RAID 5 and RAID 6 are the two main RAID levels that use striping with parity. RAID 5 stripes data across multiple disks and uses one disk for parity, while RAID 6 stripes data and uses two disks for parity. Both provide fault tolerance through the use of parity, allowing data reconstruction if a disk fails.
What is RAID?
RAID stands for Redundant Array of Independent Disks. It is a data storage technology that combines multiple disk drives into a logical unit. RAID provides increased storage capacities, speed, and redundancy for improved data protection.
The main goals of RAID are:
- Increased data reliability – RAID maintains data integrity through data redundancy.
- Improved performance – RAID allows parallel activity on multiple disk drives.
- Fault tolerance – If a disk fails, data can be reconstructed from the remaining disks.
There are several different RAID levels, each with specific data distribution and redundancy characteristics. The most commonly used RAID levels are:
- RAID 0 – Disk striping without parity or mirroring. Provides improved performance but no redundancy.
- RAID 1 – Disk mirroring without parity or striping. Provides redundancy through duplicating all data on secondary disks.
- RAID 5 – Disk striping with distributed parity. Provides fault tolerance and improved performance.
- RAID 6 – Disk striping with double distributed parity. Provides fault tolerance with two parity disks, allowing for two disk failures.
- RAID 10 – Striping over mirrored disks. Provides higher performance along with complete data redundancy.
How does striping work?
Disk striping is the technique of segmenting data across multiple physical disks in a RAID array. The data is divided into blocks called stripes that are uniformly distributed across the disks.
For example, in a 4-disk RAID 5 array, stripe 1 may be written to disks 1, 2, 3, and 4. Stripe 2 would then be written to disks 2, 3, 4, and 1. This pattern continues in a circular fashion.
Striping improves performance because multiple disks can be accessed simultaneously for each operation. It provides faster and smoother data access.
How does parity work?
Parity in RAID is a redundancy technique that allows data recovery in case of a disk failure. Parity data is calculated based on the data blocks in each stripe.
In RAID 5 and RAID 6, parity information is distributed across all the disks. If a disk fails, the missing data can be recreated using the parity block and remaining data blocks.
For example, in a 3-disk RAID 5 array, Disk 1 may contain Data A, Disk 2 may contain Data B, and Disk 3 may contain Parity A+B. If Disk 1 fails, Data A can be recalculated using Data B and Parity A+B.
The use of parity provides fault tolerance and reliability to RAID arrays.
What is RAID 5?
RAID 5 is a widely used RAID level. It uses block-level striping with distributed parity.
Key characteristics of RAID 5:
- Data is striped across multiple disks (at least 3 disks required).
- Parity information is distributed across all disks.
- Can withstand the failure of 1 disk without data loss.
- Requires a minimum of 3 disks but commonly 4+ disks are used.
- Provides good performance and storage efficiency.
In RAID 5, each stripe contains a parity block and data blocks. The parity block is rotated across the disks. If a disk fails, the parity block on the surviving disks can regenerate the missing data.
For example, in a 3-disk RAID 5 array:
- Stripe 1: Disk 1 (Data A), Disk 2 (Data B), Disk 3 (Parity A+B)
- Stripe 2: Disk 1 (Data C), Disk 2 (Parity A+C), Disk 3 (Data B)
- Stripe 3: Disk 1 (Parity B+C), Disk 2 (Data A), Disk 3 (Data C)
RAID 5 provides fault tolerance along with good performance and storage efficiency. However, write operations are slower due to parity calculation.
What is RAID 6?
RAID 6 is an advanced form of RAID 5 that uses double distributed parity.
Key characteristics of RAID 6:
- Data is striped across multiple disks (at least 4 disks required).
- Uses two parity blocks per stripe for redundancy.
- Can withstand the failure of 2 disks without data loss.
- Requires a minimum of 4 disks but commonly 6+ disks are used.
- Read performance is similar to RAID 5 but write performance is slower.
In RAID 6, each stripe contains two parity blocks P and Q. This allows the array to reconstruct missing data if up to two disks fail.
For example, in a 4-disk RAID 6 array:
- Stripe 1: Disk 1 (Data A), Disk 2 (Data B), Disk 3 (Parity P), Disk 4 (Parity Q)
- Stripe 2: Disk 1 (Data C), Disk 2 (Parity P), Disk 3 (Data A), Disk 4 (Parity Q)
- Stripe 3: Disk 1 (Parity P), Disk 2 (Data C), Disk 3 (Parity Q), Disk 4 (Data B)
RAID 6 provides excellent fault tolerance and survivability. The tradeoff is slower write speeds due to dual parity calculation.
