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mdadm - v3.2.5 - 18th May 2012

Currently have:

        Version : 1.2
  Creation Time : Tue Jun 25 19:03:31 2013
     Raid Level : raid5
     Array Size : 2930135040 (2794.39 GiB 3000.46 GB)
  Used Dev Size : 2930135040 (2794.39 GiB 3000.46 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Wed Jun 26 17:17:01 2013
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 512K

           Name : desktop:0  (local to host desktop)
           UUID : 91a6c44d:21226975:8d2dc41a:7fcff414
         Events : 7434

    Number   Major   Minor   RaidDevice State
       2       8       16        0      active sync   /dev/sdb
       1       8       48        1      active sync   /dev/sdd


/dev/sdc is full of data.

I want to grow the raid to 3 devices in degraded state (therefore 6TB), copy all the data to the raid. Then actually add /dev/sdc.

Already tried:

andey@desktop:~$ sudo mdadm --add /dev/md127 missing
mdadm: 'missing' only meaningful with --re-add

andey@desktop:~$ sudo mdadm --re-add /dev/md127 missing

No complaints from mdadm, however when I checked the RAID there wasn't a third drive

andey@desktop:~$ sudo mdadm --grow /dev/md127 --raid-devices=3
mdadm: Need 1 spare to avoid degraded array, and only have 0.
       Use --force to over-ride this check.
andey@desktop:~$ sudo mdadm --grow /dev/md127 --raid-devices=3 --force
mdadm: Need to backup 1024K of critical section..
mdadm: /dev/md127: Cannot grow - need a spare or backup-file to backup critical section
share|improve this question
up vote 1 down vote accepted

You are almost there. Now use the --backup-file option to specify a temp file for mdadm to use. Do not put it on your md array.

share|improve this answer
Thanks, worked perfectly – Andrew Wei Jun 27 '13 at 1:43

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