RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drive components into a logical unit. RAID allows data to be distributed across multiple disks, providing increased performance, capacity, and reliability.
One of the most commonly used RAID configurations is RAID 5. RAID 5 provides a great balance of performance, capacity efficiency, and redundancy for many applications. But how many disk volumes make up a RAID 5 array?
Quick Answer
A RAID 5 array requires a minimum of 3 disk drives, but can support many more drives. The data is striped across all drives in the array, providing fast performance. Parity information is also calculated and written across the drives to allow for drive failure tolerance.
What is RAID 5?
RAID 5 is what is known as a distributed parity RAID configuration. It uses block-level striping with distributed parity. This provides high performance and the ability to reconstruct data if a drive fails.
Here are some key characteristics of RAID 5:
- Data is striped across all drives in large chunks (stripes)
- Parity information is calculated and written across different drives
- Can withstand the failure of 1 drive without data loss
- Requires a minimum of 3 drives
- Very high read performance due to load balancing across drives
- Write performance is slower than RAID 0 due to parity calculation
RAID 5 is commonly used for transaction processing, relational databases, enterprise applications, and other workloads that require high random read performance along with drive fault tolerance. It offers a great balance of cost, performance, and redundancy.
RAID 5 Volumes
So how many volumes or drives make up a RAID 5 array? The minimum number of physical disk drives required for RAID 5 is 3. However, most practical implementations use many more drives.
A RAID 5 array uses the capacities of all drives to stripe data across them in chunks. It then calculates parity information based on the data chunks, and writes the parity onto a different drive. This distributes the parity evenly across all the drives.
Let’s look at an example with 3 drives:
- Drive 1 contains Data A and Parity B
- Drive 2 contains Data B and Parity C
- Drive 3 contains Data C and Parity A
If any single drive fails, the missing data or parity can be calculated from the remaining drives. This is how RAID 5 provides fault tolerance.
Minimum of 3 Drives
From this example, you can see that a minimum of 3 physical drives are required for a RAID 5 array. This allows data and parity segments to be spread evenly across the drives.
With only 2 drives, it would not be possible to have distributed parity – the loss of a single drive would result in complete data loss. So the minimum number of drives for RAID 5 is 3.
More Drives Improves Performance
While 3 drives is the minimum, most practical RAID 5 implementations use more drives. Adding more drives improves performance by distributing the load across more spindles. This allows more I/O operations to take place in parallel.
More drives also increase total storage capacity. With each drive added, the overall storage capacity of the array grows.
The performance and capacity improvements make larger drive counts ideal for most RAID 5 implementations.
Common Drive Counts
Here are some common drive counts used for RAID 5 arrays:
- 3 drives – Minimum required
- 4-8 drives – Typical small office/home office configurations
- 8-16 drives – Common for small to medium business storage
- 16-32 drives – Used to provide massive capacity and performance
Enterprise and datacenter storage systems may use even larger drive counts in the 100s for managing huge datasets.
RAID 5 Performance
One of the main benefits of RAID 5 is excellent read performance. The data is spread evenly across all the drives, allowing multiple drives to be read in parallel. This provides near linear increases in read speed as more drives are added.
However, RAID 5 write performance is slower than RAID 0. This is because with every write, parity must be calculated and written to one of the drives. This requires additional I/O operations that lower overall write speed.
Still, RAID 5 offers great performance for transactional and database workloads that are optimized for random reads. The table below compares RAID 5 performance to other common RAID levels:
| RAID Type | Read Speed | Write Speed |
|---|---|---|
| RAID 0 | Very High | Very High |
| RAID 5 | High | Medium |
| RAID 6 | Medium | Low |
| RAID 10 | Very High | High |
Based on the performance characteristics, RAID 5 works very well for read-intensive workloads. The increased drive counts also provide plenty of capacity for datasets commonly used in business applications.
RAID 5 Reliability
A key advantage of RAID 5 is its reliability and fault tolerance. The distributed parity allows the array to withstand a single drive failure without any data loss.
