Recovering RAID5 array after zeroing wrong superblock

data-recoverymdadmraid5

I have a raid5 array on 4 disks, sda1, sdb1, sdd1 and sde1. sdd1 was removed but is a healthy drive. While attempting to re-add it, I accidentally zeroed out the superblock for sdb1.

sdb1's data is consistent with sda1 and sde1, just missing it's superblock.

Is it possible to recover the superblock on that device and reassemble the array (degraded) without losing any data?

Below is the output of 'mdadm -E /dev/sd{a,d,e}1'.

Am I reading the output correctly in that sde1 is Device 0, sdd1 is Device 2 and sda1 is Device 3? Then that would mean that sdb1 was Device 1. Could I recreate the array with the following command without data loss:

$ sudo mdadm --create /dev/md0 --assume-clean --level=5 --raid-devices=4 /dev/sde1 /dev/sdb1 missing /dev/sda1

/dev/sda1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : 81a36846:cf4f0489:219e2546:b1f5b90e
           Name : cowbell:0
  Creation Time : Sun Sep 25 20:24:46 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3907025920 (1863.02 GiB 2000.40 GB)
     Array Size : 5860538880 (5589.05 GiB 6001.19 GB)
    Data Offset : 1024 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : a0abbc72:f8bb1e2b:e8f81f5c:ed62a979

Internal Bitmap : 8 sectors from superblock
    Update Time : Tue Jul 23 18:05:09 2013
       Checksum : 9f46a56 - correct
         Events : 717820

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 3
   Array State : AA.A ('A' == active, '.' == missing)
/dev/sdd1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : 81a36846:cf4f0489:219e2546:b1f5b90e
           Name : cowbell:0
  Creation Time : Sun Sep 25 20:24:46 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3907027053 (1863.02 GiB 2000.40 GB)
     Array Size : 5860538880 (5589.05 GiB 6001.19 GB)
  Used Dev Size : 3907025920 (1863.02 GiB 2000.40 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : active
    Device UUID : 79f5632d:46d5c083:a1c0130c:b83b0654

Internal Bitmap : 8 sectors from superblock
    Update Time : Tue Jul 23 17:42:02 2013
       Checksum : a78d6f5b - correct
         Events : 717392

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 2
   Array State : AAAA ('A' == active, '.' == missing)
/dev/sde1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : 81a36846:cf4f0489:219e2546:b1f5b90e
           Name : cowbell:0
  Creation Time : Sun Sep 25 20:24:46 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3907027053 (1863.02 GiB 2000.40 GB)
     Array Size : 5860538880 (5589.05 GiB 6001.19 GB)
  Used Dev Size : 3907025920 (1863.02 GiB 2000.40 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : 6abd9160:c8ae382c:e6c73d01:37ac057b

Internal Bitmap : 8 sectors from superblock
    Update Time : Tue Jul 23 18:05:09 2013
       Checksum : 9f5d8fa6 - correct
         Events : 717820

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 0
   Array State : AA.A ('A' == active, '.' == missing)

Best Answer

In addition to backups, you might also want to consider just making full dd copies of the disks before attempting any recovery.

That being said, looks like you're on the right track for recovery without data loss. You're interpreting the device numbers correctly. That command looks like what you need.

See my answer here - these things are surprisingly resilient against data loss, and the destruction of the superblock is not damaging to the ability to rebuild the array in the same geometry (see test 4 in that answer).

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