On 03/07/2022 21:07, Marc MERLIN wrote:

Is there any way around this, or not without a full reformat/rebuild?

Not sure, but I expect there is ... is this the collision between 
journal and bitmap? I know it won't let you mix those.

Look at the wiki, it tells you how to get rid of the bitmap, and then 
you can probably change it to pol.

Cheers,
Wol

https://raid.wiki.kernel.org/index.php/Linux_Raid

gargamel:~# mdadm --query --detail /dev/md5
/dev/md5:
            Version : 1.2
      Creation Time : Tue Jan 21 10:35:52 2014
         Raid Level : raid5
         Array Size : 15627542528 (14903.59 GiB 16002.60 GB)
      Used Dev Size : 3906885632 (3725.90 GiB 4000.65 GB)
       Raid Devices : 5
      Total Devices : 5
        Persistence : Superblock is persistent

      Intent Bitmap : Internal

        Update Time : Sun Jul  3 03:02:01 2022
              State : active, checking
     Active Devices : 5
    Working Devices : 5
     Failed Devices : 0
      Spare Devices : 0

             Layout : left-symmetric
         Chunk Size : 512K

Consistency Policy : bitmap

       Check Status : 99% complete

               Name : gargamel.svh.merlins.org:5  (local to host gargamel.svh.merlins.org)
               UUID : ec672af7:a66d9557:2f00d76c:38c9f705
             Events : 642977

     Number   Major   Minor   RaidDevice State
        0       8      193        0      active sync   /dev/sdm1
        6       8      177        1      active sync   /dev/sdl1
        2       8      209        2      active sync   /dev/sdn1
        3       8        1        3      active sync   /dev/sda1
        5       8       17        4      active sync   /dev/sdb1
gargamel:~# mdadm --grow --consistency-policy=ppl /dev/md5
mdadm: Current consistency policy is bitmap, cannot change to ppl

Kernel 5.16.

Thanks,
Marc