What is RAID?

RAID Explained
RAID stands for Redundant Array of Inexpensive/Independent Disks. It's a technology which combines multiple, less-expensive drives into a single, higher-capacity and faster volume. RAID was created in 1988 to deal with the increasing costs of high-performing disk drives. The inventors argued that an array of inexpensive disks could out perform a single expensive desk.

A RAID system consists of two or more drives working in parallel. It stores the same data in different places on multiple hard disks or solid-state drives to protect data in the case of a drive failure. RAID can also increase the performance and reliability of data storage.

Hardware and Software RAID
RAID storage can be either hardware or software based. It can be implemented either using a special controller (hardware RAID), or by an operating system driver (software RAID).

Hardware RAID is a dedicated processing system which uses controllers or RAID cards to manage the RAID configuration independently from the operating system. It can function on any operating system.

As hardware RAID requires additional controller hardware, it costs higher than software RAID. If your RAID controller fails, you would have to find a compatible one to replace in order to get the RAID system to perform the way you set it up.

Unlike hardware RAID, software RAID uses the processing power of the operating system where the RAID disks are installed. It costs lower because no additional hardware RAID controller is required. It also permits users to reconfigure arrays without being restricted by the hardware RAID controller.

Software RAID tends to be slower than hardware RAID. This is because some processing power is used by the software, resulting in slower read and write speeds of your RAID configuration.

Key RAID Concepts
Before exploring what different RAID levels are, it'll be helpful to know the following concepts:

Striping: data is split between multiple disks.
Mirroring: data is mirrored between multiple disks.
Parity: a calculated value used to mathematically rebuild data.

RAID Levels
RAID has different levels by which the drives are grouped together. These levels are usually referred to by number. Each RAID level provides varying amounts of reliability, availability, performance, and capacity. Originally there were five standard levels of RAID, but many variations have evolved. Below we will take a look at levels 0-6.

	RAID 0 consists of striped disks. RAID 0 gives good read and write performance but doesn't provide data redundancy. All data will be lost if one disk fails. RAID 0 should only be used if you can accept data being inaccessible during a drive failure while you restore it from other media.

	RAID 1 uses mirroring. Having multiple drives with identical data gives high availability in terms of read performance and boosts data protection. As long as one drive of a mirrored pair doesn't fail, no data is lost.

	RAID 2 consists of bit-level striping with dedicated Hamming-code parity. This level is of historical significance only. While it was used on some early machines, as of 2014 it is no longer used by any commercially available system.

	RAID 3 uses striped set with dedicated parity. This means that data is split evenly between two or more disks, plus a dedicated drive for parity storage. This level enjoys high speeds for sequential read and write operations. The downside is the poor performance for multiple simultaneous instructions.

	RAID 4 stripes the data across multiple disks just like RAID 0. In addition to that, it also stores parity information of all the disks in a separate dedicated disk to achieve redundancy. If one of the disks fails, the data can be reconstructed using the parity information of that disk.

	RAID 5 is very similar to RAID 4, but here the parity information is distributed over all the disks instead of storing them in a dedicated disk. A benefit is that there's no possibility of losing data redundancy as one disk doesn't store all the parity information.

	RAID 6 is striping with dual distributed parity. Distributed parity gives fault tolerance against two drive failures. Dual parity means that while a failed disk is being rebuilt the array is still protected by the remaining parity data.