RAID 1
RAID 1 Quick Facts
What Is RAID 1?
RAID 1 is a mirrored storage layout. The same logical data is written to multiple member drives so that another copy remains available if a supported member-drive failure occurs.
In the common two-drive implementation, both drives contain the same logical data. The usable capacity is therefore based on one drive rather than the combined raw capacity of both drives.
The Same Data Is Written to Each Mirror Member
Unlike RAID 0, which spreads different data segments across drives, RAID 1 stores redundant copies. This sacrifices raw capacity efficiency in exchange for the ability to retain a complete copy of the data when one member of a two-drive mirror fails.
Simplified conceptual illustration of a two-drive RAID 1 mirror.
RAID 1 Usable Capacity
In a standard mirror, each member must be able to hold the same logical data set. If different-capacity drives are used, the smallest participating member limits the nominal usable capacity of the mirror.
Two 4 TB Drives
4 TB mirrored to 4 TB = 4 TB nominal RAID 1 capacity .
4 TB + 8 TB
The 4 TB member limits the mirrored data set, giving 4 TB nominal RAID 1 capacity .
RAID 1 Can Survive a Member Failure
In a common two-drive mirror, the array can continue operating after one member drive fails because the surviving drive still contains a complete copy of the logical data.
A Degraded Mirror Should Be Repaired Promptly
After one member of a two-drive RAID 1 mirror fails, there is no longer a second copy inside the mirror. A subsequent failure of the remaining member before redundancy is restored can make the array unavailable.
How Mirroring Affects Read & Write Performance
RAID 1 read performance can benefit from having multiple copies of the data because a controller or software implementation may distribute read requests across mirror members. The amount of improvement depends on implementation and workload.
Writes must be committed to every required mirror member, so write throughput does not scale in the same way as RAID 0 striping. Actual performance depends on drive type, controller, cache, interface bandwidth, queue depth, filesystem, and application behavior.
RAID 1 Strengths
- Provides straightforward drive redundancy.
- No parity calculations are required.
- Rebuild logic is relatively simple because data is copied from a surviving mirror member.
- Read workloads may benefit from multiple available copies.
- Works well for smaller redundant storage sets.
RAID 1 Limitations
- Typical two-drive capacity efficiency is only 50%.
- Write performance does not scale like RAID 0.
- Usable capacity is limited by the smallest mirror member.
- A degraded two-drive mirror has no remaining in-array redundancy.
- RAID 1 does not replace an independent backup strategy.
When RAID 1 May Be Appropriate
RAID 1 is commonly considered when straightforward redundancy is more important than maximizing usable capacity.
RAID 1 vs RAID 0
| Attribute | RAID 0 | RAID 1 |
|---|---|---|
| Technique | Striping | Mirroring |
| Typical 2-Drive Capacity Efficiency | 100% | 50% |
| Drive Redundancy | None | Yes |
| Member Failure | Array fails | Two-drive mirror can continue after one member failure |
| Main Priority | Capacity and aggregate performance | Simplicity and redundancy |
Calculate Your RAID 1 Configuration
Use the Tech Supply Direct RAID Calculator to compare RAID 1 capacity, storage efficiency, redundancy, estimated performance, and cost against other RAID levels.
RAID 1 FAQ
Does RAID 1 provide redundancy?
Yes. RAID 1 stores mirrored copies of the data on multiple member drives.
How much capacity does RAID 1 provide?
A standard mirror provides nominal usable capacity equal to the smallest participating mirror member.
Can RAID 1 survive a drive failure?
A two-drive RAID 1 mirror can continue operating after one member fails because the surviving member contains a complete copy of the logical data.
Is RAID 1 faster than a single drive?
Read performance may improve in implementations that can distribute read requests across mirror members. Write performance is constrained because writes must reach the required mirror members.
Is RAID 1 the same as a backup?
No. Mirroring protects against supported drive failures, but it does not protect against accidental deletion, corruption, ransomware, or every other source of data loss.
Can different-size drives be used in RAID 1?
Some implementations allow mixed drive sizes, but usable mirror capacity is generally limited by the smallest participating member.
RAID 1 Redundancy Is Not a Backup
Mirroring can preserve availability after supported drive failures, but both mirror members contain the same logical data. Deleting, corrupting, or overwriting data can therefore affect every mirror copy. Important workloads should still use an independent, tested backup strategy.