RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drive components into a logical unit. RAID provides increased storage performance, reliability, and fault tolerance compared to single drives. One of the most commonly used RAID levels is RAID 5.
What is RAID Level 5?
RAID 5 stripes data and parity information across a minimum of three disk drives. The data is broken down into blocks which are written across the drives in the array. Parity information is used to rebuild data in case of a single disk failure. With RAID 5, data can be recovered and the array can continue operating even if one of the drives completely fails.
Some key characteristics of RAID 5:
- Requires a minimum of 3 drives
- Provides fault tolerance – can withstand a single drive failure without data loss
- Good read performance – data is striped across drives
- Write performance reduced compared to RAID 0 due to parity calculations
- Typically provides 100% of advertised capacity for data storage
Minimum Number of Drives
The minimum number of disk drives required for RAID 5 is 3. This allows striping of data across drives with a dedicated drive for parity information. With only two drives, RAID 5 would act like mirrored disks or RAID 1 which does not provide the read performance advantages of data striping.
Factors to Consider for Drive Counts
When determining the appropriate number of disk drives for a RAID 5 array, there are several factors to consider:
Storage Capacity
More disk drives allow for greater overall storage capacity in the array. Need to determine the right balance between capacity, performance and cost when adding drives.
Performance
Increasing drive counts can improve read performance by spreading data across more drives. However write performance may suffer due to parity overhead.
Fault Tolerance
More disk drives mean better fault tolerance. A RAID 5 array with 5 disks can withstand up to 2 drive failures compared to just 1 drive failure with 3 disks.
Rebuild Times
The more disk drives, the longer it takes to rebuild the array in case of a drive failure. Need to determine acceptable rebuild times based on performance needs and likelihood of second drive failure during rebuild.
Cost
More disk drives mean higher storage costs. Need to weigh the benefits of additional drives vs. the extra cost.
Recommended Drive Counts
When designing a RAID 5 array, some commonly recommended drive counts include:
| Number of Drives | Benefits |
|---|---|
| 3 | Minimum number for RAID 5. Lowest cost. |
| 4 | Improves read performance over 3-drive RAID 5. |
| 6 | Enhances performance compared to 3 or 4 drives. Provides good fault tolerance. |
| 8 | Great performance for transactional workloads. Allows for multiple drive failures. |
| 10+ | Large capacity, maximum performance and fault tolerance. But very expensive. |
Some typical drive counts for RAID 5 include 4-drive, 6-drive, 8-drive and 12-drive configurations. The optimal number depends on your specific storage needs and budget.
4 Drive RAID 5
A 4-drive RAID 5 array is a popular choice for many applications. It provides improved read performance compared to a 3-drive setup. It also allows for 1 drive failure while still protecting all data. The relatively small number of drives also keeps overall storage costs down.
6 Drive RAID 5
A 6-drive RAID 5 configuration offers faster performance and the ability to handle up to two drive failures. The increased number of drives also allow for larger overall storage capacity. Many administrators prefer 6-drive RAID 5 for critical databases and applications that require both performance and higher fault tolerance.
8 Drive RAID 5
An 8-drive RAID 5 array provides excellent performance, especially for transactional workloads involving many small writes. The larger number of drives increases fault tolerance, allowing the array to survive up to 3 drive failures with no data loss. The downside is higher cost compared to smaller arrays.
When to Choose Larger Drive Counts
Some situations where larger drive counts for RAID 5 make sense include:
- Managing large volumes of mission-critical data
- Supporting databases with high transaction rates
- Hosting virtual machines and other enterprise applications
- Providing shared storage for high-availability clustering
- Handling frequent large sequential reads for activities like data analytics
In these types of scenarios, the benefits of higher performance, capacity and fault tolerance justify the additional cost of more disk drives.
Factors that Reduce Optimal Drive Counts
In some cases, lower drive counts may be preferred for RAID 5:
- Budget constraints limit purchase of large numbers of expensive drives
- Smaller storage capacity requirements
- Older, slower drives already owned and available for the array
- Less critical data where high performance and redundancy are not required
When these factors exist, a 3 or 4 drive RAID 5 array may be sufficient.
RAID 5 Drive Count Guidelines
Based on the key considerations and scenarios above, here are some general guidelines for selecting drive counts for RAID 5 arrays:
- 3 drives – Minimum required, only for non-critical data
- 4 drives – Balance of cost and performance for most applications
- 6 drives – Sweet spot for critical data, balances performance, capacity and redundancy
- 8 drives – For transactional workloads needing very high performance and redundancy
- 10+ drives – Only for specialized apps that demand maximum capacity, speed and redundancy
Factors that can Change Recommended Counts
The drive count guidelines above assume the use of modern high capacity SATA or SAS drives. Some factors that could alter the recommended drive numbers include:
- Larger capacity drives allow lower counts to achieve needed storage space
- Faster flash drives like SSDs can provide speed with fewer disks
- Older, smaller disks would need higher counts to get capacity
- Slower RPM disks require more drives for performance
Always take the specs of your actual drives into account when designing the array.
Converting Existing Arrays
When converting existing disk arrays to RAID 5, you may be limited by the fixed drive bays and connections in your server or enclosure. Still aim for a minimum of 3 drives, and if possible expand to 4 or more drives over time as needs change.
Testing Different Drive Counts
When optimizing RAID 5 storage for your environment, it can be helpful to test arrays with different disk counts. Benchmark performance for your actual workloads. Measure rebuild times. Try storing a copy of your data on different configs.
This real-world testing can validate which drive count provides the ideal RAID 5 performance, protection and value.
Conclusion
The optimal number of disk drives for RAID 5 depends on your storage needs, but typically ranges from 4 to 8 drives. Three drives is the absolute minimum to implement RAID 5. Larger drive counts improve performance, capacity, and fault tolerance but add cost. Carefully weigh the benefits versus extra expense when choosing the right number of disks for your RAID 5 array.
