RAID 5 is a popular RAID (Redundant Array of Independent Disks) configuration that provides data redundancy and fault tolerance using parity data distributed across multiple disks. A common question for those setting up RAID 5 arrays is what the minimum number of disks required is. The quick answer is that yes, RAID 5 does require a minimum of 2 disks.
What is RAID 5?
RAID 5 is a RAID level that uses distributed parity to provide redundancy and fault tolerance. It requires a minimum of 3 disks – data is striped across disks in blocks, and parity data calculated from the data blocks is written across the disks. The parity blocks are distributed amongst the disks rather than being on a dedicated parity disk.
This distribution of parity data provides protection against loss of data due to a single disk failure. If a disk fails, the missing data blocks can be calculated from the parity blocks on the other disks. RAID 5 requires at least 3 disks since parity data needs to be distributed across multiple disks.
Why Does RAID 5 Require a Minimum of 2 Disks?
There are a few key reasons why RAID 5 requires at least 2 disks:
- Parity data distribution – As mentioned above, RAID 5 distributes parity data across multiple disks. To do this effectively it needs at least two disks for parity data to be spread across.
- Single disk fault tolerance – A key aim of RAID 5 is to provide fault tolerance in case of a single disk failure. To achieve this, data and parity must be spread across multiple disks. With just 1 disk, no fault tolerance is provided.
- Inefficiency with fewer disks – RAID 5’s parity scheme introduces some storage overhead. With just two disks, half the total capacity would be taken up by parity data which is very inefficient.
In summary, to distribute parity data, provide single disk fault tolerance, and avoid extreme inefficiency, RAID 5 requires a minimum of 2 disks.
What Happens with Just 1 Disk?
With just 1 disk, RAID 5 cannot be implemented at all. The core mechanisms of data striping with distributed parity simply do not work with a single disk.
There is no way to distribute parity data across disks if there is only one disk. There is also nowhere to obtain redundant data from in the event of a failure. As a result, a single disk RAID 5 array does not provide any fault tolerance or redundancy.
At best, a single disk RAID 5 configuration would function the same as RAID 0 – the disk is just used as standalone non-redundant storage. But it would not have any of the core functionality expected of RAID 5.
Why Single Disk RAID 5 is Not Possible
To summarize, single disk RAID 5 is not possible because:
- There is nowhere to distribute or store parity data
- No redundancy or fault tolerance is provided
- RAID 5’s parity and striping mechanisms require multiple disks
A minimum of 2 disks are required just to be able to implement the fundamental mechanisms that RAID 5 is designed around. With a single disk, it simply does not work at all.
Minimum Disk Requirements for RAID 5
To recap, the minimum disk requirements for RAID 5 arrays are:
- Minimum 2 disks – Required for parity distribution and minimal storage efficiency
- Recommended 3+ disks – Provides fully redundant array, better performance and more efficient use of capacity
While 2 disks allows RAID 5 to work, 3 or more disks is recommended for a fully functional redundant array with good performance.
2 Disks
A 2 disk RAID 5 array meets the absolute minimum requirements. Parity data can be distributed across the 2 drives providing redundancy against a single disk failure. However, with just 2 disks:
- 50% of total capacity will be taken up by parity data
- Performance will be poor due to the overhead of parity calculation
- Not fully redundant – a second disk failure will result in data loss
3+ Disks
With 3 or more disks:
- Parity overhead is lower as it is spread across more disks
- Fully redundant against a single disk failure
- Better performance by distributing disk workload
While 3 disks is the minimum recommended, the more disks the better in terms of efficiency, performance and redundancy. Large RAID 5 arrays often consist of 5+ disks.
Alternatives to RAID 5 with Fewer Disks
If you only have 1 or 2 disks available, alternatives to consider instead of RAID 5 include:
RAID 1 (Mirroring)
RAID 1 mirrors data between 2 disks. It provides redundancy with just 2 disks but has lower overall capacity.
RAID 0 (Striping)
RAID 0 stripes data across multiple disks for performance, but provides no redundancy. An option for a 2 disk array if redundancy is not needed.
JBOD (Just a Bunch of Disks)
JBOD simply combines multiple disks into a single volume. Provides more space but no performance benefits or redundancy.
Normal Non-RAID Disks
1 or 2 disks can simply be used as standalone non-RAID drives if redundancy and performance are not priorities. This avoids RAID complexity and overhead.
Software Considerations
All RAID configurations require RAID management software installed on the operating system or a dedicated hardware RAID controller. This software/hardware implements the RAID data distribution and calculations.
Many operating systems have built-in software RAID support or compatible third party tools. For hardware RAID a dedicated RAID controller is installed in the server or computer.
With software RAID, a minimum of 2 disks is still required for RAID 5 even though it is implemented in software. The software essentially virtualizes the RAID volumes but still requires the physical disks to store data and parity blocks.
Summary
To summarize the key points:
- RAID 5 requires a minimum of 2 disks to implement parity and redundancy
- 3+ disks is recommended for fully redundant array with better efficiency and performance
- With just 1 disk, RAID 5 does not work at all
- Alternatives for fewer disks include RAID 1, 0, JBOD or standalone disks
- RAID 5 requires RAID controller hardware or software
- Software RAID still requires minimum 2 physical disks
While software can virtualize and manage RAID configurations, the physical disk requirements remain. And for RAID 5 redundancy, a minimum of 2 physical disks is required.
Conclusion
In conclusion, RAID 5 does require a minimum of 2 physical disks in order to implement its distributed parity scheme. With just 1 disk, RAID 5 does not function at all. While a 2 disk RAID 5 array is possible, it is not recommended – 3+ disks should be used for fully redundant array with good performance and efficiency. Alternatives like RAID 1, 0 or JBOD can be considered if fewer than 3 disks are available.