RAID is acronym of Redundant Array of Inexpensive Disks. It is used to speed up data access and/or make it more reliable. In a raid configuration, several hard disk drives are connected together to get one.
There are various types of RAID - here are some of the more useful / common types:
RAID 0 (Stripe)
Used for speed and size: two or more hard disk drives are connected in such a way that all information is divided into logical blocks and saved on different hard disk drives simultaneously. This provides the ability to save and load the data much faster. In some cases the resulting speed can be the sum of all the hard disk drives speeds. The total size of this RAID is the size of the smallest hard disk drive multiplied by the number of the hard disk drives. A benefit of this RAID type is larger size and faster speed. A deficiency with this RAID type is that if one hard disk drive becomes damaged then you loose information on all the hard disk drives connected to this RAID.
RAID 1 (Mirror)
Used for reliability: two or more hard disk drives are connected in such a way that the same information is saved on all hard disk drives in this RAID. The speed of this RAID is the speed of the slowest hard disk drive in this RAID. The size of this RAID is the size of the smallest hard disk drive in this RAID.
RAID 0+1 (Mirrored Stripe)
Used for speed, size and reliability: two or more hard disk drives are connected as one RAID 0, and every hard disk drive is mirrored as RAID 1. A deficiency of this RAID type is that twice as many hard disk drives are required to get the same speed and size as in RAID 0, but it gives more reliability.
RAID 5 (Stripe with Redundancy)
used for speed, size and reliability: three or more hard disk drives are connected as one RAID, and one of the hard disk drives is used to keep redundancy information. Redundancy information provides the ability to restore the RAID if one of the hard disk drives is damaged. A benefit of this RAID type is that only one more hard disk drive is required for redundancy.
RAID Implementation
There are two ways to implement a RAID:
Hardware RAID: this RAID is created by a RAID controller, and appears as one hard drive to any operating system. This type requires a RAID controller but provides the ability to use it under any operating system.
Software RAID: this RAID is created by the operating system's hard disk driver, and is visible as RAID only to this operating system. Windows NT4 Server, Windows 2000, Windows XP Professional and windows 2003 (.NET) Servers have such a driver.
With hardware RAID, you can install any operating system on it, and use it for your data storage, but with software RAID, you can only use it for your data storage (you can not have the operating system on the software raid). On the other hand, with hardware RAID, the entire hard disk drives space is used, but with software RAID, you can use allocate part of a hard disk drive for the RAID. Therefore if you are using hard disk drives of different sizes, you may want to select software RAID in order to reuse unused hard disk space.
