What is RAID 5?
RAID 5 is a popular data storage configuration that combines distributed parity and striping to provide redundancy and improved performance (Source: https://www.pcmag.com/encyclopedia/term/raid-5). It requires a minimum of 3 disks, with data and parity information distributed evenly across all disks in the array. The main defining aspect of RAID 5 is how it handles parity.
Parity data is calculated from the data being stored, providing redundancy. If one disk fails, the parity information can be used to reconstruct the data that was on the failed disk. In RAID 5, parity data is distributed across all the disks, unlike other RAID configurations that dedicate an entire disk to parity. This distribution of parity provides redundancy while maximizing the usable space in the array (Source: https://networkencyclopedia.com/raid-5-volume/).
In summary, RAID 5 provides fault tolerance through distributed parity, while also improving performance via data striping across multiple disks. Its unique parity distribution maximizes available storage capacity while still allowing data recovery in case of disk failure.
Minimum Number of Disks
The minimum number of disks needed to implement RAID 5 is 3 (https://drivesaversdatarecovery.com/what-are-the-raid-5-requirements/). This allows for disk striping with distributed parity, which is the core mechanism behind RAID 5. With only two disks, you cannot achieve redundancy through parity, so three disks is the bare minimum.
Having just three disks does provide fault tolerance through the parity drive, but there are some downsides. With only two data drives, the array performance will be limited. Also, losing one drive in a three disk RAID 5 array means the array is in a degraded state with no redundancy until the failed drive is replaced.
So while three disks is the minimum required for the RAID 5 setup, most experts recommend at least five disks for performance and redundancy reasons.
How Parity Works
RAID 5 uses parity to provide redundancy and fault tolerance. Parity is extra information calculated from the data and stored alongside it on the RAID disks. Parity allows the data on a failed drive to be recreated from the data on the remaining drives.[1]
Here’s a simple example with 3 disks: Disk 1 contains data A, Disk 2 contains data B, and Disk 3 contains parity P. P is calculated by XORing A and B (P = A XOR B). If Disk 2 fails, the RAID controller can XOR A and P to recreate B. This allows the RAID to continue functioning with 1 disk failure.
With more disks, parity is more complex but works the same way. The parity is calculated across all the data disks. So if any single disk fails, the data can be recreated from the parity block and the remaining disks. This provides fault tolerance for a single disk failure.
[1] https://technote.fyi/code/sysadmin/raid-5-parity-what-is-it-and-how-does-it-work/
Optimal Number of Disks
When it comes to choosing the number of disks for a RAID 5 array, there are tradeoffs between performance, storage efficiency, and cost that need to be considered. The most commonly recommended number of disks for RAID 5 is between 3-9 drives.
With just 3 disks, you can achieve RAID 5 redundancy while maximizing storage efficiency. The tradeoff is that performance will be lower compared to a larger array, as write operations involve calculating parity across all disks. With only 3 disks, workload is highly concentrated.
As you increase to 5, 7, or 9 disks, performance improves thanks to the workload being distributed across more drives. However, storage efficiency decreases since a larger portion of capacity goes towards parity. There is also greater cost with more disks.
For most applications, 5 or 7 disks offers a good balance between performance, efficiency, and cost for RAID 5. This provides enough disks for performance while still providing decent storage utilization compared to very large arrays. However, factors like required capacity, performance needs, and budget should be evaluated to determine the ideal number of disks.
According to Dell, “optimal number of disks for RAID 5 is 3, 5 or 9” [1]. As such, these drive counts are commonly recommended configurations.
Maximum Number of Disks
The maximum number of disks that can be used in a RAID 5 configuration depends on the capabilities of the RAID controller. Most RAID controllers can support between 16-32 disks in a single RAID 5 array before performance starts to degrade.
According to Adaptec, their controllers support up to 42 disks in a single RAID 5 array before hitting the maximum number of parity groups the controller can handle. Going beyond this limit is not recommended as it can cause performance issues and instability.
It’s important to consult the specifications of your RAID controller to determine the officially supported maximum disks for RAID 5. Exceeding the recommended limit could lead to array rebuild failures, slow performance, and other issues.
In general, for optimal performance, it’s best to stay on the lower end of your RAID controller’s supported disk limit for RAID 5. The more disks in the array, the longer it takes to rebuild the array if a disk fails. Most experts recommend 16 disks or less for stable RAID 5 performance.
