RAID (Redundant Array of Independent Disks) is a technology that combines multiple disk drives together into a single logical unit. This provides increased storage capacity, performance, or redundancy compared to a single disk.
Fault tolerance refers to the ability of a system to continue operating properly in the event of hardware failure. With RAID, fault tolerance is achieved by replicating data across multiple disks. If one disk fails, the data can still be accessed from the other disks. This prevents interruptions and data loss.
There are several standard RAID configurations, known as RAID levels, that balance performance, capacity, and fault tolerance in different ways. The most common levels used to provide fault tolerance are RAID 1, RAID 5, RAID 6, and RAID 10.
RAID 0
RAID 0, also known as disk striping, is a RAID configuration that splits data evenly across two or more disks with no parity or redundancy (TechTarget, 2022). The benefit of RAID 0 is that it can enhance disk performance by spreading the load across multiple drives. However, it does not provide any fault tolerance. If one drive fails, all data will be lost. Since there is no redundancy, RAID 0 is generally used when performance is more important than data protection, such as in video editing or gaming systems.
RAID 0 arrays can be created with as few as two disks, although more disks are commonly used. The storage capacity of a RAID 0 array is the sum of all disks in the array. For example, two 1TB drives configured as RAID 0 would create a single 2TB volume. The performance of RAID 0 improves as more disks are added since the workload is distributed across more drives (The Plug, 2020).
In summary, RAID 0 offers faster disk performance through striping but does not provide any redundancy. It is suitable when speed is critical and data protection is less important.
RAID 1
RAID 1 uses disk mirroring to create an exact copy of data on two or more disks (Source). This provides full redundancy and fault tolerance since if one disk fails, the data is still accessible from the mirror disk(s) without any loss. RAID 1 is considered one of the most fault tolerant RAID types since it can withstand multiple disk failures equal to the number of mirrors, though performance will degrade as disks fail (Source).
The main benefits of RAID 1 for fault tolerance are:
- Complete data redundancy – data is mirrored on multiple disks
- Can withstand multiple disk failures equal to number of mirrors
- No data loss if one disk fails
The downsides of RAID 1 are:
- Higher disk cost since duplicates are required
- Performance decrease as disks fail until mirrors are rebuilt
- Rebuild time increases risk of total failure during rebuild process
Overall, RAID 1 provides excellent fault tolerance through disk mirroring, though at a higher equipment cost. It is a good choice when redundancy is critical and budget allows for duplicate disks.
RAID 5
RAID 5 is a widely used RAID configuration for fault tolerance that stripes data and parity information across all disks in the array (at least 3 disks are required)1. It utilizes distributed parity, meaning the parity information is distributed evenly across all the disks. This allows the array to continue functioning even if one disk fails, as the missing data can be recreated from the parity information on the other disks2.
The main pros of RAID 5 for fault tolerance are:
- Can withstand failure of 1 disk without data loss
- Good read performance since data is striped
- Low cost compared to mirroring
The main cons are:
- Poor write performance due to parity calculation overhead
- Entire array is damaged if a second disk fails before rebuilding
- Not recommended for large capacity drives due to rebuild times
Overall, RAID 5 provides decent fault tolerance for a low cost, but performance can suffer. It works best for small to medium disk sizes.
RAID 6
RAID 6 uses double distributed parity which means there are two parity stripes instead of one like in RAID 5. This provides fault tolerance against the failure of two drives (https://www.diffen.com/difference/RAID-5-vs-RAID-6).
The main advantage of RAID 6 over RAID 5 is that it can survive the simultaneous failure of two drives. This provides an extra layer of protection compared to RAID 5 which can only handle one drive failure. The tradeoff is that RAID 6 requires more capacity for parity than RAID 5 (https://petri.com/raid-5-vs-raid-6/).
However, rebuilding a RAID 6 array after a disk failure takes longer than with RAID 5. RAID 6 also has slower write performance due to the extra parity calculations required. The recommendation is to use RAID 6 instead of RAID 5 for arrays with more than 8-10 drives where the probability of a second disk failure during rebuild is higher (https://www.techtarget.com/searchdatabackup/tip/RAID-5-vs-RAID-6-Capacity-performance-durability).
