RAID 5 is a type of RAID (Redundant Array of Independent Disks) that provides fault tolerance and redundancy by using distributed parity. In RAID 5, data is striped across multiple disks like in RAID 0, but it also utilizes parity information that is distributed across the disks. This allows the array to withstand the failure of one disk without data loss.
How does RAID 5 work?
In a RAID 5 array, data is striped in chunks across multiple disks. For each set of data chunks, a parity chunk is calculated and written across the disks. For example, in a 3-disk RAID 5 array, Disk 1 may contain Data A, Disk 2 may contain Data B, and Disk 3 may contain Parity A+B.
The parity information allows RAID 5 to recover data if one of the disks fails. For example, if Disk 2 fails, Data B can be recalculated using Data A and the Parity A+B. This provides fault tolerance with minimal overhead, as only one disk worth of space is used for parity, and the remaining disks store data.
RAID 5 requires a minimum of 3 disks, but it can scale to larger arrays with more data disks. In larger arrays, the parity chunks are staggered across all the disks. This prevents the parity disk from becoming a bottleneck as all disks participate in reading and writing data.
Advantages of RAID 5
Here are some key advantages of using RAID 5:
- Fault tolerance – RAID 5 can withstand a single disk failure without data loss. The failed disk can be replaced and data rebuilt using parity.
- Good read performance – Data is striped across disks, allowing reads to occur in parallel for better performance.
- Low overhead – Only one disk worth of capacity is used for parity, minimizing overhead.
- Scalability – RAID 5 can be implemented with larger arrays of disks.
Disadvantages of RAID 5
Some disadvantages of RAID 5 include:
- Slow writes – Writes are slower as parity must be calculated and written with each write operation.
- Rebuild times – Rebuilding an array after a disk failure can take a long time for large arrays.
- Data loss risk – With larger arrays, there is a higher risk of unrecoverable read errors during rebuilds, leading to data loss.
- Poor random write performance – Due to parity calculation overhead, random writes suffer in performance.
When to use RAID 5
Here are some examples of when RAID 5 can be useful:
- Transactional databases – Provides redundancy for database workloads that are mostly read operations.
- File and application servers – Good for read intensive workloads when redundancy is needed.
- Archival storage – Useful for write-once, read-many archival data that needs redundancy.
In general, RAID 5 works well when redundant storage is required, but the workload is predominantly read operations. The parity overhead can impact performance for write-heavy workloads.
RAID 5 Implementations
There are two main implementations of RAID 5:
Hardware RAID 5
Hardware RAID 5 uses a dedicated RAID controller card that handles the RAID operations. The RAID logic is offloaded from the main CPU onto the controller. Hardware RAID provides better performance but requires purchasing a RAID card.
Software RAID 5
Software RAID 5 is implemented at the operating system level without specialized hardware. The RAID logic is handled by the CPU. Software RAID provides more flexibility and lower cost, but at the expense of some processing overhead on the main CPU.
RAID 5 Configurations
RAID 5 can be configured in different layouts depending on the number of disks and capacity requirements:
3 Disk RAID 5
A 3 disk RAID 5 array stripes data across 2 disks and uses the third for parity. It can withstand a single disk failure without data loss.
4 Disk RAID 5
A 4 disk RAID 5 array stripes data across 3 disks and uses the 4th for parity. It provides additional capacity compared to 3 disk RAID 5 while maintaining a single disk failure tolerance.
5+ Disk RAID 5
Larger RAID 5 arrays can be created for more capacity and I/O performance. The parity is distributed across all disks. A larger number of disks increases rebuild times and risk of failure during rebuilds.
RAID 50/51/53
Nested RAID levels that combine multiple RAID 5 arrays can be used to improve performance or increase fault tolerance:
- RAID 50 – Combination of multiple RAID 5 arrays striped as a RAID 0 array.
- RAID 51 – RAID 5 arrays mirrored as a RAID 1 array.
- RAID 53 – Combination of RAID 5 and RAID 3 arrays.
RAID 5 Usage Scenarios
Here are some examples of how RAID 5 can be used in real-world scenarios:
File Server Storage
A file server that needs redundant storage can use a 4 disk RAID 5 array. This provides capacity for data across 3 disks, with the 4th disk used for parity. The server can withstand a single disk failure without data loss.
Database Server
A database server can leverage a RAID 50 array for performance and redundancy. Multiple RAID 5 arrays are striped together as a RAID 0 array. This provides high read speeds from striping, as well as the ability to withstand multiple disk failures across the RAID 5 sub-arrays.
Media Archive
A media company archiving large volumes of video content can use a large 12 disk RAID 5 array. This provides high capacity redundant storage for the mostly static video files. Rebuild times are less critical since content is archival.
RAID 5 vs RAID 1
| RAID 5 | RAID 1 |
|---|---|
| Stripes data and parity across disks | Mirrors data between disks |
| Minimum 3 disks required | Minimum 2 disks required |
| Can withstand 1 disk failure | Can withstand 1 disk failure |
| Higher storage capacity | Half storage capacity utilized |
| Write performance reduced due to parity | Good write performance |
| Good for transactional workloads | Good for high write workloads |
RAID 5 vs RAID 6
| RAID 5 | RAID 6 |
|---|---|
| Single parity disk | Double parity (2 disks) |
| Withstands 1 disk failure | Withstands 2 disk failures |
| Lower overhead vs RAID 6 | Higher overhead for double parity |
| Higher capacity vs RAID 6 | Lower capacity due to 2 parity disks |
| Rebuild times similar to RAID 6 | Slower rebuilds than RAID 5 |
| Best for lower capacity/budget arrays | Best for mission critical data protection |
RAID 5 vs RAID 10
| RAID 5 | RAID 10 |
|---|---|
| Single parity disk | Mirrored stripes (RAID 1+0) |
| Minimum 3 disks required | Minimum 4 disks required |
| Sequential read/writes faster than RAID 10 | Random read/writes faster than RAID 5 |
| Rebuild times increase with larger arrays | Fast rebuilds regardless of array size |
| Lost data likelihood increases on rebuild with large arrays | Very reliable, lost data unlikely |
| Lower cost than RAID 10 | Higher cost than RAID 5 |
Conclusion
RAID 5 provides a good balance of redundancy, capacity, and performance for a variety of workloads. It works well when fault tolerance is required, but the access pattern is predominantly reads rather than writes. Larger implementations become less efficient for random access, as the parity overhead penalizes write operations. But for read-intensive workloads, RAID 5 remains a popular choice to gain redundancy without quite as much capacity sacrifice as mirroring. The distributed parity helps remove potential bottlenecks or single points of failure.
