Ubuntu – How to convert a RAID 4 array to RAID 0

diskdisk-managementmdadmraid

I had a three-disk RAID 0 array and ran the following to add a fourth disk:

mdadm --manage /dev/md127 --add /dev/xvdi

Each disk is a 1TB EC2 volume. The array took about 40 hours to reshape. About 1 hour through, reshaping stopped and the volume became inaccessible. I restarted the machine and reshaping continued then finished seemingly successfully, but the array level is now reported as RAID 4 and the useable capacity hasn't changed.

mdadm --detail /dev/md127 now reports the following:

/dev/md127:
        Version : 1.2
  Creation Time : Wed Jul  1 22:26:36 2015
     Raid Level : raid4
     Array Size : 4294965248 (4096.00 GiB 4398.04 GB)
  Used Dev Size : 1073741312 (1024.00 GiB 1099.51 GB)
   Raid Devices : 5
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Sun Oct 11 07:40:48 2015
          State : clean, degraded 
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

     Chunk Size : 512K

           Name : [removed]
           UUID : [removed]
         Events : 63530

    Number   Major   Minor   RaidDevice State
       0     202      160        0      active sync   /dev/xvdk
       1     202      144        1      active sync   /dev/xvdj
       2     202       80        2      active sync   /dev/xvdf
       4     202      128        3      active sync   /dev/xvdi
       4       0        0        4      removed

My aim here is to have a 4TB RAID 0 array. I don't need redundancy since I backup by taking volume snapshots in AWS. I'm running Ubuntu Server 14.04.3.

How do I switch to RAID 0, without losing any data, taking into account the fact that the state is clean, degraded?

Best Answer

You can change the current configuration directly to a RAID with mdadm -G -l 0 /dev/md127. Since a RAID 4 with only 4 of 5 members is essentially a RAID 0 without a parity stripe, the conversion will occur instantly. If there was a parity member, it would be dropped, but since it's already listed as "Removed", it will simply be dropped, Raid Devices decremented to 4, and state should be "clean".

From the mdadm query printed above, you can see that the member size is 1TB and the volume size is 4TB, so the volume should be usable as is, even without the parity member. You will then need to grow the partition with parted and perform filesystem resize operations per usual.

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