RAID 5 is a common RAID configuration that uses distributed parity to provide redundancy and protect against single disk failures. However, recovering data after two disk failures in RAID 5 can be challenging without proper precautions.
What is RAID 5?
RAID 5 stripes data and parity information across all the disks in the array. The parity information allows the array to reconstruct data if one of the disks fails. However, if two disks fail, the array will lose data.
A common RAID 5 configuration is 3 disks or more with one disk worth of capacity used for parity. For example, with 3 x 1 TB disks, the total capacity would be 2 TB with 1 TB used for parity. This allows the array to withstand one disk failure without data loss.
Why do two disk failures in RAID 5 lead to data loss?
When two disks fail in a RAID 5 array, the parity information is no longer sufficient to reconstruct the missing data. This leads to irrecoverable data loss and downtime.
For example, if Disk 1 and Disk 2 in a 3-disk RAID 5 fail, the parity information on Disk 3 can only reconstruct either Disk 1 or Disk 2, but not both. The data that was on the other failed drive cannot be rebuilt.
Preparing for multi-disk failure in RAID 5
To protect against two disk failures in RAID 5, proper precautions need to be taken:
- Use higher capacity drives to have a larger fault tolerance domain. For example, with five 2 TB drives, you can withstand two disk failures.
- Keep hot spares available that can automatically rebuild failed drives.
- Monitor disk health to proactively replace disks before multiple failures occur.
- Back up the RAID 5 array regularly to external media.
Steps to recover data from two failed disks
If two disks have already failed in a RAID 5 array, data recovery is complex but possible in some cases. Here are the overall steps:
- Stop all I/O activity – Prevent further data writes to avoid overwriting data.
- Replace failed disks – Install new blank disks to rebuild the array.
- Scan disks – Scan the failed disks individually to assess physical damage.
- Rebuild RAID manually – Manually rebuild the RAID by reconstructing data based on parity and remaining data.
- Extract data from disks – Use data recovery tools to extract remaining data from failed disks to external media.
- Compare recovered data – Compare data extracted from failed disks against rebuilt RAID data to find discrepancies.
- Repair RAID 5 – Repair RAID 5 array fully and restore recovered data from failed disks.
Step 1: Stop all I/O activity
The first step when recovering from multi-disk failure is to stop all read and write activity to the array. Continuing to use the RAID 5 risks further data loss if critical data needs to be rebuilt.
To stop I/O activity:
- Unmount file systems and volumes associated with the RAID 5 array.
- Shut down applications and services accessing the storage.
- Disconnect front-end servers from the storage system if possible.
This prevents writes to damaged areas during the recovery process.
Step 2: Replace failed disks
Before attempting to rebuild data, the failed disks must be replaced with new blank disks:
- Power down the storage system cleanly.
- Remove the failed disks and insert new replacement disks of equal or greater capacity.
- The new disks will normally show as failed or missing within the RAID management interface.
This provides the necessary disks for manually rebuilding the array in subsequent steps.
Step 3: Scan disks
With the disks replaced, the failed disks should be connected to a secondary system and scanned for physical damage or issues using disk diagnostic tools. Check for:
- Mechanical failures like head crashes or motor issues.
- Electronics failures like controller board damage.
- Media errors indicating platter surface problems.
This gives insight into the nature of the disk problem and determines if data recovery is possible.
Step 4: Manually rebuild RAID 5 data
With enough redundancy, the RAID 5 can manually be rebuilt using remaining data and parity information. This may require assistance from the storage vendor and involves:
- Using RAID tools, force a rebuild of the array.
- The rebuild will reconstruct data based on remaining data and parity.
- Missing data portions will be rebuilt as zeros initially.
- Log any reconstruction errors encountered.
This provides a baseline RAID 5 volume with missing data portions zeroed out.
Step 5: Extract data from failed disks
With the disks scanned and RAID partially rebuilt, data needs to be extracted from the failed disks:
- Connect failed disks to recovery system as secondary disks.
- Use data recovery software to access and extract data to image files.
- Extracted data may be fragmented or partial depending on physical disk damage.
This allows recovery of remaining data fragments from failed disks before further repair.
Step 6: Compare recovered data
Once data has been extracted from the failed disks, the files need to be compared against the rebuilt RAID 5 volume to identify discrepancies:
- Catalog and match up data fragments extracted from failed disks.
- Compare against rebuilt RAID 5 volume using diff tools.
- Note mismatches that indicate lost or corrupt data portions.
This provides a map of data loss between reconstructed RAID 5 data and recovered failed disk data.
Step 7: Repair RAID 5
With the roadmap of data loss identified, the RAID 5 can be repaired:
- Fix zeros in RAID 5 by restoring recovered data fragments from failed disks.
- Repair or regenerate any remaining corrupt data portions.
- Validate all data portions have been restored to the RAID 5.
- Restart applications and restore into production use.
Following this lengthy process, the reconstructed RAID 5 can be restored as close as possible to its original state before multi-disk failure.
Software tools for RAID 5 multi-disk failure recovery
Specialized software tools are invaluable when recovering from complex multi-disk failures in RAID 5:
RAID Recovery Tools
RAID recovery tools provide advanced capabilities to force RAID rebuilds, reconstruct missing data, and repair volumes. Examples include:
- R-Studio
- Ontrack EasyRecovery
- ReclaiMe Free RAID Recovery
Data Recovery Software
Data recovery software extracts lost data fragments from failed disks. Popular solutions include:
- R-Studio
- Disk Drill
- SpinRite
Disk Diagnostic Utilities
Disk diagnostics tools examine failed drives for physical issues. Common options are:
- Victoria for Windows
- Drive Fitness Test (DFT)
- RedHat SMARTmontools
Difference and Comparison Tools
Diff tools match up recovered data against rebuilt volumes to find discrepancies. For example:
- WinMerge for Windows
- diff and cmp for Linux
- Beyond Compare
Using the right toolset enables recovering maximum data from multi-disk RAID 5 failures.
Preventing two disk failures on RAID 5
While RAID 5 two disk recovery is possible in some cases, prevention is far better to avoid significant downtime and difficult recovery:
- Use enterprise-grade disks designed for RAID environments.
- Monitor disk health stats and temperatures for early problem detection.
- Keep hot spare drives ready to rebuild failed disks immediately.
- Consider larger capacity disks or RAID 6 to increase fault tolerance.
- Perform regular backups to external media as added protection.
With proper precautions, the chances of dual disk failures destroying a RAID 5 can be minimized.
When to use professional RAID recovery services
For business-critical RAID 5 volumes, professional data recovery may be the best option after multi-disk failure. Benefits include:
- Higher chances of data recovery using specialized tools and cleanrooms.
- Recovery possible even with major physical damage to disks.
- Experienced experts available for complex RAID rebuilds.
- Faster recovery process leaving IT staff free for other tasks.
For smaller RAID 5 volumes, DIY recovery may be acceptable. But for mission-critical systems, professional RAID 5 two disk recovery services are worth the investment.
Conclusion
Recovering from two failed disks in RAID 5 is a lengthy and complex process, but possible in many cases using the right techniques. The key steps involve stopping I/O activity, forcing a RAID rebuild, extracting data fragments from failed disks, comparing against rebuilt data, and selectively restoring recovered data.
Specialized RAID and data recovery software tools are indispensable when recovering from multi-disk failure. But prevention is always preferable through monitoring, hot spares, larger disks, and backups.
For business-critical data, engaging professional recover services can provide the expertise and capabilities to successfully restore maximum data after catastrophic RAID 5 failure.