Recently, Darth Data Recovery took over a RAID 5 array from an HP server. One of the disks was physically damaged and couldn't be recognized. As a result, we used the remaining three disks to attempt data recovery. However, the recovered data turned out to be corrupted and unreadable. At this point, I decided to address the physical issue with the hard drive. Fortunately, the drive was repaired, and we were able to create an image of it. Here's how we proceeded:
1. We attempted to rebuild the RAID 5 using the image of the failed disk along with the other three. The data was split into three different combinations.
2. We exported the files that seemed problematic and tried opening them to check for normality.
The results were unexpected. No matter which disk was missing, the combined data was still corrupted, and the files couldn’t be opened. Using a tool called "escort ship," we checked the redundancy across all four disks and found inconsistencies—some data didn’t match RAID 5’s parity rules. Based on my past experience, I initially thought the recovery might fail. But then I remembered a similar case I had handled before. This time, instead of excluding any disk, we rebuilt the RAID with all four disks. To our surprise, the recovered data was fully functional and accessible!
This case taught me a valuable lesson. My initial approach was to assume that a missing disk would cause issues, so I focused on identifying which disk was faulty. However, in reality, if one disk has incomplete or outdated data, it can lead to errors when trying to reconstruct the RAID without it. Here’s what I learned from analyzing the situation:
1. In a RAID 5 array with four healthy disks, using all four to rebuild the data should work perfectly, and even if one disk is missing, the data should still be recoverable.
2. If one disk contains outdated or incorrect data, you can usually identify it by testing different combinations where each disk is excluded one at a time.
3. However, if the data is still corrupt after excluding a disk, many professionals might give up on further recovery. This often happens when a disk had a physical issue during the initial takeover, and the remaining disks are used to recover data. If the data is wrong, it’s easy to assume that a disk is faulty. After repairing the original bad disk, the usual step is to include it in the reconstruction—but sometimes people skip this step, assuming the disk is still bad.
So why did the data become correct only when all four disks were used? Why was it always wrong when one was missing? This is something that many in the data recovery field may have encountered before, but few have fully understood. After thinking through the process, I believe the root cause lies in the RAID controller’s XOR operation. If the XOR module isn’t functioning correctly or encounters an error, the data blocks may be written properly, but the parity (check) block might not be. This leads to incorrect parity information being stored. When tools calculate the XOR of the four disks, they detect an inconsistency and wrongly assume there’s a bad drive. However, since the actual data blocks are intact, the only way to recover the data is to use all four disks in the rebuild process.
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