Common Uses of RAID Systems
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 larger 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.
Why use RAID?
Typically 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.
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.
Known as "Disk Mirroring" provides redundancy by writing twice - once to each drive. If one drive fails, the other contains an 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 higher cost, since twice the number of drives is required. 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 level 3 stripes data across multiple drives, with an additional drive dedicated to parity, for error correction/recovery.
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 recovery data if one drive fails, and is the reason 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.
This is stripping and mirroring combined, without parity. The advantages are fast data access (like RAID 0), and single ˇV drive fault tolerance (like RAID 1). RAID 0+1 still requires twice the number of disks (like RAID 1).
Data Transfer Capacity
Minimum Drive Required
Data distributed across the disks in the array. No redundant information provided.
All data duplicated
Parallel transfer disks with parity
Data sector is subdivided and distributed across all data disk.
Redundant information stored on a dedicated parity disk.
Highest of all listed alter-natives
without rotating parity
Data sectors are distributed as with disk stripping, redundant information is interspersed with user data.
Combined ˇ§striping and mirroring function without parity. Fast data access and single drive fault tolerance.