RAID 5
RAID 5 Quick Facts
What Is RAID 5?
RAID 5 is a striped RAID layout with distributed single parity. Data blocks and parity information are spread across the member drives rather than dedicating one physical disk exclusively to parity.
The parity information allows the array to reconstruct missing data if one member drive fails. While operating in this degraded condition, however, the array no longer has additional drive-failure tolerance until the failed member is replaced and redundancy is restored.
Parity Is Distributed Across the Array
RAID 5 stripes data across the member drives and distributes parity blocks across them as well. That parity can be used with the remaining data blocks to reconstruct the contents of a failed member.
Simplified conceptual illustration of distributed single parity.
RAID 5 Usable Capacity
RAID 5 has the equivalent capacity overhead of one member drive because parity information consumes space across the array. When mixed-capacity drives are used, the smallest participating drive generally determines the capacity contribution of every member.
Four 4 TB Drives
(4 − 1) × 4 TB = 12 TB nominal RAID 5 capacity .
Six 8 TB Drives
(6 − 1) × 8 TB = 40 TB nominal RAID 5 capacity .
RAID 5 Can Tolerate One Drive Failure
If one member drive fails, the remaining data and parity information can be used to reconstruct the missing data. The array can remain accessible while operating in a degraded state, depending on the implementation.
A Second Drive Failure Can Cause Array Failure
RAID 5 provides only one-drive fault tolerance. If another member fails before the array has successfully restored redundancy, the RAID set can no longer reconstruct all missing data.
Read Performance vs Write Performance
RAID 5 can deliver strong aggregate read performance because data is striped across multiple member drives. Sequential read workloads can often benefit from the combined bandwidth of the active disks.
Writes are more complex because parity must also be maintained. Small random writes can require additional read and write operations, often described as a parity write penalty. Controller cache, workload size, stripe alignment, storage media, queue depth, and implementation can significantly affect real-world results.
What Is the RAID 5 Write Penalty?
For certain small writes, RAID 5 may need to read existing data and parity, calculate updated parity, then write both the new data and updated parity. This additional work is one reason parity RAID can behave differently from RAID 0 or RAID 10 under write-heavy random workloads.
RAID 5 Rebuilds
After a failed drive is replaced, the array reconstructs the missing contents using data and parity from the surviving members. The new drive must be repopulated before full redundancy is restored.
Rebuild Duration Is Not a Fixed Number
Rebuild time depends on drive capacity and speed, controller behavior, interface bandwidth, workload activity, rebuild priority, system performance, and implementation. Large drives can take substantially longer to rebuild than smaller ones.
RAID 5 Strengths
- Provides single-drive fault tolerance.
- More capacity-efficient than two-way mirroring at larger drive counts.
- Can provide strong aggregate read performance.
- Parity is distributed rather than dedicated to one disk.
- Useful where capacity efficiency and redundancy are both important.
RAID 5 Limitations
- Only one member-drive failure can be tolerated.
- Parity creates additional work for some write patterns.
- Performance can degrade significantly during rebuilds.
- Long rebuild windows increase exposure while degraded.
- RAID 5 does not replace a backup strategy.
When RAID 5 May Be Appropriate
RAID 5 can be suitable where capacity efficiency and one-drive redundancy are priorities and the workload does not demand the strongest possible small-write performance.
When RAID 5 May Not Be the Best Choice
RAID 5 may be less appropriate when two-drive fault tolerance is required, when rebuild exposure must be reduced, or when the workload is dominated by latency-sensitive random writes. In those cases, RAID 6, RAID 10, or another storage architecture may be a better fit depending on the requirements.
RAID 5 Compared With Other RAID Levels
| RAID Level | Technique | Nominal Capacity | Drive Fault Tolerance | Primary Tradeoff |
|---|---|---|---|---|
| RAID 0 | Striping | N × smallest drive | None | Maximum nominal capacity, no redundancy |
| RAID 1 | Mirroring | Smallest mirror member | Mirror-dependent | Simple redundancy with lower capacity efficiency |
| RAID 5 | Striping + single parity | (N − 1) × smallest drive | 1 drive | Good capacity efficiency with parity write overhead |
| RAID 6 | Striping + dual parity | (N − 2) × smallest drive | 2 drives | More redundancy with additional parity overhead |
| RAID 10 | Mirrors + striping | Typically 50% | Topology-dependent | Strong performance with lower capacity efficiency |
Calculate Your RAID 5 Configuration
Use the Tech Supply Direct RAID Calculator to estimate RAID 5 capacity, storage efficiency, fault tolerance, estimated performance, hot-spare impact, and hardware cost.
RAID 5 FAQ
How many drives are required for RAID 5?
RAID 5 requires a minimum of three participating drives.
How many drives can RAID 5 lose?
RAID 5 can tolerate one member-drive failure. A second member failure before redundancy is restored can cause the array to fail.
How much capacity does RAID 5 provide?
Nominal RAID 5 capacity is generally calculated as (number of drives minus one) multiplied by the smallest participating drive capacity.
Is RAID 5 faster than RAID 1?
There is no universal answer. RAID 5 can provide strong aggregate read throughput, while parity maintenance can make some writes more expensive. Actual performance depends on the workload and implementation.
What happens when RAID 5 loses a drive?
The array enters a degraded state and reconstructs missing data from the surviving data and parity information until the failed drive is replaced and the array is rebuilt.
Is RAID 5 a backup?
No. RAID 5 provides drive-failure redundancy, but it does not protect against every cause of data loss. Important data should still be protected with independent backups.
RAID 5 Redundancy Is Not a Backup
RAID 5 can protect availability after a single member-drive failure, but it does not protect against accidental deletion, corruption, ransomware, controller failure, catastrophic system loss, or every other data-loss scenario. Important workloads should use an independent, tested backup strategy.