RAID 5 is a popular RAID configuration that provides a good balance of performance, capacity efficiency, and fault tolerance for typical server workloads. In RAID 5, data is striped across multiple drives, just like in RAID 0, but parity information is also calculated and written across the drives. This allows the array to withstand the failure of a single drive without any data loss. However, if too many drives fail, the entire RAID 5 virtual disk will fail.
What is RAID 5?
RAID 5 requires a minimum of 3 drives to implement. Data is striped across the drives in chunks or stripes, just like in RAID 0. However, for each stripe of data, a parity stripe is also calculated using XOR and written to a different drive in the array. For example, if drives 1, 2, and 3 made up a 3-drive RAID 5 array, the first stripe of data could be written to drives 1 and 2, and the parity for that stripe would be written to drive 3. The next stripe would be written to drives 2 and 3, with parity on drive 1, and so on.
This distribution of parity across all the drives allows RAID 5 to withstand the loss of any single drive without data loss. If a drive fails, the parity stripes on the remaining drives can be used to reconstruct the data from the failed drive on the fly. RAID 5 provides fault tolerance with minimal capacity overhead – only one drive worth of capacity is needed for parity. RAID 5 arrays can continue operating in a degraded mode with one failed drive.
How Many Drive Failures can RAID 5 Withstand?
RAID 5 can only withstand a single drive failure. If two or more drives fail at the same time, the RAID 5 array will fail and data will be lost. Here are some scenarios:
- If a single drive fails in a RAID 5 array, there is no data loss. The array remains in a degraded mode but continues working normally. The failed drive needs to be replaced to restore redundancy.
- If a second drive fails in a RAID 5 array before the first failed drive has been replaced, all data in the array will be lost. With two failed drives, the remaining parity data is no longer sufficient to recreate the user data.
- If multiple drives (more than 1) fail at the same time, all data will also be lost for the same reason.
Therefore, the answer is that 2 drives need to fail before a RAID 5 array will fail and lose data. A single drive failure is tolerated.
Why RAID 5 Can Withstand Only 1 Drive Failure
To understand why RAID 5 can withstand only one drive failure, you need to understand how the parity data works in RAID 5:
- Parity data is calculated by taking the XOR (exclusive OR) of all the data in a parity stripe.
- If a drive fails, the data on it can be reconstructed by taking the XOR of the data on the remaining drives and the parity drive.
- However, this requires that all the other data drives and the parity drive are still intact.
- If a second data drive fails, the XOR can no longer be calculated with the remaining data because a portion of the data is now missing from two drives.
For a more concrete example, consider a 3-drive RAID 5 array with data drives A, B and parity drive P. The parity data is calculated as:
P = A XOR B
If drive A fails, we can calculate:
A = P XOR B
However, if drive B also fails, there is no way to reconstruct A since neither A nor B is available to XOR with P. This is why the whole RAID 5 array fails with 2 drive failures.
Factors that Impact RAID 5 Reliability
While RAID 5 can only withstand a single drive failure, there are several factors that can impact the likelihood of a second drive failing during a degraded rebuild:
1. Number of Drives in the Array
The more drives in the array, the higher the probability that a second drive will fail during a RAID 5 rebuild. Rebuild times take longer with more drives, exposing the array to a greater risk of double drive failure.
2. Drive Capacity
Higher capacity drives take longer to rebuild in RAID 5 than smaller drives. The longer rebuild times increase the chance of a second failure occurring.
3. Rebuild Times
How long rebuilds take directly impacts the exposure to double drive failures. Slower rebuilds mean RAID 5 is degraded longer during more stress on the remaining drives.
4. Drive Technology
Some drive types like nearline SAS or SATA have higher annualized failure rates than enterprise class drives. This can increase the likelihood of a second failure during a RAID 5 rebuild.
5. Drive Age
Older drives are more prone to failure than newer ones. Mixing old and new drives in RAID 5 increases the chances of a double failure if an old drive fails first.
6. Temperature & Ventilation
Hot-running drives are more prone to failure. Good ventilation to keep drives running cooler reduces the chances of failures during rebuild.
7. Workload & Utilization
Heavily loaded arrays that run drives at high utilization for extended periods have a higher risk of multiple drive failures occurring.
What Happens During a RAID 5 Rebuild?
When a drive fails in RAID 5, a rebuild must take place to restore data redundancy. Here is what happens:
- The RAID controller detects the drive failure and disables access to the failed drive.
- The controller switches the RAID 5 array into degraded mode.
- A spare drive is used if available, or the failed drive is physically replaced with a new one.
- The controller begins reading all data from the surviving drives and calculating the missing data using XOR parity.
- The reconstructed data is written to the replacement drive.
- The process continues until the entire failed drive has been rebuilt.
