Redundant array of independent disks (RAID) is a data storage virtualization technology that combines multiple physical disk drive components into one or more logical units.
The purposes of a RAID is data redundancy, performance improvement, capacitiy and data security.
RAID looks like a single disk to the server.
Alternatives to RAID are JBOD, SPAN, BIG or MAID.
Standard RAID Levels
RAID Levels:
- RAID 0
- RAID 1
- RAID 2
- RAID 3
- RAID 4
- RAID 5
- RAID 6
RAID 3 and RAID 4 were quickly replaced by RAID 5.
CISSP certification asks about 0, 1, 5, 6 and 10.
RAID 0
RAID 0 (striping) requires 2 disks.
The second disk is used to increase performance, not resilience.
RAID 1
RAID 1 (mirroring) requires 2 disks.
It makes and exact copy on the second disks. If one disk breaks, there is a copy.
RAID 2
RAID 2 stripes data at the bit (rather than block) level and uses Hamming codes for error disk correction.
RAID 3
RAID 3 uses at least 3 disks, that’s why it is called like this. It uses a dedicated parity disk and bit-level striping across multiple disks.
It stripes data at byte level.
It may use the XOR operator to combines values from registers from different disks in a single register for the parity disk.
RAID 4
RAID 4 is a RAID configuration that uses a dedicated parity disk and block-level striping across multiple disks.
It needs at least 3 disks, where 1 is a dedicated parity disk.
It divides the blocks at bit level. For each “word” sent to write, it calculates a Hamming code that allows the information to be recovered in the event of a disk failure.
It may use a different coding system than RAID 4.
RAID 4 has been mainly replaced by RAID 5.
RAID 5
RAID 5 (striping with parity) requires 3 disks or more. In this case, it has no dedicated parity disk and striping is done in all disks.
It has fault tolerance for 1 disk.
It is similar to RAID 4, except that it does not use a dedicated parity disk, but instead distributes the parity blocks across all disks in the array.
RAID 6
RAID 6 (extended striping with parity) requires 4 disks or more.
It has fault tolerance for 2 disks.
Similar to RAID 5, except that it calculates two independent parity blocks for each write, sending each parity block to a different disk. There are not dedicated disks.
Nested RAID levels
RAID 01
RAID 01 or RAID 0+1, called mirroring stripe set,
It requires a minium of 4 disks.
RAID 10
RAID 10 or RAID 1+0, called striping of mirrors, combines pairs of RAID 1’s, where primary disks are mirrored and secondary disks are used only for performance.
It requires a minimum of 4 disks.
Other combined RAIDs
Some examples of combined RAIDs are: RAID 0+1, RAID 1+0, RAID 0+3, RAID 3+0, RAID 0+5, RAID 5+0, RAID 1+5, RAID 5+1.
RAID Levels Overview
| Level | Name | No. Disks Required | No. Disks with Fault Tolerance | Comment |
| 0 | Striping | 2 | 0 | Uses a 2nd disk for performance. |
| 1 | Mirroring | 2 | 1 | Uses a 2nd disk for fault tolerance. |
| 2 | Stripes data at the bit (rather than block) level. Uses Hamming codes for error disk correction | |||
| 3 | Byte-level striping with dedicated parity | ≥3 | 1 | Uses a dedicated parity for byte-level striping. |
| 4 | Block-level striping with dedicated parity | ≥3 | 1 | Uses a dedicated parity disk for block-level striping. |
| 5 | Striping with parity | ≥3 | 1 | Parity information is distributed among disks. |
| 6 | Extended striping with parity | ≥4 | 2 | Parity is distributed twice among disks. |
| 10 | Striping of mirrors | ≥4 | 1 | Multiple RAID 1 are connected with each other to make it like RAID 0 |
| 01 | Mirroring stripe sets | ≥4 | 1 | Multiple RAID 0 are connected with each other to make it like RAID 1 |
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External References
- RAID Levels
- Wikipedia community; “Standard RAID Levels“; Wikipedia
- M. Chapman et al; “CISSP Study Guide 9th Edition”; Wiley, 2021
- Anirban Das; “RAID levels explained“; BooleanWorld, 2018
- Starwind Hyperconvergence; “Understanding RAID“; Starwind Hyperconvergence
- Nested RAID
- Wikipedia community; “Nested RAID levels“; Wikipedia