Redundant Array of Independent Disks (RAID) is a storage technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy and performance improvement. There are several different RAID levels or techniques that provide varying degrees of redundancy and performance. The most commonly used RAID levels that provide redundancy are RAID 1, RAID 5, RAID 6, RAID 10, and RAID 01.
RAID 1
RAID 1, also known as disk mirroring, is the simplest RAID technique that implements redundancy. It involves duplicating or “mirroring” data across two or more disks. For example, a RAID 1 array with two disks would contain the same data on both disks. If one disk fails, the data can still be accessed from the other disk. RAID 1 provides redundancy by writing duplicate copies of data to multiple disks. This protects against data loss in the event of a single disk failure.
Some key characteristics of RAID 1:
- Implements disk mirroring to copy and maintain identical data on two or more disks
- Provides 100% redundancy by duplicating all data on a secondary disk
- Requires at least two disks
- Twice the read performance of single disk as data can be accessed in parallel
- No performance improvement for writes as data has to be written twice
- 50% storage efficiency as duplicate data takes up double the space
RAID 1 is suited for applications that require high availability and fault tolerance but do not require high input/output rates. The duplicated disks provide quick recovery from disk failures. However, the write performance does not improve due to the duplicate writes. Also, the storage overhead is high as the effective capacity is only half of the total raw capacity. Overall, RAID 1 provides simple and effective redundancy through disk mirroring.
RAID 5
RAID 5 is a redundant RAID level that stripes data and parity information across three or more disks. It requires at least three disks to implement. In a three disk RAID 5 array, user data is striped across two disks, while the third disk stores the parity information. The parity disk does not store actual data but stores computed parity values used for data recovery. For example, in a three disk RAID 5 array, Disks 1 and 2 store user data while Disk 3 stores computed parity values based on Disks 1 and 2.
Some key characteristics of RAID 5:
- Data is striped across disks with parity values calculated and stored on a dedicated parity disk
- Can withstand single disk failure without data loss as lost data can be recreated from parity info
- Requires at least three disks to implement
- Storage efficiency is N-1 disks where N is total disks
- Good random read/write performance due to disk striping
- Poor performance for large sequential writes due to parity calculations
In summary, RAID 5 provides redundancy by dedicating a disk to parity data. It provides efficient use of storage capacity while allowing recovery from a single disk failure. However, write performance suffers due to parity computations. RAID 5 is a good choice for data protection in small server environments with moderate capacity and performance needs.
RAID 6
RAID 6 provides redundancy by using double distributed parity. It stripes data across multiple disks like RAID 5, but uses two dedicated parity disks instead of one. This allows RAID 6 to withstand failures of up to two disks. For example, in a five disk RAID 6 array, two disks are used for parity while three disks store user data. If any two disks fail, data can still be recovered from the remaining disks.
Some key characteristics of RAID 6:
- Stripes data across disks with dual parity values on dedicated parity disks
- Survives up to two disk failures
- Requires minimum of four disks (two for parity)
- Storage efficiency is N-2 disks where N is total disks
- Degraded read/write performance due to dual parity calculations
In summary, RAID 6 provides excellent redundancy by using double parity, allowing recovery from up to two disk failures. The tradeoff is decreased write performance and higher storage overhead for parity. RAID 6 is suited for mission critical storage that requires high data availability and protection against multiple disk failures.
RAID 10
RAID 10 combines both disk striping and disk mirroring to provide redundancy and improved performance. It creates a striped set of mirrored disks. For example, a four disk RAID 10 array would consist of two mirrored pairs of disks with data striped across the pairs. This layout provides fault tolerance and fast read/write speeds.
Some key characteristics of RAID 10:
- Mirrors data across disks and also stripes data across mirrored sets
- Withstands multiple disk failures as long as one disk in each mirrored set survives
- Requires minimum of four disks (in pairs of mirrored disks)
- Storage efficiency is 50% due to mirroring
- Very high read/write performance due to striping and mirroring
In summary, RAID 10 provides redundancy through mirroring while also enhancing performance via disk striping. It can tolerate multiple disk failures but also has high storage overhead. RAID 10 is best for applications that require fast I/O performance along with high data availability.
RAID 01
RAID 01 is a nested RAID level that combines mirroring and striping. It is also referred to as RAID 1+0. In RAID 01, data is first mirrored onto disks and then the mirrored data is striped across multiple disk sets. For example, in a RAID 01 using four disks, data would first be mirrored across two disks. Then the two mirrors would be striped across each other.
Some key characteristics of RAID 01:
- Data is first mirrored and then striped across mirrored disk sets
- Can survive multiple disk failures as long as one disk in each mirror survives
- Requires minimum of four disks (in mirrored pairs)
- Storage efficiency is 50% due to initial mirroring
- Fast read performance due to striping of mirrors
In summary, RAID 01 provides redundancy through initial mirroring and enhances performance by striping mirrors. It offers excellent fault tolerance and fast data reads. However, the storage overhead is high at 50% due to the initial mirroring. Overall, RAID 01 is best suited for applications that demand high availability and fast reads but do not require much initial storage capacity.
Comparison of Redundant RAID Levels
Here is a comparison of some key characteristics between the main redundant RAID levels:
| RAID Type | Minimum Disks | Redundancy | Storage Efficiency | Read Performance | Write Performance |
|---|---|---|---|---|---|
| RAID 1 | 2 | 1 disk failure | 50% | High (parallel reads) | Low (duplicate writes) |
| RAID 5 | 3 | 1 disk failure | N-1 disks | High (striping) | Low (parity writes) |
| RAID 6 | 4 | 2 disk failures | N-2 disks | Medium (dual parity) | Very low (dual parity) |
| RAID 10 | 4 | Up to disks in each mirror | 50% | Very high (striping + mirroring) | High (striping + mirroring) |
| RAID 01 | 4 | Up to disks in each mirror | 50% | Very high (striping of mirrors) | Medium (writes to mirror) |
In summary:
- RAID 1 offers simple mirroring redundancy while RAID 5 uses dedicated parity
- RAID 6 provides the highest fault tolerance surviving up to 2 disk failures
- RAID 10 provides both mirroring and striping for balanced performance
- RAID 01 mirrors first then stripes for high read speeds
The choice depends on specific requirements including cost, capacity, performance, and availability needs.
Conclusion
Implementing RAID can provide important data redundancy and availability for storage systems. The main techniques that provide fault tolerance are RAID 1, 5, 6, 10, and 01. RAID 1 uses disk mirroring while RAID 5 and 6 use parity calculations. RAID 10 combines mirroring and striping. RAID 01 performs nested mirroring and striping. Each option requires different minimum disk counts and offers varying storage efficiency, performance, and fault tolerance. When choosing a redundant RAID technique, key criteria to consider include cost, capacity needs, I/O performance requirements, and availability demands. Analyzing these factors will help determine the optimal RAID solution for a particular storage environment.