RAID 5 is a popular RAID level that provides increased storage capacity and fault tolerance. In RAID 5, data is striped across multiple disks along with parity information that is distributed across the array. This allows the array to continue functioning even if one of the disks fails. When calculating the total usable capacity in a RAID 5 array, there are some key factors to consider.
What is RAID 5?
RAID 5 stands for Redundant Array of Independent Disks level 5. It is a storage technology that combines multiple disks into a logical unit and distributes parity information across the member disks.
Here are some key characteristics of RAID 5:
- Data is striped across multiple disks in blocks
- Distributed parity allows for fault tolerance
- Minimum of 3 disks required
- High read performance as data is spread across multiple disks
- Write performance reduced compared to RAID 0 due to parity calculations
- Allows for one disk failure without data loss
By distributing parity information across multiple drives, RAID 5 eliminates the single point of failure that exists in RAID 3 and RAID 4. If any single disk fails, the parity blocks on the remaining disks can be used to reconstruct the missing data.
RAID 5 Capacity Calculation
The total usable capacity in a RAID 5 array depends on the number of disks in the array and their individual capacities. Here is the formula to calculate RAID 5 capacity:
Total Capacity = (Number of Disks – 1) x Disk Capacity
Let’s break this down:
- Number of Disks – This is the total number of disks in the RAID 5 array.
- Disk Capacity – The usable capacity of each individual disk in the array.
- Total Capacity – The total space available in the RAID 5 array to store data.
Because one disk worth of capacity is used for parity information, the total usable capacity is equal to the number of disks minus one, multiplied by the capacity of each disk.
Example RAID 5 Capacity Calculation
Here is an example to illustrate the RAID 5 capacity formula:
- Number of Disks: 5
- Disk Capacity: 1 TB each
Applying the formula:
Total Capacity = (Number of Disks – 1) x Disk Capacity
Total Capacity = (5 – 1) x 1 TB
Total Capacity = 4 x 1 TB
Total Capacity = 4 TB
So in this example with 5 disks that are each 1 TB, the total usable capacity in a RAID 5 configuration is 4 TB. The fifth 1 TB disk is used for parity information.
Considerations for RAID 5 Capacity
Here are some other considerations when looking at RAID 5 capacity:
- Disk sizes – Disks in a RAID 5 array do not have to be identical. However, the total capacity will depend on the size of the smallest disk.
- Hard disk vs SSD – Both HDDs and SSDs can be used in a RAID 5 array. SSDs provide faster performance.
- RAID controllers – A hardware or software RAID controller is required to manage the RAID 5 array.
- Drive interface – Common interfaces for RAID arrays include SATA, SAS and NVMe. Faster interfaces provide better performance.
- Spare drives – Having a global or dedicated spare disk allows for quick rebuilding if a drive fails.
What Happens if a Disk Fails in RAID 5?
One of the big advantages of RAID 5 is its fault tolerance. If a single disk fails in the array, the parity information can be used to rebuild the missing data on a replacement drive.
When a disk failure is detected, the RAID software marks the disk as failed and the array switches to a degraded mode. A hot spare or replacement disk can be added to the array and the rebuild process starts automatically using the parity data to reconstruct the data that was on the failed drive.
During the rebuild process, the array is more vulnerable to data loss in case of an additional disk failure. The rebuild time depends on the storage capacity and performance of the disks. Once the rebuild is complete, the RAID 5 array goes back to normal redundant operation.
Advantages of RAID 5
Here are some of the key advantages of using RAID 5:
- Increased capacity – Total capacity is number of disks minus one, so more storage compared to mirroring.
- Fault tolerance – Can survive one disk failure with no data loss.
- High read speeds – Data is striped across multiple disks.
- Low cost – Only requires one disk worth of capacity for parity.
- Standard RAID level – Widely supported by RAID controllers.
Disadvantages of RAID 5
Some potential disadvantages of RAID 5 include:
- Slower writes – Parity calculations can impact write performance.
- No fault tolerance during rebuilds – Vulnerable to data loss during disk rebuilds.
- Potential for data loss with multiple drive failure – UREs during rebuild can cause data loss.
- Not recommended for large capacity disks – Larger rebuild times increase risk.
Alternatives to RAID 5
Some other common RAID levels to consider instead of RAID 5 are:
RAID 10 (RAID 1+0)
- Mirroring + Striping (minimum 4 disks)
- Provides fault tolerance and increased performance
- 50% storage efficiency (2x mirroring)
RAID 6
- Similar to RAID 5 but with double distributed parity
- Can withstand the loss of two disks
- Write performance impacted due to parity calculations
RAID 50
- Combination of RAID 5 and RAID 0
- Stripes data across multiple RAID 5 arrays
- Improves performance compared to RAID 5 alone
Software vs Hardware RAID
RAID can be implemented through dedicated hardware RAID controllers or in software.
Hardware RAID
- Dedicated RAID card or controller
- Handles RAID calculations on the card
- Simple configuration through RAID BIOS
- More expensive but better performance
Software RAID
- RAID is handled by the operating system
- Drives are directly connected to motherboard SATA ports
- CPU has to handle the RAID overhead
- Lower cost, but potential performance impact on the CPU
For most desktop systems, software RAID using Windows, Linux or a NAS OS is sufficient. For busy servers that demand high performance, a dedicated hardware RAID controller is a better choice.
How to Configure RAID 5
The basic steps to configure RAID 5 are:
- Select matching disks for the array – For best results all disks should be same capacity and speed.
- Connect disks to RAID controller – Either direct connect or in an enclosure.
- Enter RAID BIOS to create array – Select RAID 5 and choose disks.
- Allow RAID to initialize – Can take some time depending on capacity.
- Check disk status – Verify RAID reports disks are functioning normally.
- Create partitions/volumes on array.
Many RAID controllers include user-friendly utilities to speed up the process. Be sure to consult your specific controller’s documentation for full configuration instructions.
Software RAID configuration will vary depending on operating system. Most Linux distributions include the mdadm utility. In Windows, storage spaces can create software RAID arrays.
Increasing RAID 5 Capacity
There are a couple options to increase the storage capacity of an existing RAID 5 array:
Replace with Larger Drives
Swapping out the existing disks for larger drives allows you to grow capacity. This can be done one disk at a time by rebuilding the array after each disk replacement.
Add Additional RAID 5 Array
An additional RAID 5 array can be created and then the two arrays combined into a RAID 50 configuration. This allows scaling up capacity without having to rebuild the existing array.
Monitoring and Maintaining RAID 5
To get the most out of a RAID 5 array, it’s important to proactively monitor and maintain it:
- Watch for disk failures and replace failed drives immediately.
- Check disk smart metrics for signs of issues.
- Ensure firmware and drivers are up-to-date.
- Clean air filters and check hardware connections regularly.
- Consider periodic full array scrubbing to check for inconsistencies.
- Monitor rebuild progress when replacing failed disks.
By combining capacity, performance and fault tolerance, RAID 5 provides an appealing option for array-based storage. Taking the time to properly plan, configure and monitor RAID 5 can provide excellent long-term data storage and protection.
Conclusion
Calculating the total usable capacity of a RAID 5 array simply requires the number of disks and their individual capacities. The formula is:
Total Capacity = (Number of Disks – 1) x Disk Capacity
This accounts for the total disk space minus one disk worth used for distributed parity information. RAID 5 provides fault tolerance to drive failure, so long as the failed disk is replaced before a second one fails.
When planning a RAID 5 array, be sure to consider the performance tradeoffs with write speeds and rebuilding arrays. Routine monitoring and maintenance is necessary to gain maximum reliability and lifespan from a RAID 5 configuration.