How to repair a RAID 5 array?

What is RAID 5?

RAID 5 is a storage technology that combines multiple hard disk drives into one logical unit. Data is distributed across all the drives with parity information stored alongside. The parity allows for data recovery in case one of the drives fails. A RAID 5 array requires a minimum of 3 drives.

The main benefits of RAID 5 are:

  • Improved read performance – data can be read in parallel from multiple drives
  • Fault tolerance – the array can sustain a single drive failure without data loss

Some key characteristics of RAID 5:

  • Minimum 3 physical disks required
  • Block-level striping with distributed parity
  • Parity allows for recovery from a single disk failure
  • Read performance improved due to multiple disks
  • Write performance reduced due to parity calculation

When Do You Need to Repair a RAID 5 Array?

A RAID 5 array will need repairs in the following situations:

  • Disk failure – If one of the disks in the array fails, the RAID volume will become degraded. The data is still accessible but you are at risk until the failed drive is replaced.
  • Multiple disk failures – RAID 5 can only withstand one disk failure. If a second disk fails before the first one has been replaced, data loss will occur. The array will need to be repaired from backups.
  • Corrupted parity – If the parity information becomes corrupted, the integrity of the array is compromised. The parity will need to be recreated.
  • Accidental deletion – If disk partitions or data are accidentally deleted, the array will need to be repaired.
  • File system errors – File system corruption can render some or all data inaccessible. Repairs may involve filesystem checks and data recovery tools.
  • Controller failure – If the RAID controller fails, access to the disks is lost. The controller will need to be replaced.

Regular monitoring and maintenance of the RAID 5 array can help detect and prevent some of these scenarios. But careful repairs are required if any of these issues do occur.

Preparing for RAID 5 Repair

Before beginning repairs on a degraded or failed RAID 5 array, some preparatory steps help ensure the repair process goes smoothly:

  • Stop all I/O activity – Prevent applications, databases or users from writing further data to the array. This avoids data inconsistencies.
  • Backup critical data – If the array is still functional, back up any business critical data as a precaution.
  • Assemble tools – Gather software, spare disks and other hardware necessary for repairs.
  • Review documentation – Consult storage system manuals and procedures for the RAID configuration.
  • Notify users – Inform all affected users and teams that the storage will be temporarily unavailable.
  • Schedule downtime – Plan a maintenance window to take the storage offline for repairs with minimal impact.

With the array stable and data protected, you can proceed with the repairs.

Repairing a RAID 5 Array

The steps to repair a RAID 5 array will vary depending on the specific issue. Here are some common repair scenarios:

Repairing Degraded Array After Disk Failure

If one disk has failed in the RAID 5 array, it will switch to a degraded state. All data remains available. To repair:

  1. Physically replace the failed hard drive with a new, identical model.
  2. Insert the new drive into the RAID array enclosure in the same slot as the failed drive.
  3. The RAID controller will automatically add the drive to the array and begin rebuilding the data and parity.
  4. When the rebuild process completes, the RAID volume will return to optimal state.

The length of the rebuild depends on the size of the disks and workload. Monitor the process in the RAID management software.

Repairing Corrupted Parity

If the RAID parity gets out of sync or corrupted, repairs involve recalculating the parity:

  1. Use RAID management software to initiate a parity resync.
  2. The process will recalculate all parity and scrub the array for consistency.
  3. Monitor for any disk errors that surface during the resync.
  4. When finished, the parity will be restored without any drive replacements needed.

Parity resync times will vary based on array size and activity during the process.

Repairing After Multiple Disk Failures

If a second drive fails before the first failed drive is replaced, the RAID 5 array will suffer data loss. Repairs become more complex:

  1. Replace any failed disks with new drives.
  2. Attempt to rebuild the array from parity – some data may be recovered.
  3. Restore missing data from backups.
  4. Recalculate and restore RAID parity across the array.

With multiple disk failures, expect substantial data loss without recent backups. Prioritize critical data recovery.

Repairing Accidental Deletions

If data or disk partitions are accidentally deleted, repairs involve data recovery:

  1. Stop all activity on the array immediately to prevent overwriting deleted files.
  2. Scan disks with data recovery software to find and restore deleted files.
  3. If disk partitions are deleted, recreate the partitions.
  4. Restore data from backups as needed to fill any remaining gaps.

Act quickly when data is accidentally deleted, as the sooner recovery begins the more data can potentially be saved.

Repairing File System Errors

File system corruption requires repairs focused on checking integrity:

  1. Unmount the file system if possible to ensure read-only access.
  2. Run a file system check utility (fsck for Linux, CHKDSK for Windows).
  3. Review logs for any identified errors and attempt repairs.
  4. If critical data is corrupted, retrieve from backups.
  5. As a last resort, reformat and recreate the file system.

Always attempt non-destructive repairs first before reformatting or recreating the file system.

Repairing a Failed RAID Controller

For a failed RAID controller, the repair steps are:

  1. Replace the failed RAID controller with an identical model.
  2. Reconfigure the controller to match the original array settings.
  3. If necessary, import the existing RAID configuration from the drives.
  4. Resume normal operations once the controller rebuild completes.

Having the original controller configuration details on hand helps simplify replacement.

Repairing RAID 5 – Special Cases

Some RAID 5 repair scenarios require extra care:

  • Degraded array too long – Don’t allow a degraded array to run too long before replacing failed drives. The risk of a second disk failure increases over time.
  • Faulty controller – If disk failures are excessively common, faulty controller could be at fault. Replace controller before rebuilding array.
  • SMART errors – Review SMART diagnostic data for any indicators of pending drive issues. Address warning signs before drive failures occur.
  • Loose connectors – Loose cables or connectors can cause intermittent drive failures. Inspect physical connections.

Also watch for early signs of trouble like slower performance, increasing drive errors, and abnormal array operations. Addressing small issues early prevents bigger problems later.

Verifying Repairs

After completing RAID repairs, always verify normal operations before returning the storage to production:

  • Check management software to confirm optimal state with no warnings or alerts.
  • Look for thorough completion of any rebuild or resync processes.
  • Scan logs to ensure no recurring errors.
  • Perform read and write testing to confirm full data access.
  • Monitor performance for expected disk throughput.

Only resume normal usage after full verification to prevent reintroducing problems.

Preventing Future RAID 5 Failures

While RAID 5 provides redundancy, multiple disk failures carry substantial repair challenges. To reduce future problems:

  • Monitor closely – Utilize RAID monitoring tools and watch for early signs of disk problems.
  • Use hot spares – Designate hot spare disks that can automatically rebuild failed drives.
  • Shorten rebuild times – Keep RAID 5 arrays small and use faster drives to reduce exposure during rebuilds.
  • Check fans and temperature – Ensure adequate airflow and cooling to reduce risk of failures.
  • Test backups – Validate backup systems frequently for reliable restores when needed.
  • Consider RAID 6 – Adds an additional parity drive to withstand loss of two disks.

With proper ongoing maintenance and monitoring, RAID 5 arrays can provide many years of reliable storage performance.


Repairing degraded or failed RAID 5 arrays involves assessing the cause, replacing components if needed, restoring data from backups, and recalculating parity. Careful verification is required before putting the storage back into production use.

To reduce repair needs in the future, monitoring disk health, ensuring proper cooling, validating backups, and considering additional redundancy with RAID 6 can all help maximize uptime and reliability. With proper care, a RAID 5 array can deliver many years of relatively worry-free operations.