Comparison between RAID 5 and RAID 6
| Feature | RAID 5 | RAID 6 |
|---|---|---|
| Minimum disks | 3 | 4 |
| Parity disks | 1 | 2 |
| Withstand disk failures | 1 | 2 |
| Read performance | Excellent | Excellent |
| Write performance | Good | Slower than RAID 5 |
| Storage efficiency | Good | Lower than RAID 5 |
| Fault tolerance | Moderate | Excellent |
In summary, both RAID 5 and RAID 6 provide fault tolerance through striping with parity. RAID 5 uses single parity and can withstand one disk failure. RAID 6 uses double parity and can withstand two disk failures. RAID 6 provides excellent survivability at the cost of slower writes and lower storage efficiency.
When to use RAID 5 vs RAID 6?
Choosing between RAID 5 and RAID 6 depends on your specific requirements for performance, storage capacity, and fault tolerance.
Reasons to use RAID 5:
- You need moderate fault tolerance with improved storage efficiency and performance.
- Your disks are very reliable with a low chance of second disk failure during rebuild.
- You have a limited number of disks available.
Reasons to use RAID 6:
- Maximum fault tolerance and reliability are critical.
- You have a large number of low-cost consumer grade disks with higher failure rates.
- Your RAID group is very large, increasing likelihood of dual disk failures.
- Your application has long rebuild times, increasing risk of second failure.
In general, RAID 6 is preferable for mission critical data or large disk arrays where higher fault tolerance is required. RAID 5 may be used in smaller arrays where performance and capacity are bigger considerations.
Advantages of RAID 5
Some key advantages of using RAID 5 include:
- Good performance – Read speeds are very fast since data is striped across multiple disks. Write speeds are slower than RAID 0 but faster than RAID 6.
- Decent storage efficiency – RAID 5 arrays require 1 disk worth of space for parity, so storage efficiency is 1/n where n is number of disks.
- Standard fault tolerance – Can survive a single disk failure without data loss. Provides a balance of redundancy and efficiency.
- Wide compatibility – RAID 5 is supported by most RAID controllers. Drivers are mature and stable.
- Minimum 3 disks – Can be implemented on a small number of disks unlike RAID 6 which needs a minimum of 4.
For most general purpose use cases where performance and capacity are important considerations, RAID 5 provides a good blend of attributes. It has been a popular choice for many years.
Disadvantages of RAID 5
Some potential disadvantages of using RAID 5 include:
- Risk of data loss – Only single disk failure is tolerated. Second disk failure during rebuild can lead to data loss.
- Slower rebuilds – Rebuilding large RAID 5 arrays can take days, increasing risk of failure during rebuild.
- Slower writes – Write performance takes a hit due to parity calculation compared to RAID 0.
- Not ideal for large arrays – Very large arrays increase the risk of UREs and potential data loss.
- Rebuild impact – Performance and fault tolerance are impacted during rebuilds. Entire arrays can become offline if rebuild fails.
Due to the risk of a second disk failure during long rebuilds, RAID 5 may not provide adequate protection for critical data or very large disk arrays. The rebuild impact also affects performance till rebuild completes.
Advantages of RAID 6
Some major advantages provided by RAID 6 include:
- Excellent fault tolerance – Can withstand double disk failures. Provides enhanced redundancy for better data protection.
- Safer rebuilds – parity and rebuild mechanisms minimize risk of data loss during rebuilds.
- Ideal for large arrays – Provides protection against higher URE risks in very large high-density disk arrays.
- Handles latent errors – Better at eliminating data corruption undetected errors.
- Less rebuild impact – Can remain online with close to normal performance during rebuilds.
The double parity provided by RAID 6 offers superior protection for important data. The additional parity disk also reduces rebuild times and impact. This makes RAID 6 well suited for large arrays with higher potential for disk failures.
Disadvantages of RAID 6
Some disadvantages of using RAID 6 can include:
- Higher minimum disks – RAID 6 requires a minimum of 4 disks unlike RAID 5 which can work with 3 disks.
- Lower storage efficiency – The use of double parity means storage overhead is 2/n compared to 1/n for RAID 5.
- Slower writes – Writes take longer due to double parity calculation reducing performance.
- Higher cost – Requires more disks than equivalent RAID 5 array leading to higher costs.
- Complex configurations – RAID 6 setups can be more complex to configure optimally compared to RAID 5.
The performance and storage capacity tradeoffs may make RAID 6 unsuitable for some use cases where these attributes need to be optimized. The cost and complexity considerations also apply in smaller implementations.
Conclusion
In conclusion, RAID 5 and RAID 6 provide varying levels of fault tolerance for RAID storage environments through the use of distributed parity.
RAID 5 is the more popular implementation that offers a single parity disk. It provides a good balance of performance, capacity, and redundancy for general use cases.
RAID 6 builds on RAID 5 by adding a second dedicated parity disk. This offers excellent protection against dual disk failures along with safer rebuilds. However, the tradeoff is decreased performance, storage efficiency, and increased complexity.
RAID 6 is preferable for mission critical data or very large arrays where reliability is a higher priority than capacity or performance. RAID 5 remains a good option for smaller arrays where fault tolerance requirements are more moderate.
The choice between RAID 5 and 6 will depend on the specific priorities, size, cost constraints, and data protection needs of each storage environment.