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What is RAID?
Last updated: March 28, 2011
RAID stands for Redundant Array of Inexpensive (or Independant) Disks.
It is a technology that allows multiple physical hard disks to be seen by the operating system as a single disk and depending of the type of RAID that is configured will determine how the array performs. Different types of RAID are suited for different working conditions or expectations.
The main advantages of a RAID are increased performance and increased data integrity. The increase in performance comes from the data being spread across multiple devices and each physical device does not need to do as much work to read or write the data. The increase in data integrity comes from the redundancy built into RAID technology. This means that if one (or more depending on RAID type) of your drives fails you can replace it with a new one on a live system and the array will recognize the new drive and start to use it as a data store filling it with data from the rest of the array. Due to this it is obvious that if you have 2 10GB hard disks in an array you will not get an array with 20GB capacity so you sacrifice some space for the advantages that come with using a RAID.
There are two styles of RAID. Hardware RAID and software RAID. A hardware RAID requires a RAID controller which is usually a PCI card in a server. This card handles all the computing required to manage the array and the data. It is the preferred method as with this your computer's processors are free to work on other things. A software RAID is software that does what a hardware RAID does but uses the computer's processors to do the work.
Common types of RAID are:
RAID 0 - This does not support data integrity and is not recommended in a server envioronment.
RAID 1 - Requires at least two disks and functions as a mirror with identical data written to both disks. Allows for single drive failure.
RAID 5 - Requires at least three drives. Uses block level striping and distributed parity. Allows for single drive failure.
RAID 6 - Requires at least 4 drives. Uses block level striping and double distributed parity. Allows for two drive failure.