What is RAID?
The basic idea of RAID is to combine multiple small, inexpensive disk drives into an array of disk drives which yields performance exceeding that of a Single Large Expensive Drive (SLED). Additionally, this array of drives appears to the computer as a single logical storage unit or drive.
The Mean Time Between Failure (MTBF) of the array will be equal to the MTBF of an individual drive, divided by the number of drives in the array. Because of this, the MTBF of an array of drives would be too low for many application requirements. However, disk arrays can be made fault-tolerant by redundantly storing information in various ways.
RAID Levels:
RAID 0 is the fastest and most efficient array type but offers no fault-tolerance.
RAID 1 is the array of choice for performance-critical, fault-tolerant environments. In addition, RAID-1 is the only choice for fault-tolerance if no more than two drives are desired.
RAID 2 is seldom used today since ECC is embedded in almost all modern disk drives.
RAID 3 can be used in data intensive or single-user environments which access long sequential records to speed up data transfer. However, RAID-3 does not allow multiple I/O operations to be overlapped and requires synchronized-spindle drives in order to avoid performance degradation with short records.
RAID 4 offers no advantages over RAID-5 and does not support multiple simultaneous write operations.
RAID 5 is the best choice in multi-user environments which are not write performance sensitive. However, at least three, and more typically five drives are required for RAID-5 arrays.
RAID 50 is more fault tolerant the Raid level 5 but has twice the parity overhead. High read and write rates are achieved thanks to its RAID 5 segments.
RAID 6 is similar to RAID level 5 however it allows extra fault tolerance by using a second independent parity scheme.
RAID 10 is implemented as a striped array whose segments are RAID 1 arrays.
RAID 0+1 is a mirrored array whose drives are in a RAID 5 array.
All the above RAID information was compiled by Broadberry