RAID 5 is a type of redundant array of independent disks (RAID) that uses distributed parity to provide fault tolerance and improve performance. In RAID 5, data is striped across all the disks in the array, and parity information is distributed across the disks as well. This allows the array to continue functioning even if one disk fails. The total storage capacity of a RAID 5 array depends on the number of disks in the array and the size of each disk.
RAID 5 Storage Capacity Calculation
The total storage capacity of a RAID 5 array can be calculated with the following formula:
Total Capacity = (Number of Disks – 1) x Size of Smallest Disk
For example, let’s say you have 5 disks in your RAID 5 array, each with a capacity of 2TB. Using the formula:
Total Capacity = (5 – 1) x 2TB = 8TB
So in this example, the total usable capacity of the RAID 5 array is 8TB. One disk’s worth of capacity is used for parity information, leaving the remaining disks’ capacity for data storage.
Factors Affecting RAID 5 Capacity
There are several factors that can affect the total usable capacity of a RAID 5 array:
- Number of disks – More disks means higher capacity, but one disk is used for parity
- Disk capacity – Larger disks increase total capacity
- Unallocated spare disks – Hot spares don’t add to capacity
- File system overhead – File system structures use a small percentage of space
In general, make sure to account for parity, hot spares, and file system overhead when calculating the usable capacity of a RAID 5 array.
RAID 5 Capacity Calculator
To make it easy to figure out the total capacity of a RAID 5 array, you can use an online RAID calculator. Here are the steps:
- Go to a RAID calculator like https://wintelguy.com/raidmttdl.pl
- Enter the number of disks in your planned array
- Enter the capacity of each disk
- Select “RAID 5” as the RAID level
- The calculator will output the total capacity
This can save you having to do the math yourself. Be sure to use the same capacity for each disk to get an accurate estimate.
RAID 5 Capacity Examples
Here are some examples of RAID 5 array capacities with different disk configurations:
| Number of Disks | Disk Capacity | Total Capacity |
| 3 | 1 TB | 2 TB |
| 4 | 2 TB | 6 TB |
| 5 | 4 TB | 16 TB |
| 6 | 8 TB | 40 TB |
As you can see, both increasing the number of disks and the capacity of each disk results in higher total capacity for the array.
RAID 5 Capacity vs. Other RAID Levels
Compared to other common RAID levels, RAID 5 provides a middle-ground of capacity efficiency:
- RAID 0 – Full capacity of all disks, no redundancy
- RAID 1 – 50% capacity (2 disks mirror each other)
- RAID 5 – 1 disk worth of capacity lost for parity
- RAID 6 – 2 disks worth of capacity lost for dual parity
- RAID 10 – 50% capacity (mirrored pairs in a stripe set)
RAID 5 requires a minimum of 3 disks, since parity information needs to be spread across multiple disks. It provides a good balance of redundancy and storage utilization compared to other RAID levels.
Real-World RAID 5 Sizes
In practice, 12-24 disk RAID 5 arrays with large capacity (4TB+) nearline SAS or SATA disks are common in business storage systems. This allows total capacities in the 100TB+ range.
For example, a 24 disk array with 8TB disks would provide:
Total Capacity = (24 – 1) x 8TB = 184TB
Larger arrays are certainly possible as disk densities continue to increase over time. When deployed with hot spares, the usable capacity would be slightly less.
RAID 5 and Larger Disks
As hard disk capacities increase, the rebuild time for a failed drive also increases proportionally. This increases the risk of a second disk failure during a RAID 5 rebuild, which would result in data loss.
For this reason, larger RAID 5 implementations may incorporate additional fault tolerance like:
- Larger arrays (24+ disks)
- More hot spares
- Higher reliability enterprise HDDs
- Periodic scrubbing to check data integrity
Alternatives like RAID 6 or RAID 10 may also be considered for better performance and fault tolerance when using large capacity disks.
