RAID 0, also known as Striping, is not redundant at all and is not RAID in a pedantic sense. What RAID 0 does is to increase performance significantly by breaking up data blocks into smaller equally sized chunks which are then distributed across two or more physical disks. The performance enhancement is brought about by the fact that data is being written to or read from all disks in the array or RAID set nearly simultaneously as opposed to sequentially from a single disk. In general, the higher the number of disks in the RAID 0 array, the better the performance. Another advantage of RAID 0 is that the total capacity of all disks in the RAID set is available to use for data storage.The performance and capacity advantages do come at a price however. In fact, following on from our initial example in the introduction above, a single disk failure in a RAID 0 array will render all data in the array irretrievable - with the only recourse being restoration from backups. And as there are now more disks being used simultaneously, the chances of a failure occurring increase.

RAID 0 is excellent for applications that require ultimate I/O performance with a caveat of the ability to commit less dynamic data to longer term storage at certain intervals. Applications such as image editing, pre-press and digital rendering can benefit greatly form RAID 0 I/O performance and capacity characteristics.

RAID 1 or Mirroring is the opposite of RAID 0. Rather than chunking data bits and spreading them across two or more physical disks, mirroring writes identical data bits to two or more physical disks so that in the event of a disk failure, at least one disk in the RAID set still has a complete copy of the data. RAID 1 confers true redundancy and is generally achieved and implemented as a mirror of two disks. Mirrors can however be created with disks numbering multiples of two. The main disadvantages of RAID 1 are that cost and capacity. Cost doubles and capacity is halved in comparison to a single disk non-RAID configuration. Performance on writes can also slightly degrade as the data needs to be written at least twice for the write operation to be considered complete.

RAID 1 is excellent for applications where data integrity is absolutely critical and the inconvenience of restoring from backups is to be avoided at all cost. Accounting and financial applications are two typical application scenarios where RAID 1 would be ideal.

This RAID level, as the name suggests, combines the attributes of RAID 0 and RAID 1 to gain benefits of both levels; performance and redundancy. RAID 0+1 requires a minimum of four disks to implement and is a mirrored stripe set. That is to say, a RAID 1 array is layered over two RAID 0 arrays. While getting the performance benefits of RAID 0, RAID 0+1 increases reliability as well by keeping a mirror of the data striped data. Naturally, as multiple copies of the data is kept, the cost of the solution is double that of a RAID 0 array. A major disadvantage of this RAID level is that a single drive failure will cause the array to become a RAID 0 array.

RAID 3 uses byte level striping with parity information stored on a dedicated disk. RAID 3 has very high read and write data transfer rates and single disk failures do not impact throughput significantly. RAID 3 stripes data blocks and stores the striped information in the exact same location on the individual disks that make up the array - so parallel I/O is not possible as data requests require seeks on all disks simultaneously to the same position.

RAID 3 is excellent for media applications such as image editing, digital pre-press and live streaming. The total capacity of a RAID 3 array is sum(N-1) and requires a minimum of three disks to implement. 

RAID 4 algorithm is similar to RAID 3 except that striping is done at the block rather than byte level. This has the advantage of blocks requests being serviced by a single disk if the controller supports that functionality. With single disk block request serving, multiple block requests may be services simultaneously in parallel so long as the individual blocks reside on separate disks.

The total capacity of a RAID 4 array is sum(N-1) and requires a minimum of three disks to implement.

RAID 5 or Striping with Parity is implemented with a minimum of three disks. In a typical three-disk RAID 5 array the data is striped across two disks and parity information is written to the third. This scheme is extended to any further number of disks in the array. For every stripe of data that is written to disks on a RAID 5 array, a special parity bit is calculated and stored in a round-robin fashion. The parity information is therefore distributed and any disk in the array can fail and data can then be restored from the remaining set of disks in the array using the parity information. The total capacity available in a RAID 5 array is sum(N - 1), where N is the number of disks in the set.

A RAID 5 array cannot handle more than a single disk failure without being corrupted. If two disks fail within a short time of one another, i.e insufficient time has elapsed for the parity calculations to rebuild the data blocks of the failed disk before another failure occurs, then the array and its data will be lost. It is useful therefore to have a dedicated hot-spare^* disk in the array so that a rebuild can start immediately upon a disk failure. With such a configuration, in the event of a single disk failure the RAID controller will rebuild the data on the failed disk on the hot-spare disk using the available parity information on the remaining array members. Once the rebuild has finished, the array will operate as normal.

In terms of disks, a RAID 5 array is cheaper to implement than a RAID 0+1 array. RAID 5 data reads are also slightly faster than single (standalone) disk reads. The main disadvantage of RAID 5 compared to RAID 0+1 or RAID 0 is that a disk failure has medium to significant impact on throughput performance. RAID 5 also consumes a lot of resources in rebuild operations, meaning implementation in software as opposed to a dedicated hardware controller would impact the host system processor and applications more than is desired.

RAID 5 is an excellent option for general file and application servers, database servers and web/email/news servers. To that end, RAID 5 is the most commonly deployed RAID level in network server environments.

RAID 6 is an extension of RAID 5 and provides added redundancy by using two parity sets instead of one. The advantage here is that up to two disks can fail in the array without compromising data integrity. RAID 6 requires a minimum of four disks as opposed to three disks for RAID 5. As it requires quite powerful computational resources, few hardware RAID controllers  have this algorithm implemented. However it is quite common in software RAID implementations that make use of the host system processing facilities. The total storage capacity for RAID 6 arrays is sum(N - 2) where N is the number of disks in the set.

Like RAID 5, RAID 6 is great for database servers, file and print applications and web and email serving. It provides an excellent amount of fault-tolerance with very little overhead when compared to other resilient RAID levels such as RAID 5 and RAID 10.

RAID 10 is a nested RAID level and can be described as striped mirroring. Like RAID 0+1, RAID 10 provides the benefits of both resiliency and performance. Multiple RAID 1 arrays are grouped into a single RAID 0 array and the striping of blocks is mirrored via the child arrays. A RAID 10 array can lose all but one drive in each of the child RAID 1 arrays without compromising data integrity. However, if all the drives in one child RAID 1 array should be lost, the entire RAID 10 array will be compromised as would be the case for a single drive loss in a RAID 0 array.

RAID 10 is very popular for high transaction applications such as databases as write speeds are very good with quite acceptable levels of data security and integrity. The total capacity of a RAID 10 array is sum(N/2) where N is the number of drives in the array and count(N) is even.

Like RAID 10, RAID 50 is a nested RAID level. It consists of striping (RAID 0) over two or more RAID 5 arrays. RAID 50 gives an added performance boost over RAID 5 with the caveat of being twice as expensive (assuming two RAID 5 sets are being combined into a RAID 50). RAID 50 provides better performance than RAID 5 with limited loss in capacity. RAID 50 is able to achieve high data transfer rates as a result of the RAID 5 segments and good I/O rates for small requests due to the RAID 0 striping layered over the RAID 5 segments.

RAID 50 suffers from a similar intolerance as RAID 10 in terms of degradation of a child RAID 5 set. A failed child RAID 5 set, which can occur if two drives from within the same RAID 5 set fail, in a RAID 50 array will bring down the entire array resulting in loss of all data.