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Assume the following situation: An administrator was not careful enough maintaining a NAS server (QNAP QTS 4.3.3) using RAID5 and it has already reached a situation in which two (out of four) drives contain bad blocks, however no drive failed completely. There's no external backup.

Assuming that the bad blocks on the two drives don't overlap, it makes sense that 100% data recovery should be possible, but is that really so? How should one approach this?

Thanks all!

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    How should one approach this? By not wasting any further time and restoring from backup to a new and better RAID. You also lack details like the type of RAID and platform you use. Commented Jan 1, 2018 at 11:01
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    Please stop using R5 too, it's 2018 ffs Commented Jan 1, 2018 at 20:00

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Backup ASAP. If you want to be 100% sure you'd better hire some recovery service. It's going to cost you some $$$ but you decide does it worth it or not.

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  • I think you need a few more "$" for the recovery service! <grin> Commented Jan 2, 2018 at 4:18
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    This is true! That's why backup is king LOL. Commented Jan 2, 2018 at 11:17
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  1. As BaronSamedi1958 stated, backup now, all that can still be read. If that's not enough you need to hire a costly recovery specialist.
  2. Replace the failed disks, first one and after (hopefully successfull) rebuild the other. Depending on the exact controller logic, a full data check may be a better solution to repair the damage before replacing the disks. You should contact vendor support to help you decide on that.
  3. Set up a regular backup, according to your scenario and restore requirements.
  4. Reconsider whether a low-cost NAS fits your requirements in performance and reliability - few, large disks are cheap but cause extensive rebuild periods when one fails.
  5. Make sure you set up regular disk scrubbing aka media patrol. This makes sure no stale sectors build up that pop up on a rebuild and make it fail.
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  • Step 1, okay. Step 2, maybe not so much. If he does get a good backup, surely he should not be rebuilding a RAID 5! Commented Jan 1, 2018 at 20:56
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    RAID5 is dead in the water... Commented Jan 1, 2018 at 20:59
  • With only four drives and weak NAS hardware there's little choice (that's where step 4 becomes vital). RAID 5 can be OK if you've got a solid backup/restore concept - which you need anyway - and understand its limited reliability. Commented Jan 1, 2018 at 21:07
  • Either way, although it there appear to be references to RAID 6 in documentation from at least as early as QTS 4.3.3.0154. And if half the drives are crapping out, probably not a bad time to consider something better. Commented Jan 1, 2018 at 21:21
  • Four-bay QNAP NASes usually support RAID 6 but lacking decent hardware support this can hurt performance significantly. Plus, with four drives you can just as well use RAID 10. Commented Jan 1, 2018 at 21:25

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