How to repair a RAID 5 array?

What is RAID 5?

RAID 5 is a redundant array of independent disks (RAID) configuration that distributes data and parity blocks across multiple disks (TechTarget, 2023). It requires a minimum of three disks, with data blocks striped across disks along with a parity block distributed for each stripe. Unlike other RAID levels like RAID 0 or RAID 1, RAID 5 provides fault tolerance and improved performance without requiring mirroring (IONOS, 2023).

In a RAID 5 array, data is divided into blocks that are striped and written across all the disks, similar to RAID 0. However, for each set of data blocks written, a parity block is calculated using XOR and also written to the array. The parity block allows for data recovery in case of disk failure, providing fault tolerance (Wikipedia, 2023).

The key benefits of RAID 5 include:

  • Improved read performance compared to a single disk, since data can be read in parallel from multiple disks.
  • Fault tolerance – the array can sustain a single disk failure without data loss.
  • Efficient use of storage space, as only a single parity disk is required unlike mirroring in RAID 1.

When Do RAID 5 Arrays Need Repairing?

There are several common reasons why a RAID 5 array may require repair:

Disk failure – If one disk in the RAID 5 array fails, the array will continue functioning in a degraded state. However, the failed disk needs to be replaced and the array rebuilt to restore full redundancy. [1]

Multiple disk failures – RAID 5 can only withstand a single disk failure without data loss. If two or more disks fail, data may be lost and extensive repairs required. Multiple concurrent disk failures are more likely with larger arrays.

Controller failure – The RAID controller can also fail, potentially leading to inaccessibility or corruption of the array. The controller will need to be replaced and the array imported or rebuilt.

Accidental deletion of disk – If a disk is accidentally deleted from the array, the parity information and data layout will be disrupted. The missing disk will need to be re-added and the array rebuilt.

File system corruption – File system errors can occur if there are disk read/write errors. In this case the file system will need to be repaired using CHKDSK or a similar utility.

RAID 5 arrays require repair most commonly due to disk failures. However controller failure, accidental deletion of disks, multiple disk failures and file system corruption can also necessitate RAID 5 repair.

Preparing for RAID 5 Repair

Before attempting to repair a RAID 5 array, it is important to take some preparatory steps to ensure the process goes smoothly and avoid potential data loss. Some key preparation tips include:

Assembling the proper tools – Having the right tools on hand will make the RAID repair process easier. Useful tools include new compatible hard drives for replacing failed disks, a PC with RAID configuration software installed, anti-static wrist straps, and a Phillips-head screwdriver for opening up the server or external enclosure to access the disks (

Backing up critical data – If possible, create backups of your most important data on the RAID 5 array before attempting repairs. Backups provide protection in case anything goes wrong during the rebuild process. Consider cloning all disks or imaging the RAID array to preserve data (

Checking RAID card functionality – Prior to rebuilding, verify that the RAID card is fully functional. Check that the existing disks are detected, visit the manufacturer’s website for card driver updates if needed, and inspect logs for any controller errors which could impact the rebuild (

Identifying and Replacing Failed Disks

When a disk fails in a RAID 5 array, it will need to be replaced to restore full redundancy. There are a few ways to identify which specific disk has failed:

  • Check indicator lights on the disk enclosures – failed disks will often display a red or amber light.
  • Look for errors in OS/RAID management utilities – this will specify the disk with issues.
  • Run S.M.A.R.T. diagnostics to spot problems.

Once the failed disk is identified, it can be hot swapped for a new one without powering down the full array (according to this guide). Simply remove the failed disk and insert a new blank one of equal or greater capacity in the same bay. The RAID controller will automatically rebuild the data across the new disk using parity information. Monitor the rebuild process until completion. Then verify the disk was fully integrated by checking the RAID management utility.

It’s crucial to swap out failed disks promptly to avoid the risk of a second failure during rebuild, which would lead to data loss. Regular monitoring and hot spares can help minimize downtime when disks fail.

Rebuilding the Array

Once any failed disks have been replaced, the next step is to rebuild the RAID 5 array. This involves the array controller using the parity data to reconstruct the missing data and write it to the new replacement disk(s). Here are the key steps for rebuilding the array:

Initiating the rebuild process: After inserting the new disk(s), the rebuild process can be started through the RAID controller’s management software. Make sure to initiate a rebuild and not a reformat, as rebuilding will preserve your data. The controller will start reading data from the surviving disks and parity in order to reconstruct the data for the replacement disk(s) [1].

Monitoring rebuild progress: The management software will show the rebuild progress and estimated time to completion. A RAID 5 rebuild can take several hours or longer depending on the size of the disks and the performance of the controller. Be patient and avoid unnecessary disk activity during this process. Expect reduced performance on the array while rebuilding [2].

Verifying completed rebuild: Once finished, verify that the array is fully rebuilt and operating normally. Check the management software to confirm all disks are now showing as healthy. It’s also a good idea to run read/write tests on the array to validate full functionality.

