What happens if a drive fails in RAID 10?

RAID 10, also known as RAID 1+0, is a hybrid RAID configuration that combines disk mirroring and disk striping to provide redundancy and improved performance. RAID 10 requires a minimum of four drives and stripes data across mirrored pairs. This means data is written identically to two drives, while the array is striped across two pairs of drives.

What is RAID 10?

RAID 10 provides a combination of data protection and increased input/output performance. The data protection comes from the mirroring, which duplicates data across paired drives. The performance benefits come from the striping, which segments and distributes data across multiple drives.

In a minimum four-drive RAID 10 configuration, the array stripes data across a pair of mirrored drives. For example, data may be written to drives 1 and 2, which are mirrored, and also written to drives 3 and 4, which are mirrored. This layout provides fault tolerance and fast throughput.

How does drive failure affect a RAID 10 array?

One of the key benefits of RAID 10 is its ability to withstand a single drive failure without data loss. If a single drive fails in a RAID 10 array, the data remains intact on the mirrored drive within the pair. For example, if Drive 2 fails in the example above, the data is still available on Drive 1.

When a drive failure occurs in RAID 10, the array is considered “degraded” but remains operational. Read and write operations can continue as normal, although performance may be impacted. The failed drive will need to be replaced to restore full redundancy.

Rebuilding RAID 10 after a drive failure

After replacing the failed drive in a RAID 10 array, the rebuild process restores the mirror and fully redistributes data across the array. The rebuild is performed automatically, although the process can take time depending on the size and number of drives.

The rebuild progresses in the following general stages:

  1. The new replacement drive is identified by the RAID controller.
  2. Data from the surviving mirror drive is copied to the replacement drive.
  3. When the mirror rebuild finishes, any data that was inaccessible due to the drive failure is reconstructed through parity.
  4. The replacement drive completes a full synchronization with its drive pair to restore fault tolerance.

During the rebuild, the RAID 10 array remains available. However, performance can be degraded depending on the controller, drives, and rebuild prioritization. The total rebuild time depends on the storage capacity and performance of the drives.

Getting notifications for RAID 10 drive failure

Most RAID controllers provide notifications when a disk failure event occurs. This may include:

  • On-screen pop-up alerts
  • Event log entries
  • Email alerts
  • SNMP traps

Enabling failure notifications can alert IT administrators that a drive needs to be replaced. Most controllers allow administrators to configure the alert methods and notifications settings.

Monitoring RAID 10 rebuild progress

RAID management utilities allow monitoring the progress of a RAID 10 rebuild. This is important to track, since rebuild times can be lengthy for large drive capacities. Monitoring options include:

  • RAID controller menus and status screens
  • Vendor-specific management software
  • Operating system utilities like mdadm on Linux

Tracking progress ensures the rebuild is making forward advancement and not stalled. If the rebuild stalls, it could indicate problems with the new drive or array configuration issues.

Importance of hot spares for RAID 10

Designating hot spare drives allows RAID 10 arrays to automatically rebuild after a drive failure. The hot spare begins rebuilding the degraded array as soon as a failed drive is detected. This minimizes the time spent in a degraded state.

Hot spares are unused, standby drives the RAID controller can automatically call into service. These are commonly configured at the hardware or firmware level when creating the array.

Using hot spares provides several advantages:

  • Faster rebuilds since no manual drive swapping is needed.
  • Reduced risk from running in degraded mode.
  • Greater fault tolerance for mission critical storage.

The tradeoff is that hot spares occupy unused storage capacity and have an added hardware cost. Still, they provide significant benefits for RAID 10 availability and performance.

Steps for replacing a failed drive in RAID 10

Manually swapping out a bad drive in a RAID 10 array involves just a few steps:

  1. Identify the failed drive by its indicator lights, array status, or drive numbering.
  2. Remove the failed drive from the RAID enclosure or server.
  3. Insert the replacement drive into the same drive bay.
  4. Allow the RAID controller to automatically detect the new drive and start the rebuild process.
  5. Monitor the rebuild status until completion.

Most RAID implementations make it straightforward to hot swap the physical failed drive. The RAID controller automatically integrates the new disk and syncs the data without requiring a full array rebuild.

Recovering data after multiple drive failures

RAID 10 can only withstand a single drive failure without data loss. Multiple drive failures within the same mirrored set will cause array degradation and potential data loss.

However, it is sometimes possible to recover data if both mirrored drives fail in a RAID 10 array. This requires immediately powering down the array to prevent further corruption. The recovery process may involve:

  • Rebuilding the array with new replacement drives.
  • Using data recovery software to reconstruct data from the failed drives.
  • Sourcing data from backups created before the failure.

While challenging, data recovery methods can sometimes retrieve data after dual drive failures on RAID 10. But restoring from backup is the most reliable method.

Preventing drive failures on RAID 10

Some best practices can help minimize drive failures and improve RAID 10 resiliency:

  • Use enterprise-class drives designed for RAID environments.
  • Ensure proper cooling and ventilation for drives.
  • Regularly monitor drive SMART health statistics.
  • Clean air filters in servers and disk enclosures.
  • Consider using hot spare drives.
  • Always gracefully shutdown RAID arrays before powering off.

Careful drive monitoring coupled with enterprise-class components will help avoid performance issues or failures during rebuilds.


RAID 10 provides strong protection against disk drive failure thanks to mirrored pairs. With RAID 10, a single drive failure only degrades the array until the failed drive is replaced and rebuild completes. Using hot spares can automate the rebuild process for maximum uptime.

RAID 10 retains data accessibility after a single drive loss. But multiple drive failures in the same mirror will cause data loss unless recovered from backups or via data recovery techniques. Following best practices for drive maintenance, monitoring, and replacement will minimize the chances of failure.