RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drive components into a logical unit. RAID 5 is a common RAID configuration that provides data redundancy and fault tolerance using distributed parity.
The minimum number of disks required for a RAID 5 configuration is 3 (three).
What is RAID?
RAID (Redundant Array of Independent Disks) is a technology that combines multiple physical disk drive components into one or more logical units for the purposes of data redundancy, performance improvement, or both.
RAID configurations come in different standardized levels or types, numbered RAID 0, RAID 1, RAID 5, RAID 6, etc. Each RAID level provides different tradeoffs between key factors like data reliability, fault tolerance, I/O performance, and efficient use of storage capacity.
Key Advantages of RAID
- Increased data reliability and fault tolerance – RAID allows recovery of data if one drive fails.
- Improved I/O performance – Spreading I/O across multiple drives improves speed.
- Increased storage capacity – RAID combines smaller drives into a larger logical unit.
What is RAID 5?
RAID 5 is one of the most popular RAID levels used in business and enterprise settings. It strikes a balance between data reliability, fault tolerance, and efficient storage capacity usage.
RAID 5 works by distributing parity information evenly across all the disks. Parity refers to calculated error-correcting information that can be used to reconstruct data in case of disk failure. With RAID 5, the parity information is spread evenly across all drives.
Key Characteristics of RAID 5
- Data is striped across all disks, providing fast I/O performance.
- Parity is distributed across all disks for redundancy.
- Can withstand the failure of 1 disk without data loss.
- Minimum 3 disks required.
Minimum Number of Disks for RAID 5
The minimum number of disk drives required for RAID 5 is 3 (three). Here’s why:
RAID 5 works by striping data evenly across all disks in the array. It also writes parity information that gets distributed across all the disks. The parity allows the array to tolerate the failure of 1 disk drive by reconstructing the lost data from the remaining data and parity.
For this distributed parity to work properly, the RAID 5 array needs a minimum of 3 disks:
- Disk 1 – Holds actual data
- Disk 2 – Holds actual data
- Disk 3 – Holds distributed parity information
If any 1 of the 3 disks fails, the missing data or parity can be calculated from the remaining 2 disks.
With just 1 disk, RAID 5 would not provide any parity or redundancy. With 2 disks, there is no location to store the distributed parity.
Therefore, 3 disks is the absolute minimum required to implement RAID 5’s distributed parity scheme and gain fault tolerance against a single disk failure.
RAID 5 Example With 3 Disks
Here is a visual example to illustrate how data and parity is laid out in a 3-disk RAID 5 array:
This shows how a RAID 5 array with the minimum 3 disks would distribute the data and parity. Data blocks A1, A2, A3, B1, B2, and B3 are striped across the first two disks. The parity blocks P1, P2, and P3 are spread evenly across all three disks.
If any of the 3 disks were to fail, the missing data or parity blocks could be recalculated from the remaining disks. This is how RAID 5 provides fault tolerance with the minimum number of disks.
When More Disks Are Required
While the technical minimum number of disks for RAID 5 is 3, most practical implementations use more disks. Here are some reasons why:
- Larger overall storage capacity – More disks allow for larger arrays.
- Increased performance – More disks allow higher I/O bandwidth.
- Greater fault tolerance – Can sustain multiple disk failures with enough parity.
For business critical systems that require high capacity, performance and maximum fault tolerance, RAID 5 arrays are typically deployed with 5, 6, 7 or more disks.
Using more disks also enables RAID 5 to recover from multiple disk failures. With the minimum 3 disks, only 1 disk failure can be tolerated. But with additional disks for parity, RAID 5 can withstand 2 or even 3 concurrent disk failures before data loss occurs.
Typical Number of Disks in RAID 5
- Minimum: 3 disks
- Small/entry level: 4 to 6 disks
- Medium business use: 6 to 8 disks
- Large business/enterprise use: 8+ disks
While not a hard requirement, most real-world RAID 5 implementations will use more than the theoretical 3 disk minimum for increased storage capacity, performance and redundancy.
Differences from RAID 1
It’s worth comparing the minimum disks required for RAID 5 versus RAID 1, another popular RAID level.
RAID 1 uses disk mirroring to create an exact copy of data from one drive to another. This provides full redundancy but doubles the required storage capacity.
The minimum number of disks required for RAID 1 is 2 – one as the primary disk, the other as a mirrored copy.
Compared to RAID 1, RAID 5 is more efficient in its use of disks since it doesn’t require full duplication. The distributed parity approach provides redundancy without as much capacity overhead.
However, RAID 1 offers faster read performance and more straightforward drive rebuilds when a disk fails. RAID 5 rebuild times can be more complex.
