RAID Basics

RAID Basics

aa RAID Concept

RAID (Redundant Array of Independent Disks) is an acronym first used in a 1988 paper by Berkeley researchers Patterson, Gibson and Katz. It described array configuration and applications for multiple inexpensive hard disks, providing fault tolerance (redundancy) and improved access rates.

RAID provides a method of accessing multiple individual disks as if the array were one large disk, spreading data access out over these multiple disks, thereby reducing the risk of losing all data if one drive fails, and improving access time.

aa Why use RAID

Typically the RAID is used in large file servers, transaction of application servers, where data accessibility is critical, and fault tolerance is required. Nowadays, RAID is also being used in desktop systems for CAD, multimedia editing and playback where higher transfer rates are needed.

aa RAID Levels

RAID 0: Also known as "Disk Striping", this is technically not a RAID level since it provides no fault tolerance. Data is written in blocks across multiple drives, so one drive can be writing or reading a block while the next is seeking the next block.

The advantages of striping are the higher access rate, and full utilization of the array capacity. The disadvantage is there is no fault tolerance - if one drive fails, the entire contents of the array become inaccessible.

RAID 1: Known as "Disk Mirroring" provides redundancy by fully duplicating drive data to all other drives in the array. If one drive fails, the others contain exact duplicate of the data and the RAID can switch to using the mirror drive with no lapse in user accessibility. The disadvantages of mirroring are no improvement in data access speed, and capacity is low. However, it provides the best protection of data since the array management software will simply direct all application requests to the surviving disk members when a member of disk fails.

RAID 3: RAID level 3 stripes data across multiple drives, with an additional drive dedicated to parity, for error correction & recovery.

RAID 5: RAID level 5 is the most popular configuration, providing striping as well as parity for error recovery. In RAID 5, the parity block is distributed among the drives of array, giving a more balanced access load across the drives. The parity information is used to recover data if one drive fails, and this method is the most popular. The disadvantage is a relatively slow write cycle (2 reads and 2 writes are required for each block written). The array capacity is N-1, with a minimum of 3 drives required.

RAID 0+1: This is stripping and mirroring combined, without parity. The advantages are fast data access (like RAID 0), and single ¡V drive fault tolerance (like RAID 1). RAID 0+1 still requires twice the number of disks.

RAID Level	Common Name	Description	Array's Capacity	Data Reliability	Data Transfer Capacity	Minimum Drive Required
0	Disk striping	Data distributed across the disks in the array. No redundant information provided.	(N) disks	Low	Very High	2
1	Disk mirroring	All data duplicated	1* disks	Very High	High	2
3	Parallel transfer disks with parity	Data sector is subdivided and distributed across all data disk. Redundant information stored on a dedicated parity disk.	(N-1) disks	Very High	Highest of all listed alter-natives	3
5	Independent access Array with rotating parity	Data sectors are distributed as with disk stripping, redundant information is interspersed with user data.	(N-1) disks	Very High	Very High	3
0+1	Disk-Striping + Disk-Mirroring	Combined ¡§striping and mirroring function without parity. Fast data access and single drive fault tolerance.	(N/2) disks	Very High	High	4