RAID 5 is a type of RAID (Redundant Array of Independent Disks) that provides data redundancy and fault tolerance by using distributed parity. According to PCMag, RAID 5 stripes data and parity information across a minimum of three disks. If one disk fails, the parity blocks from the remaining disks can be used to reconstruct the data from the failed drive. This provides redundancy and fault tolerance.
RAID 5 works by breaking data into blocks and writing blocks across multiple disks in the array. As it writes block, it calculates and writes parity information for each block. The parity block is written to a different disk than the data block. If a disk fails, the parity blocks on the other disks can be used to reconstruct the data from the failed drive (Network Encyclopedia). This provides protection against disk failure without having to duplicate all data like in RAID 1.
A key benefit of RAID 5 is this distributed parity provides redundancy while also allowing for increased disk capacity compared to RAID 1 mirroring. RAID 5 requires a minimum of three disks, allowing it to dedicate full disk capacity toward storing data unlike RAID 1. The tradeoff is write performance may be slower due to the parity calculation.
RAID 5 Rebuild Process
The rebuild process for RAID 5 involves the controller scanning the remaining drives and using the parity information to recreate the data that was on the failed drive onto the replacement drive. As explained by IBM, “The RAID 5 recovery process uses the parity disk to re-create the data from the failed hard disk drive onto the hot-spare disk drive.” (IBM)
The controller prioritizes rebuilding the most critical data first during this process. As noted on Quora, “The rebuild process prioritizes rebuilding the most frequently accessed data first. This helps restore full redundancy and data protection to the most important data as quickly as possible.” (Quora)
By leveraging parity information, the RAID 5 array can reconstruct the data from a failed drive onto a replacement drive, prioritizing critical data first to restore redundancy.
Factors Affecting Rebuild Times
There are several key factors that impact how long it takes to rebuild a RAID 5 array after a disk failure:
The number of drives in the array is a major factor. RAID 5 requires parity data to be recalculated and written across all drives except the failed drive during a rebuild. Therefore, the more drives in the array, the longer the rebuild takes (ServerFault).
Drive capacity also affects rebuild times. Larger capacity drives take longer to rebuild as there is more data to read, calculate parity for, and rewrite across the array. Rebuilding a 4TB drive will take longer than a 2TB drive, for example (LinkedIn).
Finally, the performance characteristics of the drives impacts rebuild times. Slower RPM hard disk drives will have longer rebuild times compared to faster solid state drives. The controller and connection interfaces to the drives also play a role (TechTarget).
Rebuild Times for 4TB Drives
According to recent benchmarks, rebuilding a RAID 5 array with 4TB drives takes approximately 24-48 hours depending on the controller and system load. In ideal conditions with a high-end RAID controller and minimal system activity, some report rebuild times as fast as 18 hours for 4 x 4TB drives in RAID 5 (Source: https://serverfault.com/questions/967930/raid-5-6-rebuild-time-calculation).
However, most real-world tests put RAID 5 rebuild times for 4TB drives around 1-2 days. Slower and older RAID controllers may require up to 3 days for 4TB RAID 5 rebuilds (Source: https://ipvm.com/discussions/what-is-the-maximum-number-of-drives-you-would-use-in-a-raid-5-with-regards-to-rebuild-times).
Compared to smaller drives, 4TB RAID 5 rebuilds take significantly longer. A RAID 5 array with 1TB drives may only require 6-12 hours to rebuild. The larger capacity drives means more data to reconstruct in the event of a failure (Source: https://www.reddit.com/r/DataHoarder/comments/ud8olz/raid_options_and_rebuild_times/).
Best Practices
When using RAID 5, there are several best practices that can help mitigate risks and improve performance:
Use hot spares – Having one or more hot spare drives that can immediately take over if a drive fails reduces rebuild times and the risk of data loss during rebuilds. Dell recommends using hot spares with RAID 5 arrays. (1)
Monitor drive health – Keeping an eye on hard drive health metrics like read error rates allows you to replace questionable drives before they fail completely. This avoids lengthy rebuilds if multiple drives fail in close succession. (2)
Maintain backups – Having recent backups available reduces the impact of drive failures, as data can be restored if needed before a lengthy rebuild completes. Backups also protect against data corruption. (3)
Alternative RAID Options
There are a few alternative RAID options that can be used instead of RAID 5 to provide increased redundancy and performance.