- When finished, the array goes back into normal redundant operation.
The rebuild process puts additional strain on the surviving drives as they are accessed continuously to reconstruct the data for the replacement drive. This is why rebuild times are a critical factor.
How to Improve Reliability in RAID 5
If uptime and redundancy are critical, RAID 5 may not provide adequate protection in large arrays. However, there are things that can improve reliability:
- Use enterprise-class drives designed for RAID environments.
- Reduce rebuild times by limiting drive capacities in the array.
- Use hot spares to reduce rebuild times.
- Use RAID 6 dual parity for added protection.
- Monitor drive health and replace deteriorating drives early.
- Ensure proper ventilation, cooling and temps in the server.
- Consider replacing RAID 5 with RAID 10 for better performance and redundancy.
When to Avoid RAID 5
Due to the risk of data loss from double failures, RAID 5 may not be suitable in certain scenarios:
- Arrays with a very large number of high capacity drives – the probability of 2 failures during rebuild gets too high.
- Arrays using consumer-grade SATA or nearline SAS drives which have higher failure rates.
- Mission-critical data that cannot have any downtime or risk of data loss.
- Drive rebuild times exceed 24 hours – this exposes the array to failures too long.
- No hot spares or inability to replace failed drives quickly.
In these situations, RAID 6 or RAID 10 would be a better choice than RAID 5 despite the increased cost.
Differences Between RAID 5, RAID 6 and RAID 10
| RAID 5 | RAID 6 | RAID 10 | |
|---|---|---|---|
| Minimum Drives | 3 | 4 | 4 |
| Available Capacity | (n-1) Capacity | (n-2) Capacity | (n/2) Capacity |
| Drive Fault Tolerance | 1 Drive | 2 Drives | 1-2 Drives (depends on config) |
| Rebuild Times | Longer | Longest | Faster |
| Performance | Medium | Medium | Best |
| Best Use Cases | Archival storage, media libraries | Large arrays needing high redundancy | Performance-critical applications |
In summary, RAID 6 provides an extra parity drive for dual fault tolerance but at a capacity penalty. RAID 10 provides faster performance by mirroring but uses 50% of capacity for redundancy. For better performance and redundancy than RAID 5, consider RAID 10 or RAID 6.
Should I use RAID 5 or RAID 10?
Choosing between RAID 5 and RAID 10 depends on your specific needs:
Reasons to use RAID 5:
- Lower cost as it requires fewer drives.
- Higher storage efficiency – RAID 10 has a 50% capacity overhead for redundancy.
- Sufficient redundancy for some applications.
Reasons to use RAID 10:
- Faster performance – RAID 10 allows multiple drive access for reads and writes.
- Can survive multiple drive failures if in the right configuration.
- Rebuilds are faster as there are fewer drives to process.
- Higher reliability – avoids rebuild issues associated with RAID 5.
In summary:
- Use RAID 5 if cost and storage efficiency are the primary goals.
- Use RAID 10 for better performance and redundancy for critical data.
RAID 5 Performance Issues and Optimizations
RAID 5 can suffer from performance issues that get worse as arrays grow larger:
- Slower random writes – Extra parity calculations can significantly slow small random writes.
- Slow reads during rebuilds – Rebuilds block services reading data until the rebuild completes.
- Slower sequential reads – Reading large sequential blocks incurs extra I/O for parity drives.
However, there are techniques to optimize RAID 5 performance:
- Use battery-backed write caches to speed up random writes.
- Enable read-modify-write optimization to avoid full stripe writes.
- Spread the parity evenly across all drives.
- Keep drives defragmented and use proper stripe sizes.
- Ensure sufficient disk controllers and backend I/O bandwidth.
Benchmarking tools can help analyze performance bottlenecks. Profiling access patterns allows optimizing RAID parameters.
Newer Alternatives to RAID 5
Due to the rebuild issues with large RAID 5 implementations, newer technologies have emerged as alternatives:
Erasure Coding
More efficient mathematical techniques to generate and distribute parity data across many drives. Allows faster rebuilds.
RAID 6
Dual parity provides fault tolerance up to 2 drive failures. Useful for large arrays.
Distributed Software RAID
Software-defined RAID across networked drives and nodes. No single point of failure. Self-healing capabilities.
Object/Cloud Storage
Distributed storage usingtechnologies like erasure coding under the hood. High redundancy without RAID.
Conclusion
In summary, RAID 5 requires at least 2 drive failures before complete array failure, as it can withstand a single drive failure through parity. However, the risk of a second failure during rebuild increases with more drives. Newer technologies like RAID 6, better erasure coding, and distributed software RAID provide more reliable options. In general, avoid RAID 5 for large, mission-critical arrays unless steps are taken to minimize rebuild times and improve fault tolerance.