If a drive fails, the RAID controller can instantly re-calculate the missing data using the parity segments on the other drives. This allows normal operation to continue until the failed drive is replaced.
RAID 6 offers double parity and can withstand 2 drive failures. But for most small to medium applications, the single drive redundancy of RAID 5 is sufficient. Larger arrays may prefer RAID 6 for the added protection.
Rebuilding RAID 5 Arrays
When a failed drive gets replaced in a RAID 5 array, the controller initiates a rebuild process. This reads all the data and parity from the good drives to reconstruct the data onto the replacement drive.
The larger the RAID 5 array, the longer the rebuild takes. Large arrays may take many hours to completely rebuild onto the new drive.
During this time, the array is vulnerable to a second drive failure. If that happens, data loss can occur. This vulnerability window gets larger as the array size increases.
Preventing Drive Failures
To minimize the chances of drive failure, it’s important to:
- Use enterprise-grade drives designed for RAID
- Monitor drive health statistics
- Follow a regular replacement schedule (every 3-5 years)
- Ensure proper cooling and ventilation
- Use an uninterrupted power supply (UPS)
- Keep firmware up to date
Taking these precautions will help maintain maximum uptime and availability.
RAID 5 Array Management
Managing RAID 5 arrays requires careful consideration. The operating system, RAID controller, and management tools all play a role.
Hardware vs Software RAID
RAID can be implemented via:
- Hardware RAID – Uses a dedicated RAID controller card with onboard processor
- Software RAID – Managed by OS and drivers, uses CPU
Hardware RAID provides the best performance since it has dedicated processing. But software RAID can work well for smaller arrays. The choice depends on budget, performance needs, and other factors.
RAID Controller Features
When selecting a hardware RAID controller, look for features like:
- Support for large drive capacities
- Cache to improve write performance
- Battery backup for cache data
- RAID management software
- Monitoring capabilities
- Support and warranty services
Leading vendors include Dell, HP, Adaptec, and LSI Logic. Select an appropriate RAID controller based on your budget, platform, and performance requirements.
OS and Filesystem Selection
The operating system and filesystem also factor into managing RAID arrays. Considerations include:
- OS support for software vs hardware RAID
- Filesystem maturity and RAID integration
- Volume management flexibility
- Compatibility with virtualization
Windows, Linux, and other enterprise OSes include utilities for managing RAID. The choice depends on infrastructure and application needs.
Choosing RAID Levels
When planning a RAID implementation, there are several factors to consider when choosing the RAID levels:
- Read/write performance – RAID 0 provides the best overall throughput. RAID 5 optimizes reads.
- Redundancy – RAID 1 and 5 provide single drive fault tolerance. RAID 6 allows two drive failures.
- Cost – RAID 0 provides the maximum usable capacity for the cost. Mirroring and parity lower usable capacity.
- Number of drives – Minimum drives are required for different RAID levels.
For most general-purpose applications, RAID 5 or RAID 10 provide a good blend of performance, capacity, and redundancy. But the exact choice depends on budget, I/O patterns, and availability needs.
Combining RAID Levels
In larger storage environments, it may make sense to combine multiple RAID levels to create tiered storage pools.
For example, combining:
- RAID 10 for critical databases
- RAID 5 for virtual machine storage
- RAID 6 for archival data
This allows you to optimize each storage pool for specific needs. Tiering can provide the ideal balance of cost, performance, and redundancy.
Conclusion
To summarize, a RAID 5 array requires a minimum of 3 drives, but typically uses more drives for enhanced performance and capacity. Data is striped across the drives for high speed access, while parity provides fault tolerance from drive failures.
RAID 5 works very well for transactional databases, enterprise applications, virtualized servers, and other environments that can take advantage of its distributed performance and redundancy. It offers an excellent combination of cost efficiency and data protection for small to large-scale storage needs.
By understanding how RAID 5 works and the factors involved in managing arrays, you can determine if it is a good solution for your storage and workload requirements.