What happens if a RAID 1 drive fails?

RAID 1, also known as disk mirroring, is a storage technology that protects data by writing identical copies of data to two or more disks. If one disk fails, the data can still be accessed from the other disk(s). This provides fault tolerance and protection against data loss in the event of a drive failure.

What is RAID 1?

RAID 1 involves duplicating data across multiple disks, creating an exact copy or “mirror” of the data. For example, in a 2-drive RAID 1 array, the data from drive 1 is identically copied to drive 2 in real-time. All reads and writes are performed on both drives simultaneously. The drives are said to be “mirrored”.

The key benefits of RAID 1 include:

  • Fault tolerance – if one drive fails, the data is still accessible from the other mirrored drive.
  • No single point of failure – since the data is copied to multiple disks, the failure of any one disk will not result in data loss or downtime.
  • Read performance is improved – data can be read in parallel from both mirrored drives.

The main limitation of RAID 1 is that it doubles the hardware cost since all data is duplicated. Effective storage capacity is also reduced by 50% since two drives are required to store the same data as a single drive.

What happens if a drive fails in RAID 1?

If one of the mirrored drives in a RAID 1 array fails or becomes corrupted, the system continues operating using the surviving drive. All reads and writes will continue as normal using the functional drive only. This is known as operating in a “degraded” state.

For example, in a 2-drive RAID 1 setup where Disk 1 fails:

  • The RAID controller detects that Disk 1 has become inaccessible and is marked as failed.
  • The system switches to using Disk 2 only for all I/O operations.
  • Performance may be impacted slightly as the surviving disk now handles all reads and writes alone.
  • The data remains accessible and the system stays operational.

Operating in a degraded state exposes the data to risk, since if the second drive also fails before the first drive is replaced, data loss will occur. Therefore, the failed drive should be replaced as soon as possible to restore fault tolerance.

Rebuilding the RAID 1 array

Once the failed drive has been replaced with a new, blank drive, the RAID 1 array begins rebuilding. This synchronizes the data from the surviving drive onto the new replacement drive, restoring full redundancy.

The rebuild process works as follows:

  1. The new replacement drive is inserted into the RAID array and designated as the new mirror.
  2. The drive firmware or OS RAID software initiates the rebuild process.
  3. All data blocks are copied from the surviving drive to the replacement drive.
  4. The RAID 1 array goes into a rebuilding state until the process is 100% complete.
  5. Once finished, the array is fully redundant again with two mirrored drives.

The time required to rebuild a failed RAID 1 array can vary greatly depending on the storage capacity and performance of the drives. For large arrays, rebuilds can take many hours to complete.

During the rebuild, it’s recommended to avoid heavy write activity to minimize impact on performance. The system remains vulnerable to data loss until redundancy is restored.

Mitigating RAID 1 disk failures

While RAID 1 offers good protection against drive failure, there are steps that can be taken to reduce the likelihood of drives failing:

  • Use enterprise-grade drives designed for 24/7 operation in RAID environments.
  • Monitor drive health metrics using RAID software tools.
  • Replace aging drives before they exceed manufacturer lifespan recommendations.
  • Maintain proper datacenter temperature, humidity and airflow.
  • Install uninterruptible power supplies to prevent damage from power issues.
  • Perform regular backups in case multiple drives in the array fail.

Monitoring drive SMART attributes like reallocated sectors, spin-up time and drive temperature can provide early warnings about potential drive issues.

Recovering from catastrophic RAID 1 failure

If both mirrored drives in a RAID 1 array fail simultaneously or before the degraded array can be rebuilt, complete data loss will occur. The RAID 1 array will be rendered inoperable.

There are several options for attempting to recover data after this type of catastrophic failure:

  • Restore from backups – Having good backups is essential for recovery from any complete RAID failure scenario. Restore your data from your backup tapes or disks.
  • Send drives to a data recovery service – Specialist data recovery firms may be able to repair drives and reconstruct arrays from severely damaged media.
  • Rebuild array from scratch – If possible, install new replacement drives, reconfigure RAID 1 and restore data from backups.

