RAID overview

Redundant Array of Independent Drives , or also known as Redundant Array of Inexpensive Drives (or Disks), and in either respect it is referred to as RAID. RAID is a term used for computer data storage schemes that spread and or replicate data among multiple hard disk drives. Raids were designed with two key goals: to increased data reliability and increased I/O (input/output) performance.

A number of standard configurations were designed which are referred to as levels. There were five RAID levels originally created, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary).

A raid combines physical hard disks into a single logical unit by using either special hardware or software. Hardware Raid solutions can come in a variety styles, from built onto the motherboard or add in cards, up to large enterprise NAS or SAN servers. With these setups the operating system is unaware of the technical workings or the Raid. Software solutions are typically implemented in the operating system.

There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Different RAID levels use one or more of these techniques, depending on the system requirements.

RAID is traditionally used on servers, but can be also used on workstations. The latter is especially true in storage-intensive computers such as those used for video and audio editing.

Commonly used RAID terms

• RAID - is a technology that supports the use of 2 or more hard drives in various configurations for the purposes of achieving greater performance, reliability and larger volume sizes through the use of consolidating disk resources and parity calculations.

• Parity – A mathematical calculation which allows drives within a Raid array to fail without the loss of data. The simplest way to show this is the equation:

A + B = C

You can remove anyone of the letters from above and work out its value from the 2 remaining. I.e. if B was removed so the equation looked like A + ? = C, then B’s value can be worked out by moving the A, so B = C – A.

This is obviously a simplistic way of describing it, to fully understand it in a Raid sense, knowledge of binary and the logical XOR expression is required.

Learn more:

http://en.wikipedia.org/wiki/Parity_bit
http://en.wikipedia.org/wiki/Exclusive_or

• Mirroring – The data from 1 or more hard drives is duplicated onto another physical disk(s).

• Striping – The method that data and parity can be written across multiple disks. In the example below the data is written across the drives in an sequential order until the last drive, it then jumps back to the first and starts a 2 nd stripe.

• Block – A block is the logical space on each disk where the data is written, the amount of space is set by the Raid controller and most commonly would be 16KB to 256KB in size. The data will fill up the space until the limit is reached and then move onto the next drive, until the last drive when it will jump to the start of the next stripe.

• Left / Right Symmetry – The symmetry in a Raid controls how the data and parity are distributed across the drives. There are 4 main styles of symmetry, which one is used depends on the raid vender. Some companies also make proprietary styles depending on their business needs.

See Raid 5 for more information on Symmetry.

• Hot Spare – There are a few different methods for dealing with drive failures within Raids, one is the use of a Hot Spare. It is a spare disk which can be used in place of the failed one.

• Degraded Mode – This happens when a drive in the Raid becomes unreadable, the drive is then considered bad and is withdrawn from the Raid. The new data and parity are then written to the remaining drives within the raid, if any data is requested from the failed drive it is worked out with the parity on the others. This degrades the performance of the Raid, hence degraded mode.