Both RAID 5 and RAID 1 require a minimum of 3 total disks to allow for a failed drive to be hot swapped with a new replacement.
RAID 5 with Hot Spare Drives
Many RAID 5 implementations will designate one additional drive as a hot spare. This is an extra standby disk that can automatically replace a failed drive and start rebuilding the data.
The advantage of having a hot spare is that it saves the delay of waiting for an admin to manually swap out the bad drive. The rebuild process can start immediately, minimizing downtime.
With a hot spare drive, the minimum number of physical disks needed for a RAID 5 array becomes 4:
- Disks 1 & 2: Active data disks
- Disk 3: Active parity disk
- Disk 4: Hot spare disk
The hot spare itself does not contribute storage capacity or I/O performance until it is swapped in. But it provides an automatic failover safeguard.
Software vs Hardware RAID 5
RAID 5 can be implemented either through dedicated hardware RAID controllers, or via software-based RAID in the operating system.
Hardware RAID 5 uses physical RAID controller cards with a processor dedicated to managing the RAID calculations and redundancy. This provides the best performance but at increased cost.
Software RAID 5 is configured at the operating system level, without specific RAID hardware. This uses the system CPU for processing so has slightly lower performance, but costs less.
In both hardware and software RAID 5, the minimum number of physical disks remains 3. The implementation method does not change the fundamental calculation and parity requirements.
Minimum Disk Size for RAID 5
There is no minimum disk size requirement as part of the RAID 5 specification itself. However, factors like performance, cost efficiency, and rebuilding considerations impact the practical minimum disk size:
- Performance – Larger disks typically have better performance characteristics.
- Cost – Smaller disks tend to have lower cost efficiency in terms of $/GB.
- Rebuild time – Rebuilding small disks can take proportionally longer.
As a result, most RAID 5 implementations use enterprise class disks in the 100GB to 1TB+ size range. Using consumer-grade disks below 100GB is not ideal, but technically possible in smaller RAID 5 configurations.
Minimum Disk Performance for RAID 5
There are no hard technical requirements for minimum disk performance speeds in a RAID 5 array. However, there are practical considerations:
- Rotational disks should be 7200 RPM for adequate performance.
- SATA disks are suitable for low to medium workloads.
- SAS or NVMe SSD disks recommended for high workload systems.
- Mix of disk types in one array can impede performance.
Using slow disks in RAID 5 can lead to perceived sluggishness as the increased I/O of the array exposes disk weaknesses. Enterprise class disks with higher spindle speeds, low latency and high IOPS capabilities are strongly encouraged for RAID 5 installs under heavy workloads.
Can SSDs Be Used for RAID 5?
Solid State Drives (SSDs) can absolutely be used as part of a RAID 5 array. Here are some considerations when using SSDs for RAID 5:
- Use enterprise SSDs for reliabilty and sustained performance.
- Consumer SSDs work but have higher failure rates.
- SSDs eliminate seek time delays for faster response.
- Cost is higher than traditional hard disks.
- SSDs can shorten rebuild times due to speed.
The superfast seek times of SSDs can mitigate some of the write penalty intrinsic to RAID 5 parity calculations. All-flash RAID 5 arrays are excellent for applications requiring very high IOPS, throughput and low latency.
Ideal RAID 5 Scenarios
Due to its balance of performance, capacity efficiency and cost, RAID 5 is well suited for a variety of applications including:
- Database servers and OLTP environments
- Business application servers
- Virtual machine storage and hypervisors
- Medium sized web and app servers
- Disk-based backup environments
Scenarios that demand high read IOPS performance along with excellent sequential speeds work well with RAID 5. The array can utilize multiple disks in parallel to improve responsiveness under high traffic loads.
When Not to Use RAID 5
RAID 5 may not be the best choice in these situations:
- Very high write-heavy workloads – parity penalty can impact performance
- Very large arrays – rebuild times become extremely long
- Critical systems needing maximum fault tolerance
- Distributed applications with very large files
Due to the nature of RAID 5’s parity scheme, write-heavy workflows can lead to slower performance. And large arrays with a high number of disks increase the risk of multiple concurrent failures as well as skyrocketing rebuild times. For these scenarios, other RAID levels like RAID 10 are more appropriate.
The minimum number of disks required for RAID 5 is 3. This allows the distributed parity scheme to function and provide redundancy and fault tolerance against a single disk failure.
While viable, most real-world deployments will use a larger number of disks for increased capacity, performance and redundancy. Typical RAID 5 arrays have between 4 to 8+ disks depending on the required workload capabilities and storage needs.
By balancing performance, capacity efficiency and cost, RAID 5 is a popular choice for a wide variety of business IT applications and data storage use cases where uptime and reliability are important factors.