Recently, Darth Data Recovery was assigned a RAID 5 array from an HP server. One of the drives in the array could not be physically recognized, so we proceeded with 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 investigate the physical issue with the faulty drive. Fortunately, after some repairs, the drive became functional again, and we were able to create a disk image. Here are the steps I followed:
1. I used the image of the failed drive to rebuild the RAID 5 array, splitting the data into three different combinations based on the other working disks.
2. I exported the files that seemed problematic and tried opening them to check for normality.
The result was unexpected: no matter which disk was missing, the combined data remained corrupted and couldn't be opened. When using the "escort ship" tool to verify RAID 5 redundancy across all four disks, we found inconsistencies—some data didn't match the expected RAID 5 parity information. Based on past experience, I initially thought the recovery might be a failure. But then I remembered a similar case I had handled before. This time, instead of trying to reconstruct the array without a disk, I attempted to combine all four disks together. To my surprise, the recovered data was fully readable and intact!
This case taught me an important lesson. Initially, I assumed that the problem stemmed from a missing or faulty disk, and I focused on eliminating bad drives through disk omission. However, the real issue wasn't about identifying a bad disk—it was about understanding how the RAID controller handled parity calculations.
Let’s break it down:
1. In a RAID 5 setup with four disks, if all disks are healthy, combining all four should yield correct data. If any one disk is missing, the data should still be recoverable.
2. If one disk has outdated or incorrect data, you can identify it by testing the array with each disk omitted.
3. However, when a disk is missing and the data is wrong, many technicians might give up at this point, assuming the missing disk is the cause. This often happens when a physical issue exists on one of the disks during the initial takeover. We may try to recover data using only the remaining healthy disks. If the data is corrupt, we assume there's a disk with outdated data, and the original bad disk isn’t functioning properly. After repairing the faulty disk, the common approach is to include it in the array while omitting one of the good ones, which is a step many overlook.
So why does omitting a disk from a 4-disk RAID 5 array lead to corrupted data, but combining all four disks works? This situation is not uncommon in data recovery. Some professionals have encountered it before, and I think I finally figured out what was going on. The key lies in the RAID controller’s XOR operation module. If the controller fails to write the parity block correctly—either due to a hardware error or a software glitch—the data blocks may be written properly, but the parity information becomes invalid. As a result, when you try to calculate the XOR using the remaining disks, the system incorrectly identifies a bad drive. However, since the actual data blocks are intact, the only way to recover the correct data is to use all four disks in the array.
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