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RAID Defined

RAID stands for Redundant Array of Independent Disks. RAID is a method of combining several hard drives into one unit. It offers fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives. RAID levels 0,1, 10 and 5 are the most popular.

The acronym RAID, originally coined at Berklee University in 1987, stood for Redundant Array of Inexpensive Disks.

RAID Configurations

RAID 0 (stripe)
Raid level 0 splits data across drives, resulting in higher data throughput. The performance of this RAID is great, but a loss of any drive in the array will result in data loss. This level is commonly referred to as striping. Minimum number of drives required: 2
Advantages
High performance Easy to implement No parity overhead
Disadvantages
No fault tolerance
RAID 1 (mirror)
RAID Level 1 writes all data to two or more drives. The performance of a level 1 array tends to be faster on reads and slower on writes compared to a single drive, but if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required; however, since one drive is used to store a duplicate of the data, the cost per megabyte is high. This level is commonly referred to as mirroring. Minimum number of drives required: 2
Advantages
Fault tolerant Easy to recover data in case of drive failure Easy to implement
Disadvantages
100% parity overhead Becomes very costly as number of disks increase Inefficient
RAID 5
RAID Level 5 stripes data at a block level across several drives, with parity equality distributed among the drives. The parity information allows recovery from the failure of any single drive. Write performance is rather quick, but because parity data must be skipped on each drive during reads, the performance for reads tends to suffer. The low ratio of parity to data results in low redundancy overhead. Minimum number of drives required: 3
Advantages
High efficiency Fault tolerant The best choice in multi-user environments which are not write performance sensitive.
Disadvantages
Disk failure has a medium impact on throughput Most complex controller design
RAID 0+1 (Stripe+Mirror)
RAID Level 0+1 is a mirror (RAID 1) array whose segments are striped (RAID 0) arrays. It is a great alternative for users that like the security of RAID 1 but need some additional performance boost. Minimum number of drives required: 4
Advantages
Fault tolerant Very High I/O rates
Disadvantages
Very expensive High overhead Very limited scalability
RAID 10 (Mirror+Stripe)
RAID Level 10 is a striped (RAID 0) array whose segments are mirrored (RAID 1). It is similar in performance to RAID 0+1, but with better fault tolerance and rebuild performance. Minimum number of drives required: 4
Advantages
High fault tolerance High I/O rates Faster rebuild performance than RAID 0+1 Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures
Disadvantages
Very expensive High overhead Very limited scalability
RAID 50 (Raid 5 + Stripe)
RAID Level 50 is a striped (RAID 0) array which is striped across a RAID 5 array. Performance is improved compared to RAID 5 because of the addition of the striped array. Fault tolerance is also improved. Minimum number of drives required: 6
Advantages
Higher fault tolerance than RAID 5 Higher efficiency than RAID 10 Higher I/O rates
Disadvantages
Very complex and expensive to implement