RAID 6 Benefits
RAID 6 is considered an alternative to RAID 5 that provides better fault tolerance by using a dual distributed parity scheme. This means RAID 6 can sustain up to two simultaneous drive failures without data loss (whereas RAID 5 can only handle one drive failure). The tradeoff is somewhat reduced write performance compared to RAID 5. However, RAID 6 offers excellent read speeds. Overall, RAID 6 offers excellent redundancy for critical data at the cost of some write performance. It is ideal for large arrays where drive failures are more likely over time. See this article for more on the benefits of RAID 6 over RAID 5.
RAID 10 Benefits
RAID 10 provides fault tolerance by combining mirroring and striping. It offers high performance by spreading data across multiple drives similar to RAID 0, but also duplicates the data for redundancy like RAID 1. The result is excellent read and write speeds combined with the ability to withstand multiple drive failures so long as no more than one drive fails per mirrored set. RAID 10 is ideal for applications requiring very high performance and redundancy. However, it does require at least 4 drives to implement. See this comparison of RAID 6 vs RAID 10 for more details.
Mitigating Failure Impact
While RAID 5 rebuild times can be lengthy for large arrays, there are ways to mitigate the impact of drive failures and reduce the risk of data loss (1). The two main techniques are:
- Scheduled rebuilds – Periodically rebuilding RAID arrays during maintenance windows can help identify bad sectors or predict drive failures before they actually occur. This reduces the likelihood of a second drive failing during a prolonged rebuild.
- Proactive drive replacement – Drives can be replaced preemptively once they reach a certain age, even if they haven’t yet failed. This avoids putting older, riskier drives through the stress of a full-speed rebuild.
Proactively swapping out potential problem drives ahead of failure events greatly decreases the chances of experiencing an array-level outage. Regular rebuilds also help limit recovery times by ensuring any bad blocks are remapped.
When to Upgrade RAID
There are a few key signs that indicate it may be time to upgrade your RAID 5 array:
- The array is frequently reaching full capacity – If you find yourself constantly deleting files or migrating data off the array to make room, it’s undersized for your storage needs. Upgrading allows you to add more overall storage.
- Rebuilds are taking longer – As drives get larger, rebuild times also increase. If rebuilds are taking multiple days and impacting performance, upgrading can help by allowing you to use newer drives with faster rebuild times.
- Your storage needs are increasing – If your business or workload is generating more data over time, your storage requirements will grow. Upgrading RAID 5 allows you to expand capacity to meet increasing demands.
- Newer drives offer better performance – New drive technologies like SATA SSDs provide faster access times and IOPS compared to older mechanical drives. Upgrading allows leveraging newer drive performance.
By monitoring your storage usage patterns and rebuild times, you can determine when increased capacity and performance justify upgrading your RAID 5 array.
Newer Technologies
Traditional RAID solutions like RAID 5 and RAID 6 are declining in popularity as newer technologies emerge. One of the most promising newer technologies is erasure coding.
Erasure coding works by breaking data into fragments, encoding it with redundant data pieces, and storing those encoded fragments across a set of different drives. This provides data protection similar to RAID 5/6 but requires less overhead. With erasure coding, you only need about 1.5x the amount of total storage as usable storage, compared to 2x for RAID 6. This improves storage efficiency.
Erasure coding also scales better than RAID as you add more drives. More drives means more redundancy and protection. Software solutions like CepH utilize erasure coding and allow massive scalability. As a result, erasure coding is growing in popularity for large scale storage systems.
For home and small business NAS setups, traditional RAID still has advantages of maturity and hardware support. But for larger deployments, erasure coding provides more efficient redundancy and better scalability. As the technology matures more, erasure coding may eventually supersede RAID solutions.
Sources:
[Modern alternatives to raid 5 reddit](https://www.reddit.com/r/DataHoarder/comments/7fteu6/im_told_raid5_isnt_viable_anymore_whats_an/)
[raid 10](https://www.open-e.com/blog/raid-5-raid-6-or-other-alternativee/)
Conclusion
In summary, RAID 5 rebuild times for 4TB drives can vary greatly depending on several factors. The number of disks in the array, disk interface, controller performance, workload during rebuild, and drive specs all play a role. While larger arrays and higher capacity disks lengthen rebuild times, newer interface technologies like 12Gbps SAS can help accelerate the process. Rebuild times of 24 hours or more are not uncommon for 4TB drives in larger arrays.
To mitigate lengthy rebuilds and potential for multiple disk failures, admins should follow best practices like using enterprise-class drives, monitoring disk health, providing adequate ventilation, balancing rebuild load, and considering alternative RAID levels. Periodically upgrading to higher capacity drives and expanding the array can also help maintain manageable rebuild times.
Though RAID 5 rebuilds on 4TB disks can be lengthy, proper precautions can minimize disruption. Understanding the factors that influence rebuild times allows admins to make informed decisions when architecting storage and recovery solutions.