RAID products can be grouped into the categories described in the following sections.

RAID Level 0

Technically, these products are not RAID products at all, because they do not offer parity or error- correction data to provide redundancy in the event of system failure. Although data striping is performed, it is accomplished without fault tolerance. Data is simply striped block-by-block across all the drives in the array. There is no way to reconstruct data if one of the drives fails.

RAID Level 1

These products duplicate data that is stored on separate disk drives. Also called mirroring, this approach ensures that critical files are available in case of individual disk drive failures. Each disk in the array has a corresponding mirror disk, and the pairs run in parallel. Blocks of data are sent to both disks simultaneously. Although highly reliable, Level 1 is costly because each drive requires its own mirror drive, which doubles the hardware cost of the system.

RAID Level 2

These products distribute the code used for error detection and correction across additional disk drives. The controller includes an error-correction algorithm, which enables the array to reconstruct lost data if a single disk fails. As a result, no expensive mirroring is required. The code, however, requires that multiple disks be set aside to do the error-correction function. Data is sent to the array one disk at a time.

RAID Level 3

These products store user data in parallel across multiple disks. The entire array functions as one large, logical drive. Its parallel operation is ideally suited to supporting imaging applications that require high data-transfer rates when reading and writing large files. RAID Level 3 is configured with one parity (i.e., error-correction) drive. The controller determines which disk has failed by using additional check information recorded at the end of each sector. However, because the drives do not operate independently, every time an image file must be retrieved, all the drives in the array are used to fulfill that request. Other users are put into a queue.

RAID Level 4

These products store and retrieve data using independent writes and reads to several drives. Error-correction data is stored on a dedicated parity drive. In RAID Level 4, data striping is accomplished in sectors rather than bytes or blocks. Sector-striping offers parallel operation in that reads can be performed simultaneously on independent drives, which allows multiple users to retrieve image files at the same time. Although multiple reads are possible, multiple writes are not because the parity drive must be read and written to for each write operation.

RAID Level 5

These products interleave user data and parity data, which are then distributed across several disks. Because data and parity codes are striped across all the drives, there is no need for a dedicated parity drive. This configuration is suited for applications that require a high number of input/output operations per second, such as transaction processing tasks that involve writing and reading large numbers of small data blocks at random disk locations. Multiple writes to each disk group are possible because write operations do not have to access a single common parity drive.

RAID Level 6

These products improve reliability by implementing drive mirroring at the block level so data is mirrored on two drives instead of one. Up to two drives in the five-drive disk array can fail without loss of data. If a drive in the array fails with RAID Level 5, for instance, data must be rebuilt from the parity information spanned across the drives. With RAID Level 6, however, the data is simply read from the mirrored copy of the blocks found on the various striped drives. No rebuilding is required. Although this results in a slight performance advantage, it requires at least 50% more disk capacity to implement.

Vendors continually tout the effectiveness of various RAID solutions. In truth, the choice among RAID solutions involves tradeoffs between cost, performance, and reliability. Rarely can all of these requirements be satisfied simultaneously, especially when trying to address high-availability, large-scale storage needs.