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I’ve got a Synology NAS that I’m trying to reduce a RAID6 array from 12 drives down to 11 drives.

I’ve already reduced the size of the file system and logical volume, but when I try to reduce the size of the RAID array to 11 drives, mdadm just gives me an error.

root@DiskStation:~# mdadm -V
mdadm - v3.4 - 28th January 2016

root@DiskStation:~# mdadm -D /dev/md2
/dev/md2:
        Version : 1.2
  Creation Time : Sat Jul 12 13:24:29 2014
     Raid Level : raid6
     Array Size : 19487796480 (18585.01 GiB 19955.50 GB)
  Used Dev Size : 1948779648 (1858.50 GiB 1995.55 GB)
   Raid Devices : 12
  Total Devices : 12
    Persistence : Superblock is persistent

    Update Time : Tue Feb  4 10:14:55 2020
          State : clean
 Active Devices : 12
Working Devices : 12
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           Name : DiskStation:2  (local to host DiskStation)
           UUID : 263bdad7:eed15299:ff0c7a70:c97ee595
         Events : 414186

    Number   Major   Minor   RaidDevice State
      15       8        5        0      active sync   /dev/sda5
       1       8       21        1      active sync   /dev/sdb5
       8       8       37        2      active sync   /dev/sdc5
       9       8       53        3      active sync   /dev/sdd5
       4       8       69        4      active sync   /dev/sde5
       7       8      117        5      active sync   /dev/sdh5
       6       8      101        6      active sync   /dev/sdg5
       5       8       85        7      active sync   /dev/sdf5
      11       8      133        8      active sync   /dev/sdi5
      12       8      149        9      active sync   /dev/sdj5
      13       8      165       10      active sync   /dev/sdk5
      14       8      181       11      active sync   /dev/sdl5

root@DiskStation:~# mdadm --grow -n11 /dev/md2
mdadm: Not allowed to reduce the number of devices.

I have very little experience with this sort of thing, and the fact that I can’t find anything online regarding that particular error has left me at a dead end.

I’ve been following these instructions here, and they say I should be prompted to reduce the array size. Do I just need to go ahead and reduce the array size anyway without being prompted? How do I know what size to make it?

Scibit
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  • My guess is Synology is using a custom build of the mdadm tool and blocked this action. Connect the disks to a different Linux system and try from there. You could also try calculating the new array size by using "Array size" from the output above divided by 12 multiplied by 11 then use that with `--array-size`. However, might be that Synology also blocked this. – mbirth Feb 23 '21 at 00:37

0 Answers0