To reduce the chance of a catastrophic RAID 1 failure occurring:

  • Use RAID 1+0 (RAID 10) which mirrors stripe sets for added redundancy.
  • Keep drives from the same manufacturer and batch when building arrays.
  • Stagger drive replacements to avoid aging drives failing simultaneously.

Software vs Hardware RAID 1 Implementation

RAID 1 can be implemented in two ways – via OS software or a dedicated hardware RAID controller:

Software RAID

Many operating systems like Windows, Linux and macOS include built-in software RAID capabilities that can create mirrored arrays without additional hardware. This uses the OS to manage the array and CPU for parity calculations.


  • No additional cost for a RAID controller.
  • OS integration and management tools.
  • Flexibility in RAID level selection and drive mixes.


  • CPU overhead for parity calculation.
  • Limited drive failure recovery options.
  • Operating system manages rebuild so can impact performance.

Hardware RAID

Dedicated RAID controller cards with onboard processors handle RAID calculations and management. The RAID firmware and tools provide monitoring and recovery options.


  • Offloads RAID processing overhead from main CPU.
  • May support advanced recovery options like drive cloning.
  • Operating system sees a single logical volume vs. individual disks.


  • Added cost for RAID controller hardware.
  • Less flexible than software RAID implementations.

For mission critical data protection in enterprise environments, hardware RAID 1 is typically recommended over software RAID 1 implementations. The dedicated RAID processor helps improve performance and availability.

RAID Type Controller Location Pros Cons
Software RAID CPU No hardware cost, OS integration CPU overhead, limited recovery
Hardware RAID RAID card Dedicated processor, advanced recovery Added hardware cost

RAID 1 Performance Considerations

RAID 1 performance characteristics include:

  • Excellent read speed – Reads can be distributed across mirrored drives for potentially 2x throughput.
  • Average write speeds – Writes must go to both drives before completion so can be bottlenecked by slowest drive.
  • Minimal CPU impact (with hardware RAID) since parity calculations are offloaded.
  • High fault tolerance – array stays operational if one drive fails.

RAID 1 is preferred in environments where read performance and fault tolerance are more important than storage efficiency or cost. Its mirrored redundancy introduces minimal latency or bottlenecking issues.

RAID 1 Rebuild Performance Impact

During RAID 1 rebuilds, the reconstruction process running in the background does consume some CPU/storage resources. This may impact performance:

  • Increased latency for applications accessing the array during rebuild.
  • Temporarily reduced write speeds until rebuild completes.
  • Prolonged rebuild time if array has heavy workloads.

Managing client workloads and avoiding unnecessary disk activity during rebuilds can help minimize performance impact.

Alternatives to RAID 1

While RAID 1 is well-suited for performance and high availability, other RAID levels offer different advantages that may better match requirements:

RAID 0 (Disk Striping)

RAID 0 stripes data across multiple disks with no redundancy. This can provide better write speeds and storage efficiency but has no fault tolerance if a drive fails.

RAID 5 (Distributed Parity)

RAID 5 stripes data across drives and dedicates one drive worth of capacity for parity information. It provides redundancy for one drive failure with higher storage efficiency than RAID 1 but requires parity overhead on writes.

RAID 10 (Nested Mirroring)

RAID 10 mirrors multiple RAID 0 stripe sets to provide the performance benefits of RAID 0 and redundancy of RAID 1. However, at least 50% of total capacity is lost to redundancy.


In summary, RAID 1 delivers excellent availability and read performance by mirroring data across drives. If a drive fails, the system stays online using the surviving disk. After replacing the failed drive, RAID 1 automatically rebuilds the mirror to restore full redundancy.

To protect against data loss, RAID 1 should be combined with comprehensive backups, drive health monitoring and proper datacenter conditions. Other RAID levels like RAID 5 or RAID 10 may be preferable depending on cost, performance and redundancy requirements.