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turbo
Asked: 2011-06-22 09:06:18 +0800 CST

How does btrfs RAID work in degraded mode?

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My idea was that (using loopback devices) it works like this

  1. Create the raid array sudo mkfs.btrfs -m raid1 -d raid1 /dev/loop1 /dev/loop2
  2. You mount them sudo mount /dev/loop1 /mnt and mark them touch goodcondition
  3. You unmount and simulate disk failure (remove disk or delete loopback device loop2 in my case)
  4. You mount degraded -o degraded and mark again touch degraded
  5. You add the bad disk again sudo btrfs dev add /dev/loop2
  6. You rebalance sudo btrfs fi ba /mnt

And Raid 1 should work again. But that's not the case. sudo btrfs fi show:

Total devices 3 FS bytes used 28.00KB
devid    3 size 4.00GB used 264.00MB path /dev/loop1
devid    2 size 4.00GB used 272.00MB path /dev/loop2
*** Some devices missing

The file degraded lives on loop1 but not on loop2 when loop2 is mounted in degraded mode. Why is that?

mount

2 Answers

  1. In this situation, you need to do two things. First, you need to indicate to btrfs that the missing device is permanently gone: btrfs dev delete missing /mnt (missing is a keyword indicating any missing devices). Second, you need to rebalance to ensure that the data is properly replicated: btrfs fi balance /mnt.

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  2. Best Answer

    It looks like you added a third device and so the original second is still missing. I guess you need to remove the missing device before adding the new one. The brtfs mailing list might also be a better place to ask this question.

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