When RAID arrays were first proposed by the minds at Berkeley in the 1980s, there were five levels of RAID array. Today, researchers and manufacturers have devised other variations of RAID levels, but the original five are considered, by purists, “true” levels of RAID.
Of course, there's also RAID “0,” which many experts don't consider to be a RAID array but it's worth discussing in this context. Read on to find out more about the five levels of RAID, plus one level that really isn't.
RAID 0 - RAID stands for Redundant Array of Independent (or Inexpensive) Drives. Therefore, RAID 0, which contains no redundancy, isn't really a RAID level. RAID 0 stripes data across two or more drives, providing the best storage efficiency and fast performance, but no protection against data loss.
RAID 1 - RAID 1 provides the best protection against disk failure since all data is mirrored on an identical drive. A RAID 1 array needs at least two hard drives, and can use as many drives as you'd like, always added in pairs.
RAID 2 - RAID 2 has fallen out of use since it tends to be more complicated than other RAID levels; it uses a combination of data disks and Error Correcting Code disks to protect against data loss.
RAID 3 - RAID 3 stripes data across a number of disks, using a separate parity disk for data redundancy. RAID 3 provides flexible storage capacity since you can use a large number of drives in the array, as long as you have at least three. A RAID 3 array can survive the loss of any one drive in the array. The parity disk tends to slow down write speeds, but even with this loss of performance, RAID 3 has become a popular choice for many applications.
RAID 4 - RAID 4 employs similar methods to RAID 3, but uses block level striping rather than byte level striping for data distribution. This permits users to change the stripe size for more efficient data storage. Because of its good fault tolerance, RAID 4 is also a common choice.
RAID 5 - RAID 5 is one of the safest, most secure, and fastest RAID levels available, making it one of the most popular solutions for file servers, databases and other data storage needs.
Unlike RAID 3 and RAID 4, level 5 RAID distributes parity across the drives and employs block level striping for better performance. RAID 5 can tolerate the failure of one drive without the need for emergency data recovery services.