Redundant Array of Independent Disks (RAID) is a data storage technology that combines multiple disk drive components into a logical unit. RAID provides fault tolerance by replicating and distributing data across multiple disks. If one disk fails, the data can still be accessed from the remaining disks. There are several RAID levels that provide varying degrees of fault tolerance:
RAID 0
RAID 0 stripes data across multiple disks for improved performance, but does not provide fault tolerance since there is no data redundancy. If any disk in a RAID 0 array fails, all data will be lost. RAID 0 is generally not recommended for mission critical data that requires high availability.
RAID 1
RAID 1 mirrors data between two disks. If one disk fails, the data can still be accessed from the other disk. RAID 1 provides good performance for read operations since data can be accessed in parallel from both disks. However, write performance is slower since data must be written to both disks. RAID 1 requires at least two disks and provides a fault tolerance of 1 disk failure.
RAID 5
RAID 5 stripes data and parity information across 3 or more disks. If any single disk fails, the missing data can be recreated from the parity information on the other disks. RAID 5 provides good read performance and reasonable write performance. However, rebuilding data after a disk failure can be slow due to the parity calculation overhead. RAID 5 requires at least 3 disks and can tolerate 1 disk failure.
RAID 6
RAID 6 is similar to RAID 5, but uses double distributed parity to protect against two simultaneous disk failures. RAID 6 stripes data and parity across 4 or more disks. If up to two disks fail, data can still be recreated from the remaining parity information. RAID 6 provides fault tolerance of up to two disk failures but has slower write performance due to the double parity overhead. RAID 6 requires a minimum of 4 disks.
RAID 10
RAID 10 combines mirroring and striping for both performance and fault tolerance. Data is striped across two mirrored disk pairs. If any single disk fails, data can be accessed from the mirror. RAID 10 provides faster rebuild times than RAID 5 or 6 after a failure. However, RAID 10 requires at least 4 disks to provide fault tolerance for 1 disk failure.
Choosing a RAID Level for Fault Tolerance
The RAID level that provides the best fault tolerance depends on your specific requirements:
- RAID 1 provides the simplest mirroring for a two disk system.
- RAID 5 provides single disk fault tolerance in a minimum 3 disk array.
- RAID 6 provides two disk fault tolerance with a minimum of 4 disks.
- RAID 10 provides mirroring and striping for enhanced performance and 1 disk fault tolerance in a 4 disk array.
When choosing a RAID level, you must also consider the performance characteristics. RAID 5 and 6 provide good overall performance but rebuilding data after a failure can be slow. RAID 10 provides faster rebuilds but requires more disks to provide the same level of fault tolerance as RAID 6.
The size of your disks is also a factor. Larger capacity disks take longer to rebuild in the event of a failure. With larger disks, a RAID 6 array may be preferable over RAID 5 to limit exposure to a second disk failure during a prolonged rebuild.
Considerations for RAID Setup
There are some additional factors to consider when planning your RAID implementation:
- RAID Controller: A dedicated hardware or software RAID controller is required to manage the RAID array. Hardware controllers provide better performance but cost more. Many server motherboards have integrated RAID support.
- Hot Spare: Having a hot spare disk on standby allows the RAID array to automatically rebuild using the spare after a disk failure. This avoids the need to replace a failed drive first.
- Drive Types: Matching drives from the same manufacturer and model is recommended. Mixing drive types and capacities in a RAID array can negatively impact performance.
Software vs. Hardware RAID
RAID can be implemented in software or hardware:
- Software RAID is implemented at the operating system level, without additional hardware. Software RAID provides basic fault tolerance but has relatively poor performance since it consumes CPU resources.
- Hardware RAID uses a dedicated RAID controller card with onboard processor and memory. Hardware RAID offloads RAID processing overhead from the main CPU for enhanced performance.
For mission critical systems that require the best performance, hardware RAID is preferable over software RAID. The exact RAID controller feature set can vary between models in terms of things like cache memory size, supported drive types, and number of channels.
RAID in the Cloud
Cloud storage providers such as Amazon AWS, Microsoft Azure, and Google Cloud offer managed RAID-like redundancy options without having to directly configure RAID arrays yourself. These include:
- Data replication – Automatically maintaining multiple copies of data in different availability zones or regions.
- Erasure coding – Data & parity fragments distributed across devices. Can tolerate 2-3x failures for the same overhead as simple replication.
Cloud RAID offers easier management, fault tolerance, and potentially lower costs than on-prem RAID setups. But there is less control over the specific implementation details. Performance can also be inconsistent depending on infrastructure utilization.
Conclusion
To achieve the best fault tolerance for data availability, RAID levels 5, 6, or 10 are preferable depending on your redundancy and performance needs. The minimum disks required varies between 3 disks for RAID 5, 4 disks for RAID 6, and 4 disks for RAID 10. Hardware RAID provides better performance than software RAID but at increased cost. When planning your RAID implementation, also factor in the RAID controller, hot spares, and drive types.
Cloud providers offer transparent redundancy options without direct RAID configuration. But on-prem RAID still grants you greater control and predictability in the fault tolerance design. Overall, the RAID level choice depends on your availability requirements, performance needs, budget, and manageability preferences.
