performance.

System Performance +
System Resilience =
Cost +
Complexity =
Minimum Drives 2

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[expand title=”Level 1″]

This is also known as ‘Drive Mirroring’ and again uses 2 drives to write data to. This time however, the data is only read from one drive, the other drive is a ‘mirror’ of the first drive and only used to recreate the first drive in the event of a failure.

System Performance –
System Resilience +
Cost +
Complexity =
Minimum Drives 2

[/expand]

[expand title=”Level 2″]

This level introduces ‘Striping’, or writing data across all of the drives at once, and ‘Parity’ technology. Imagine cutting your data in to chunks the same as the number of drives in the system and writing those chunks to each disk, then doing some clever maths (known as ‘Hamming Code’) to enable rebuilding the data if a drive fails.

This system tends not to be used, as later systems give better results.

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[expand title=”Level 3″]

This is the same as using RAID level 2 but with bigger chunks of data.

Again, this system is rarely implemented.

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[expand title=”Level 4″]

This system collates all the parity bits and puts them on to one drive.

As above, this is rarely used.

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[expand title=”Level 5″]

This level is the most regularly used, as it gives good resilience and good performance, for not too much money.

RAID 5 is also known as striping with distributed parity. In this case data and parity code, is striped across three or more drives. If a single drive fails, data can be recovered from the remaining data blocks and the parity information, the array will fail in the event of a double drive failure.

This level also features improved read and write performance because data can be read or written simultaneously across multiple drives.

System Performance +
System Resilience ++
Cost ++
Complexity +
Minimum Drives 3

[/expand]

[expand title=”Level 6″]

This level includes ‘Double data parity’, that writes the correction bits twice.

In this case the array will survive a double drive failure.

System Performance +
System Resilience +++
Cost +++
Complexity ++
Minimum Drives 4

[/expand]

There are several other hybrid and nested levels of RAID – such as RAID 01, 10, 50 etc. all giving slightly different levels of protection and all increasing cost and complexity.

To read more about RAID pop over to Wikipedia here