Performance Impact
Adding more disks to a RAID 5 array can increase performance by distributing reads and writes across more drives. However, there is also additional overhead from the parity calculations that must be performed with each write in order to protect against drive failure. According to a Serverfault discussion, RAID 5 performance tends to peak at around 3-5 drives[1]. Beyond that, the parity overhead starts to outweigh the benefits of more disks.
A Reddit thread echoes this, noting that the most optimal RAID 5 configurations are 3, 5, or 9 drives plus a hot spare[2]. The performance sweet spot is where the stripe size aligns well with the underlying block size. Too many drives introduces more frequent parity calculations that bog down write performance.
In general, adding more disks improves read speed since the workload is distributed. But writes slow down due to the parity overhead, especially with an odd number of drives. The optimal balance depends on the specific workload and storage requirements.
Failure Tolerance
One of the key considerations with RAID 5 is its failure tolerance. RAID 5 can only tolerate a single disk failure without data loss. This is because RAID 5 uses distributed parity, meaning the parity information is spread across all the disks in the array.
If a single disk fails, the parity information on the remaining disks can be used to reconstruct the lost data. However, if a second disk fails before the RAID 5 array is rebuilt, data will be lost. This is known as an unrecoverable read error.
Therefore, it is crucial to replace failed disks and rebuild RAID 5 arrays quickly to avoid the risk of a second disk failure. The rebuilding process puts additional strain on the remaining disks, so failure risk is increased during this time. As drive sizes grow larger, rebuild times also increase, expanding the window of vulnerability.
Overall, the single disk failure tolerance of RAID 5 provides basic protection but still carries substantial risk if rebuilding is not completed quickly after a failure. For greater protection, RAID 6 is a better choice as it can survive up to two disk failures.
Alternatives to Consider
When evaluating RAID 5, it’s important to also consider some alternative RAID configurations that offer different tradeoffs. Two common alternatives to RAID 5 are RAID 6 and RAID 10.
RAID 6 requires a minimum of 4 drives and uses double distributed parity to protect against the failure of up to two drives. This provides better fault tolerance than RAID 5, but write performance suffers due to the double parity calculation. RAID 10 mirrors data across drives and also stripes across mirrored sets, providing faster performance but less overall storage capacity.
The choice between RAID 5, RAID 6, and RAID 10 depends on your specific priorities like performance, capacity, and redundancy needs. RAID 6 offers better protection than RAID 5 while RAID 10 can provide faster performance. Evaluating your options can help select the right RAID level for your environment.
Implementation Considerations
When implementing RAID 5, there are some key factors to consider for optimal performance:
Disk Size
Larger capacity disks can impact rebuild times when recovering from a failed drive. Consider using smaller sized disks (e.g. 600GB – 1TB) to minimize potential downtime. See Best Practices for Installing and Configuring RAID Arrays.
RAID Controller Caching
Having an appropriate RAID controller with a battery-backed write cache can significantly boost performance for random writes. Ensure your controller has enough cache for your workload.
Benchmarking
Test the actual performance of your planned RAID 5 array under realistic workloads. Use tools like fio to simulate your actual I/O patterns and measure throughput and latency. Adjust configuration if needed.
When to Use RAID 5
RAID 5 can be a good option in certain scenarios where read performance and storage efficiency are priorities, but it may not be the best choice for performance-critical systems.
RAID 5 offers decent read speeds since data is striped across multiple disks. Writes are slower due to the parity calculation requirement, but RAID 5 still provides better overall performance compared to RAID 1 or RAID 10 with the same number of disks. https://petri.com/raid-5-vs-raid-10/
In terms of storage efficiency, RAID 5 is excellent – you only lose the capacity of 1 disk for parity, compared to 50% capacity loss for RAID 1 mirroring. This makes RAID 5 cost-effective for bulk storage uses where performance is not the top priority.
However, RAID 5 is not recommended for transactional databases or other applications requiring high write performance and fault tolerance. The parity calculation can become a bottleneck for write-intensive loads. Also, rebuilding a failed RAID 5 array is more complex than with RAID 6 or RAID 10.
Overall, RAID 5 offers a good balance of performance, capacity and cost-efficiency for general purposes like file servers and backup systems. But for mission-critical systems where uptime and speed are crucial, RAID 6 or RAID 10 may be better options despite higher hardware costs.