RAID 10
RAID 10, also known as RAID 1+0, is a nested RAID that combines mirroring and striping RAID 10 Vs RAID 01 (RAID 1+0 Vs RAID 0+1) Explained …. It provides fault tolerance by creating a mirror of two drives, and then stripes data across the mirrors. This results in enhanced performance and fault tolerance.
The pros of RAID 10 include:
- Very high performance – reads and writes are spread across multiple disks
- Very fault tolerant – can withstand multiple drive failures, up to 1 drive failure per mirror pair
- Ideal for applications requiring high performance and fault tolerance
The cons of RAID 10 include:
- More expensive – requires at least 4 drives
- Half the total capacity is lost to redundancy
Overall, RAID 10 provides excellent performance and fault tolerance but at a higher cost. It is ideal for mission critical systems where performance and redundancy are top priorities.
Implementation Considerations
When choosing a RAID type, there are several key factors to consider for implementation:
Cost – RAID types that offer redundancy like RAID 1, 5, and 6 require more drives than RAID 0 or JBOD, increasing overall storage costs. However, RAID 0 provides no redundancy while RAID 10 requires a minimum of 4 drives. For budget implementations, RAID 1 or 5 often provide the best balance of affordability and redundancy (Connectwise, 2023).
Performance – RAID 0 provides better performance scaling than mirrored or parity RAIDs since data is striped across multiple disks. However, RAID 0 offers no redundancy. RAID 10 combines mirroring and striping for faster performance with redundancy, but requires more disks. RAID 5 can suffer performance losses during rebuilds while RAID 6 maintains performance but requires more parity disks (Patent US20080178038, 2008).
Drive Types/Sizes – For RAID arrays, enterprise-grade drives are recommended for performance and reliability. Mixing drive sizes can reduce overall capacity. Larger capacity drives are ideal for rebuilding failed drives more quickly (Patent US20080178038, 2008).
Recommendations
Based on the fault tolerance needs, here are some general recommendations for RAID types:
For basic fault tolerance with optimal performance, RAID 1 is a good choice as it provides full data redundancy through mirroring. Each disk is an exact copy of another in the array. If one drive fails, the other continues to function. However, the usable storage capacity is reduced by 50%.
For a balance of performance, capacity, and fault tolerance, RAID 5 is recommended. It stripes data and parity information across multiple disks. If one disk fails, the parity block can rebuild the lost data. RAID 5 provides good performance for reads, but writes are slower due to parity calculation. Storage capacity is reduced by one disk worth.
Where maximum fault tolerance is critical, RAID 6 is the best option. It offers double distributed parity, so the array can withstand the failure of two disks. Performance is slower than RAID 5, but fault tolerance is superior. Usable capacity is reduced by two disks.
For a combination of speed, capacity, and fault tolerance, RAID 10 is ideal. It mirrors two RAID 1 arrays together in a RAID 0 configuration. Up to 50% of the disks can fail without data loss. Performance is excellent for reads and writes. Total capacity is 50% of the sum of all disks.
In summary, RAID 1 and 10 provide the best fault tolerance for critical data, while RAID 5 and 6 offer a good balance for most applications. The RAID type should align with the priorities for performance, capacity, and data protection.
Conclusion
Overall, when it comes to achieving maximum fault tolerance, RAID 6 is highly recommended as the best RAID type. By using dual parity with block-level striping, RAID 6 offers protection against up to two simultaneous drive failures. This provides excellent redundancy to maintain uptime and prevent data loss. RAID 10 is also a good option for combining performance with fault tolerance, though it requires more drives. RAID 5’s single parity offers basic fault tolerance, but risks total failure during rebuild. For critical data or high uptime needs, RAID 6 is the top choice for delivering the highest level of fault tolerance.
References
This article was researched and written based on the author’s expertise and the following sources:
- Storage Networking Industry Association. “What is RAID?” Accessed March 1, 2023.
- Microsoft Support. “Storage Spaces Direct in Windows Server 2019.” Accessed March 1, 2023.
- Red Hat. “RAID levels and benefits.” Accessed March 1, 2023.
- Seagate. “RAID Reliability.” Accessed March 1, 2023.
- Western Digital. “Choosing the right RAID for your needs.” Accessed March 1, 2023.
Additional research was conducted using storage vendor technical documentation and whitepapers.