Repairing Deleted Disks

If one or more disks have gone missing from a RAID 5 array due to accidental removal or failure, it is possible to repair the array and rebuild the missing data without data loss. The key steps are:

Identify which specific disks are missing from the array. This can usually be determined by looking at the RAID management utility or controller BIOS. Missing disks will typically show an error or offline status.

Reinsert or replace the missing disks. The new disks must match the original size and type. Once added back to the system, the disks should automatically be detected and show back up in the RAID management screen.

Rebuild the array. In the RAID utility, select the option to rebuild or reactivate the degraded array. This will synchronise the disks and reconstruct the missing data using parity information spread across the remaining disks.

The rebuild process can take hours or days depending on the RAID 5 array size and the amount of data that needs to be recreated. The array is vulnerable during rebuilding, so avoid heavy disk usage until it is completed.

Following a successful rebuild, the RAID 5 array should be fully operational again and all data should be intact and accessible. Regular backups are still recommended to protect against future disk failures.


Troubleshooting Issues

Even when following proper RAID 5 repair procedures, rebuilding a RAID 5 array can run into problems. Troubleshooting issues depends on understanding what the underlying hardware problems are.

The most common issues faced when rebuilding RAID 5 arrays are:

Long Rebuild Times

RAID rebuilds normally take several hours, with larger arrays and more disks taking longer. But if rebuild times stretch to multiple days or even weeks, it likely indicates disk hardware problems slowing the process. Check SMART data for high pending and uncorrectable sector counts on disks. Replacing disks proactively can help avoid failures during long rebuilds.

Inaccessible Disks

If disks disappear from the array or become inaccessible during the rebuild, there are likely physical connection issues. Check cables and connectors to ensure stable disk connections. Replace damaged cables and connectors. In some cases, disk controllers or backplanes fail causing accessibility issues.

Other Hardware Problems

Beyond disks and connections, other RAID controller, server and enclosure hardware problems can disrupt RAID 5 rebuilds. Issues like faulty controller caches, overheating and power supply failures have been known to interfere with rebuilds. Monitor system logs for hardware alerts and failures during RAID 5 rebuilds.

Replacing defective hardware proactively is key to avoiding failures during rebuilds. When issues do occur, consult server and storage vendors for troubleshooting and recovery steps specific to the hardware.

Recovering Corrupted Data

If the RAID 5 array becomes corrupted and inaccessible, there are several methods for recovering the data:

Performing a file system check can help repair issues that prevent the RAID from mounting properly. On Linux, this can be done using the fsck command. On Windows, chkdsk can scan and fix errors (Diskinternals, 2024).

Specialized data recovery software like R-Studio or DiskInternals RAID Recovery can read and reconstruct RAID arrays, even with multiple failed disks. They rebuild the array virtually so the data can be recovered (Diskinternals, 2024).

If the RAID configuration data gets corrupted or deleted entirely, the array will have to be manually rebuilt from scratch. This requires restoring data from a previous backup.

Preventing Future Failures

There are several best practices that can help prevent failures and data loss in RAID 5 arrays:

Monitoring health – Use disk monitoring tools to keep an eye on disk health and receive early warnings about potential failures. This allows you to replace disks before they actually fail and avoid downtime. See Three key strategies to prevent RAID failure for monitoring recommendations.

Hot spares – Configure hot spare disks which can automatically rebuild the array if a disk fails, minimizing downtime. The array can keep operating normally during the rebuild process. Refer to Tips to Prevent Data Loss in a RAID array for how to set up hot spares.

Upgrading disks – Gradually upgrade disks to larger ones with better performance and reliability. This improves overall array robustness. Plan upgrades ahead of time to avoid unpredictable failures of older disks.

Backup policies – Maintain regular backups of the RAID 5 data, ideally both onsite and offsite/cloud backups. This provides recovery options if the array experiences total failure. Test backups periodically to verify their integrity. See 10 Tips to Protect Your RAID Device for backup tips.

When to Call a Professional

In some cases, it’s best to leave RAID 5 repairs to the professionals. Severe hardware damage, advanced recovery needs, or tight time constraints may warrant outside expertise.

If multiple disks have failed or the RAID controller itself is damaged, repairs could require skills and tools the average user lacks. Professional data recovery services are equipped to handle severe hardware issues. They have access to specialized equipment for analyzing drives and repairing controllers.

Likewise, if critical data has been corrupted or deleted from the array, professionals can often recover it when DIY software fails. With advanced forensic tools, they may extract data even if drives won’t mount or initialize. This level of recovery is beyond most users’ means.

Finally, if time is of the essence, professional services can expedite the repair process. While rebuilding a degraded array takes hours or days, technicians can often get systems back online faster. Their expertise can minimize downtime.

So for major damage, valuable data loss, or urgent needs, it’s wise to let the professionals handle RAID 5 repairs. Their skills, tools, and experience justify the cost.