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New to RAID and Linux. I grew my RAID5 array from 3 to 4 devices. For other reasons, I had to fail and remove one of the 4 drives - sda1. I'd like to shrink it now back to 3 devices, but when trying to do so, I'm getting a new_offset error:

# mdadm --grow --raid-devices=3  /dev/md127
mdadm: Cannot set new_offset for /dev/sdb1

Some notes:

  • The array is mounted at /, so I'd like to avoid having to unmount it.

  • There is data on the array, so I'd like to preserve it.

RAID details:

/dev/md127:
        Version : 1.1
  Creation Time : Mon Oct 22 16:20:37 2012
     Raid Level : raid5
     Array Size : 1953518592 (1863.02 GiB 2000.40 GB)
  Used Dev Size : 976759296 (931.51 GiB 1000.20 GB)
   Raid Devices : 4
  Total Devices : 3
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Tue Nov  8 17:09:28 2016
          State : active, degraded
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 512K

  Delta Devices : 1, (3->4)

           Name : xxx
           UUID : xxx
         Events : 275192

    Number   Major   Minor   RaidDevice State
       3       8       49        0      active sync   /dev/sdd1
       2       0        0        2      removed
       4       8       33        2      active sync   /dev/sdc1
       6       8       17        3      active sync   /dev/sdb1

Can I fail and remove /dev/sdb1 from the array? I imagine that would solve the error message, but I worry that in growing the array to 4 devices, failing 2 of them will render it irreparable. If that can't be done, what can be?

Update

Worked around the headache. rsynced the contents of the degraded array to a backup drive, replaced the RAID with fresh-baked drives, then rsynced back.

Voriki
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  • hmm... I never knew that you can shrink the number of disks in a RAID with a command line in Linux. In a Windows world, taking an image of the data to an external location, rearranging the RAID and then copy back the image would do it. – Noor Khaldi Nov 08 '16 at 22:24
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    @NoorKhaldi: You're right. There is no way in Linux. – Ipor Sircer Nov 08 '16 at 22:26
  • @NoorKhaldi I'm growing more convinced that that's the way to do it. But it'd be nice if there was a way that didn't involve shutting things off while the array was being set up – Voriki Nov 08 '16 at 22:27
  • It seems you forgot to use resize2fs first – kay27 Nov 08 '16 at 23:12

1 Answers1

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You can do it but you need to specify a backup file in the command line. In your case something like 

mdadm --grow --raid-devices=3  /dev/md127 --backup-file /root/md127.backup

should work.

If you have a usb stick or some other storage that isn't in the array, use it for backup file.

Gary Dale
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