|
|
|
RAID systems are differentiated by a level which indicates the type of RAID in place. RAID 0, 1 and 5 are the most common levels.
RAID Level 0 - Disk striping without parity
Level 0 is a volume stretched across several drives, it offers no redundancy rather just the ability to create a larger single volume than the individual disks.
RAID Level 1 - Disk mirroring
Data is written simultanously to two disks. Performance is on a par with a single drive and it offers a level of redundancy. In the event of data corruption, it will be written to both drives.
RAID Level 2 - Hamming Code ECC
Each bit of data word is written to a data disk drive. Each data word has a Hamming Code or Error Correction Code (ECC) word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.
RAID Level 3 - Parallel transfer with parity
RAID 3 adds redundant information by adding parity to a parallel access striped array, permitting reconstruction of data in the event of a disk failure. One strip of parity protects data striped on the remaining disks. RAID 3 provides high data transfer rate and high data availability, at a lower cost than mirroring. Its transaction performance is poor, however, because the array member disks operate in lockstep. A RAID Level 3 system requires at least 3 drives.
RAID Level 4 - Independent Data disks with shared parity disk
As with RAID 3, this uses parity on a single disk to protect data. However RAID4 disks are independently accessible making it more suited to transactional I/O activity rather than large files transfers. The dedicated parity disk creates a throughput bottleneck so level 4 is rarely used without some form of sophisticated caching. RAID Level 4 requires a minimum of 3 drives.
RAID Level 5 - Independent Data disks with distributed parity blocks
RAID 5 is probably the most common level. Data and parity are stripped across 3 or more disks reducing the write bottleneck of RAID 4. If a drive fails with RAID 5 the parity bit data can reconstruct the information on the lost drive. Where two or more drives in a RAID 5 implementation fail the services of a data recovery expert are required.
 Read about RAID combination levels
|
|
|
