OS6 Data Recovery - How to Mount BTRFS Volumes

Attempting to recover a RAID 6 volume with a missing hard disk

To attempt this recovery, the configuration is the same as listed in the above entry. I just physically unplugged the ReadyNAS drive number 1 (on the motherboard SATA connector 2) to simulate a completely failed drive.

I have been attempting to use mdadm to assemble the array in a degraded mode with just the remaining 3 hard disks. So far, no luck, but here's what I found.

First, the information about the array as I collected previously:

cat /proc/mdstat
Personalities:
md125 : inactive sdd1[3] (S) sdc1[2] (S) sdb1[1] (S)
             12576768 blocks super 1.2

md126 : inactive sdb2[2] (S) sdd2[3] (S) sdc2[2] (S)
            1572096 blocks super 1.2

md127 : inactive sdd3[3] (S) sdb3[1] (S) sdc3[2] (S)
            5845994613 blocks super 1.2

unused devices: <none>

Here is the result of information gathering about individual drives separately:

mdadm --examine /dev/sdd3
/dev/sdd3:
                 Magic : a92b4efc
               Version : 1.2
          Feature Map : 0x0
       Array UUID : be4dd378:31e311ab:9af53399:976b5245
                    Name : 5e276850:TESTFLX6-0
       Creation Time : Sun Jul 20 07:45:00 2014
       Raid Level : raid6
               Raid Devices : 4

 Avail Dev Size : 3897329742 (1858.39 GiB 1995.43 GB)
    Array Size : 3897329408 (3716.78 GiB 3990.87 GB)
 Used Dev Size : 3897329408 (1858.39 GiB 1995.43 GB)
   Data Offset : 262144 sectors
   Super Offset : 8 sectors
   Unused Space : before=262064 sectors, after=334 sectors
         State : clean
  Device UUID : fa36d5e1:76f3276a:e10be6e3:535a21bb

   Update Time : Mon Jul 21 00:53:10 2014
       Checksum : 21bec207 - correct
         Events : 19

     Layout : left-symmetric
  Chunk Size : 64K

Device Role : Active device 3
  Array State : AAAA ('A' == active, '.' == missing, 'R' == replacing)

I'm only listing one drive, but the other two report in a similar fashion.



Here is the result of information gathering about the array:

mdadm --examine /dev/sdd3 --scan
ARRAY /dev/md/TESTFLX6-0 metadata=1.2 UUID=be4dd378:31e311ab:9af53399:976b5245 name=5e276850:TESTFLX6-0

After investigating how to force the array to assemble, I tried this:

mdadm --create md127 --chunk=64 --level=raid6 --raid-devices=4 --run /dev/sdd3 /dev/sdc3 /dev/sdb3 missing --verbose
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: super1.x cannot open /dev/sdd3: Device or resource busy
mdmon: ddf: Cannot use /dev/sdd3: Device or resource busy
mdmon: Cannot use /dev/sdd3: It is busy
mdadm: cannot open /dev/sdd3: Device or resource busy

Any ideas about getting this to assemble and mount up as a degraded array with one disk missing? :shock:

Since RAID 6 should allow an array to be available with up to 2 disks missing, the array should be able to mount with just one missing. The mdadm man information says to use the word "missing" in place of the device that is missing and use the command "--run" to force the assembly of the incomplete array. I tried several variants of the commands including the following (but all state the device is busy):

mdadm -- assemble --scan --run (FAILED - mdadm: No arrays found in config file or automatically)

mdadm --assemble --uuid=be4dd378:31e311ab:9af53399:976b5245 --run (FAILED - Device busy)

mdadm --assemble md127 --uuid=be4dd378:31e311ab:9af53399:976b5245 --run (FAILED - Device busy)

mdadm --assemble --uuid=be4dd378:31e311ab:9af53399:976b5245 /dev/sdd3 /dev/sdc3 /dev/sdb3 --run (FAILED - Device busy)

I'd like to find out how to do this with test data first, before I have to try it with real data at risk.

Thanks for any ideas!

- Stan