RAID 5 requires a minimum of 3 disks to implement, but most implementations use 4 or more disks for improved performance and capacity. The key characteristics of RAID 5 are:
What is RAID 5?
RAID 5 is a storage technology used to provide fault tolerance and improve performance in disk arrays. It uses distributed parity and striping to achieve redundancy and speed.
The main features of RAID 5 are:
- Data striping – Data is split across multiple disks in blocks.
- Distributed parity – Parity information is distributed across all the disks.
- Block-level striping – RAID 5 stripes data at the block level.
With these features, RAID 5 provides fault tolerance by allowing data recovery in case of a single disk failure. It also improves disk I/O performance by spreading reads and writes across multiple disks.
Why does RAID 5 require at least 3 disks?
RAID 5 requires a minimum of 3 disks because of how it implements distributed parity and striping:
- One disk is used to store parity information.
- The remaining disks store data in stripes.
- To implement striping and have parity, you need at least 3 disks – one for parity and two for data.
With just two disks, you cannot stripe data. And with no parity disk, there is no fault tolerance. At minimum 3 disks are needed for the core RAID 5 functionality.
Typical RAID 5 implementations
While RAID 5 technically requires only 3 disks, most implementations use 4, 6, 8 or more disks.
There are several reasons to use more disks:
- Improved capacity – More data disks increase total storage capacity.
- Higher performance – More disks allow greater parallelism.
- Overhead factor – With small disk counts, parity disks take up a large percentage of total disks.
Some common RAID 5 scenarios:
- 4 drives – Typically 3 data disks and 1 parity disk. Lowest cost option while still providing good performance.
- 6 drives – 5 data disks and 1 parity disk. Improved capacity and performance.
- 8+ drives – Most enterprise implementations. Excellent performance and capacity.
While adding more disks improves RAID 5, it also increases rebuild times and potential for multiple disk failures. So very large arrays can be problematic.
How parity works in RAID 5
Parity in RAID 5 provides redundancy and fault tolerance. Here is a high level overview of how it works:
- Parity information is calculated using XOR operations across data on the strips.
- The parity strips are distributed evenly across all the disks.
- If a disk fails, the parity can rebuild the missing data.
- Only one parity strip per stripe set. So RAID 5 can only tolerate a single disk failure.
Some key advantages of distributed parity in RAID 5:
- Allows single disk failure recovery.
- More efficient than mirrors – No need to duplicate all data.
- Better balanced load – Parity is spread out across disks.
The main limitations are:
- RAID 5 arrays can only handle a single disk failure. With more failed drives, data recovery becomes impossible.
- Parity calculation can impact write performance.
- Rebuild times are long for large arrays as all disks must be read to reconstruct lost data.
RAID 5 performance characteristics
RAID 5 provides significantly better performance than a single disk. But it does have some constraints relative to other RAID levels.
The performance profile of RAID 5 arrays includes:
- Reads are fast and efficient since data is striped.
- Writes are slower than RAID 0 due to parity calculation.
- Not well suited for random writes due to Parity Write Penalty.
- Large stripe size improves sequential I/O performance.
In general, RAID 5 excels at operations like data warehousing, streaming, and backups. But it is not ideal for high-volume random write workloads like busy transactional databases.
Alternatives to consider
While RAID 5 is a proven technology, there are some alternatives worth considering:
RAID 6
RAID 6 is like RAID 5 but uses double distributed parity. This allows it to tolerate failure of up to two disks.
RAID 10
RAID 10 combines mirroring and striping for both redundancy and high performance. But it requires more disks.
RAID 50/60
Nested RAID levels that combine multiple RAID 5 arrays into a larger RAID 0 or RAID 6 array.
Erasure Coding
More advanced technique that breaks data into fragments and encodes into a wider array of disk drives.
Storage Spaces
Microsoft’s software-defined storage solution that can use various resiliency mechanisms like parity.
When should you choose RAID 5?
RAID 5 offers a good balance of cost, performance and redundancy for many scenarios. It works well when:
- You need more I/O performance than a single disk can provide.
- Seeking fault tolerance for disk failures.
- Disk capacity efficiency is important.
- Rebuild times are acceptable.
Typical uses cases include:
- File servers and data shares.
- Database servers (non-critical OLTP).
- Data warehousing and analytics.
- Disk-based backups.
When to avoid RAID 5
There are some scenarios where alternatives to RAID 5 would be more appropriate:
- Mission critical transactional databases – Use RAID 10 for better performance.
- High risk of multiple disk failures – Choose RAID 6 instead.
- Need more capacity efficiency – Look at erasure coding solutions.
- Concerned about long rebuild times – Keep RAID 5 arrays small.
- Mainly random writes – Performance will suffer due to parity writes.
Best practices when using RAID 5
Some best practices to follow for RAID 5 deployments:
- Use at least 4 disks to start – Provides good capacity and overhead factor.
- Benchmark workloads – Measure performance to pick optimal stripe size.
- Monitor disk health – Replace failed drives immediately.
- Consider hot spares – Speeds up rebuilds.
- Schedule patrol reads – Identify bad sectors.
- Scrub parity – Detect and correct silent errors.
Following best practices helps ensure RAID 5 can deliver on its potential for performance, redundancy and efficient capacity.
Conclusion
While RAID 5 technically requires only 3 disks, most implementations use 4 or more disks to provide good storage capacity, performance and overhead factor. The distributed parity in RAID 5 provides efficient redundancy along with improved read speeds due to striping. But write speeds can suffer due to parity calculations. Overall, RAID 5 offers a great combination of cost, performance and fault tolerance for many applications. But alternatives like RAID 6, RAID 10 or erasure coding may be better choices for certain workloads or higher availability needs.
