The different types of RAID is typically referred to as ‘levels’. This FAQ will focus on level 0, 1 and 0+1 since these are what is most often supported by embedded RAID controllers.
Level 0 : Striping.
Level 0 provides increased performance by writing alternating blocks of data (referred to as the stripesize) to 2 or more drives simultaneously. Read performance is also improved since data is read from all drives at the same time. No redundant information is stored and failure of a SINGLE drive will cause all data to be lost. The number of drives in a level 0 array is sometimes also referred to as the stripe width.
Level 1 : Mirroring.
Level 1 provides redundancy by writing all data to 2 or more drives. Level 1 provides no increase in write performance (it may even be a bit slower). Read performance tend to be faster than a single drive, but not as fast as level 0. Level 1 provides excellent data security since ALL drives has to fail before any data is lost.
Level 2 : Bit interleaving with Hammingerror correction code.
Data is striped across multiple disks at the BIT level. Dedicated drives are used for Hamming error correction. Hamming error correction is a forward error correction code capable of correcting any single bit error or detecting any double bit error within the code word.hard drive repair.
Level 3 : Striping with parity.
Data is striped across 2 or more disks and parity is written to a dedicated drive. Level 3 is typically implemented at the BYTE level.
Level 4 : Striping with parity.
Data is striped across 2 or more disks and parity is written to a dedicated drive. Level 4 is typically implemented at the block (stripesize) level.
Level 5 : Striping with distributed parity.
Data and parity is striped across 3 or more drives. Parity is distributed to each drive. Level 5 is the most widely used RAID for servers and other highperformance storage solutions. Any single drive can fail without data loss, ie. at least two drives must fail before any data is lost.
Level 6 : Striping with dual distributed parity.
Essentially the same as level 5, but two sets of parity is calculated in order to improve data security.
Level X+Y
It is possible to combine various RAID levels to optimise data security and/or performance. E.g. Level 0+1 and 1+0 as explained below.
Level 0+1 : Striping and Mirroring.
Level 0+1 combines level 0 and level 1 by mirroring a striped volume. Level 0+1 provides read and write performance very close (or equal) to level 0. Level 0+1 should not be confused with level 1+0. If there is 1 mirror set, a single drive failure will cause the whole array to become, in essence, a level 0 array. Level 0+1 requires an even number of drives and minimum 4.
Level 1+0 : Striping and Mirroring.
Level 1+0 (sometimes referred to as level 10) combines level 0 and level 1 by striping a mirrored volume. Level 1+0 has better data security than level 0+1. The reason for this is that the level 1+0 controller can take advantage of a partial mirror set, but the level 0+1 controller cannot take advantage of a partial stripe set.
JBOD : Just a Bunch Of Drives.
Not actually RAID, but some RAID controllers support this. In JBOD, 2 or more drives, which can be of any size, are put together so it appear as a single drive whose capacity is the sum of the individual drives. Since JBOD provides no performance increase and reduced data security, it is seldomly used.
