RAID 0 with 3 disks: md/raid0: please set raid.default_layout to 1 or 2

Today was time for a backup. I've recently increased my RAID 0 with a third disk for more space. Adding the disk was already a pain in the ass and it seemed that I fucked up again. Last time I did a backup was round about a month ago. So today I attached the three disks again and the RAID would not build. A mdadm --detail gave me:

[root@homeserver ~]# mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Mon Oct  7 18:11:26 2019
        Raid Level : raid0
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Mon Oct  7 18:11:26 2019
             State : active, FAILED, Not Started
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

        Chunk Size : 512K

Consistency Policy : unknown

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       -       0        0        1      removed
       -       0        0        2      removed

That's nasty. When I tried to assemble no error appeared on scree. But dmesg logged something:

[869705.131868] sd 15:0:0:0: [sdm] Attached SCSI disk
[869705.248265] md127: ADD_NEW_DISK not supported
[870085.207371] md: md127 stopped.
[870260.516069] md: md127 stopped.
[870260.523859] md/raid0:md127: cannot assemble multi-zone RAID0 with default_layout setting
[870260.523861] md/raid0: please set raid.default_layout to 1 or 2

Thankfully some people on the internet (🖇️ 🔐) digged into the C code and found a solution:

After that the detail page looks promising:

[root@homeserver ~]# mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Mon Oct  7 18:11:26 2019
        Raid Level : raid0
        Array Size : 8790295552 (8383.08 GiB 9001.26 GB)
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Mon Oct  7 18:11:26 2019
             State : clean
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

        Chunk Size : 512K

Consistency Policy : none

              Name : homeserver:127  (local to host homeserver)
              UUID : c1e68c25:0cdaa7e9:3d0a14a0:5717a5f1
            Events : 0

    Number   Major   Minor   RaidDevice State
       0       8      177        0      active sync   /dev/sdl1
       1       8      161        1      active sync   /dev/sdk1
       2       8      193        2      active sync   /dev/sdm1

Time to start the backup!


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