Software vs. Hardware RAID 5
RAID 5 can be implemented via hardware RAID controllers or in software at the operating system level. Hardware RAID provides better performance, while software RAID offers lower cost and platform flexibility.
For software RAID 5, a minimum of 1GB RAM per 1TB of storage is recommended. Hardware RAID 5 controllers handle the parity calculations on their own processors and memory.
RAID 5 Performance
RAID 5 provides better performance than RAID 1 or RAID 10 for large sequential reads and writes. However, small random I/O performance can suffer due to the parity calculation overhead. Databases and other applications requiring high random I/O may benefit from RAID 10 instead.
Still, for general file and application storage, RAID 5 offers a good balance of capacity, performance, and redundancy for most use cases.
RAID 5 Rebuild Time
When a disk in a RAID 5 array fails, the controller must rebuild the data and parity on the new replacement disk. The rebuild time depends on the total capacity as well as the controller and disk performance. As a general guideline:
- HDD arrays – 24 hours per TB
- SSD arrays – 1-2 hours per TB
So a 10TB HDD based RAID 5 would take approximately 10 days to rebuild after a disk failure. This is why it’s crucial to minimize the chance of a second disk failure during this time.
Improving RAID 5 Reliability
If uptime and data protection are critical, here are some ways to improve RAID 5 reliability:
- Enterprise HDDs with longer MTBF ratings
- Hot spare drives to immediately start rebuilding
- Disk scrubbing to proactively detect errors
- Monitoring tools to get early warnings
- Smart problem detection and automated alerts
- Redundant controllers for no single point of failure
Additional RAID best practices like using homogeneous disks and scheduled drive replacements will also maximize availability.
When to Use RAID 5
RAID 5 is ideal for use cases that require:
- Redundancy for one drive failure
- Better read performance for large transfers
- More affordable capacity compared to mirrors or dual parity
Common RAID 5 applications include:
- File servers
- Backup storage
- Media servers / NAS
- Archival storage
Any environment that needs capacity protection without the 50% overhead of RAID 1 or RAID 10 mirrors can benefit from deploying RAID 5.
When to Avoid RAID 5
RAID 5 may not be the best choice when:
- Uptime and fault tolerance are critical
- Rebuilding large arrays would take too long
- Frequent random writes are needed
Mission critical transactional databases, virtualization hosts, and high performance applications may benefit more from RAID 10 or RAID 6 instead.
RAID 5 vs. RAID 6
Compared to RAID 6, RAID 5 offers more usable capacity but less redundancy. RAID 6 can withstand two simultaneous disk failures by using a second distributed parity disk.
RAID 5 rebuild times also tend to be faster than RAID 6, as there is only one parity disk to reconstruct in the event of a failure. However, with larger arrays and drive capacities, RAID 6 may provide more protection.
RAID 5 vs. RAID 10
RAID 10 combines mirroring and striping for performance and redundancy. However, 50% of the total capacity is lost to redundancy. RAID 5 provides better storage efficiency than RAID 10 in most cases.
However, RAID 10 can outperform RAID 5 for write intensive workloads, as write operations only affect a single mirror drive. RAID 5 requires parity recalculation on writes across all the drives.
Migrating from RAID 5 to RAID 6 or 10
To change redundancy levels, data will need to be migrated from an existing RAID 5 array to a new RAID 6 or RAID 10 array. This involves:
- Provisioning new RAID 6 or RAID 10 array
- Copying data from RAID 5 to new array
- Redirecting applications to new array
- Decommissioning old RAID 5 array
Migrating the data while both arrays are live minimizes downtime. Use of backup snapshots can further reduce risk.
Conclusion
RAID 5 provides a good balance of performance, capacity, and fault tolerance for general storage needs. Total usable capacity is calculated based on the number and size of disks in the array. Larger arrays with higher capacity disks can achieve hundreds of TBs of storage.
Careful RAID design along with reliability best practices allows RAID 5 to be used successfully even with larger disk capacities. When higher availability or performance is required, RAID 6 or RAID 10 may be better options.