6 RAID

RAID is a technique that combines multiple hard disk drives for the purpose of reduncy, reliability and availability. There are multiple levels of RAID, the most common are the following ones:

• RAID0: combines the storage of two or more drives to one logical block device. This is accomplished by striping. Striping is a technique that distributes data at a block level evenly across multiple devices. RAID0 does not offer any redundancy.
• RAID1: mirrors the storage of one drive to one or more other drives. RAID1 does offer redundancy.
• RAID5: combines the storage of three or more drives by means of striping, but also incorporate and distribute parity across the component drives. RAID5 combines the benefits of RAID0 and RAID1.

RAID can be implemented by hardware or software. When RAID is implemented by hardware, a specialized hard drive controller interfaces to a number of hard disk drives. Such a configuration usually needs to format the drives in a special way so that the RAID configuration is stored on the hard disk drives. To set up a RAID system this way, the BIOS of the controller includes an interactive program to specify the configuration and perform low level operations (like drive duplication, repair, and removal).

RAID can also be implemented by software. In this case, a special driver (kernel module) does the job of a specialized RAID controller. Software RAID works on partitions, whereas hardware RAID works on drives. As a result, a drive that participates in software RAID needs to be properly partitioned first. In Linux, software RAID requires that a partition use a type of hexadecimal fd.