The levels of RAID

Level 0

A.K.A striping.  No data recovery available.  Data is striped across all disks equally and sequentially trhough the array.  This is the fastest of all the levels, because it only combines the drives into one drive without error checking.

RAID 0 A Disk B Disk C Disk D Disk E Disk
Stripe 0 A0 B0 C0 D0 E0
Stripe 1 A1 B1 C1 D1 E1
Stripe 2 A2 B2 C2 D2 E2
Stripe 3 A3 B3 C3 D3 E3
Stripe 4 A4 B4 C4 D4 E4

 

Level 1

Also called duplex or disk mirroring.  Each disk in the array has an error checking disk with a duplicate set of information.  When a disk fails, it's clone takes it place.  This is the most expensive version of RAID since it requires the same number of disks for error checking as it does for holding the data.  Performance and dependability are high.  Though the most expensive RAID level, it use is common in applications where large disk space is not needed.

RAID 1 A B C D
Stripe 0 A0 B0 A0 B0
Stripe 1 A1 B1 A1 B1
Stripe 2 A2 B2 A2 B2
Stripe 3 A3 B3 A3 B3
Stripe 4 A4 B4 A4 B4

In this case blocks A and B would each represent a physical disk and C and D would be their respective mirror disks.

Level 2

Hamming Code ECC.  Use of Hamming Codes require that all disks be access during an I/O operation.  Built-in error correction being common in today's disks and having no significant advantages over the other levels make this level rarely used.  Requires 30% more disk space than level 0.

Level 3

The data is striped across the disks by the byte with one disk holding error checking data.  Only one operation can be performed at a time.  Primarily for data intensive or single user applications.

RAID 3 A Disk B Disk C Disk D Disk Parity Disk
Stripe 0 A0 B0 C0 D0 0 Parity
Stripe 1 A1 B1 C1 D1 1 Parity
Stripe 2 A2 B2 C2 D2 2 Parity
Stripe 3 A3 B3 C3 D3 3 Parity
Stripe 4 A4 B4 C4 D4 4 Parity

Each stripe spans multiple disks.  The parity disks hold the parity of the other disks in the same stripe or rank.

Level 4

The data is striped by the block across multiple drives with and extra error checking drive.  Disks can be accessed in parallel, increasing read performance.

RAID 4 A Blocks B Blocks C Blocks D Blocks Parity Disk
Stripe 0 A0 B0 C0 D0 0
Stripe 1 A1 B1 C1 D1 1
Stripe 2 A2 B2 C2 D2 2
Stripe 3 A3 B3 C3 D3 3
Stripe 4 A4 B4 C4 D4 4

Each block of data is written to a data disk.  Blocks in the same stripe or rank generate the parity disk information.

Level 5

The most widely used RAID level.  Disks can be hot-swapped without stopping the system.  Data is striped accros multiple disks, with a section of each disk being used for error checking.  Multiple reads and writes can be performed at the same time.  As with level 1 it is best used for multi-user, and networked environments( NT, Unix).

RAID 5 A Blocks B Blocks C Blocks D Blocks E Blocks
Stripe 0 A0 B0 C0 D0 0 Parity
Stripe 1 A1 B1 C1 1 Parity E1
Stripe 2 A2 B2 2 Parity D2 E2
Stripe 3 A3 3 Parity C3 D3 E3
Stripe 4 4 Parity B4 C4 D4 E4

The parity information for each block of the same rank is distributed across the disks.

 

Other levels exist, but mainly only in theory. RAID 6 is the only one occasionally implemented due to the costs of the other levels.

RAID 6 - the paranoid version.  RAID 6 can sustain 2 disk failures without losing data.  Parity is distributed as with RAID 5 but 2 blocks in each rank contain parity information.  Requires an 2 extra disks as opposed to the one needed for levels 3-5.