testsvr:~ #   mdadm -D  /dev/md0
mdadm: md device /dev/md0 does not appear to be active.


testsvr:~ # mdadm --detail  /dev/md0  
mdadm: cannot open /dev/md0: No such file or directory


testsvr:~ # mdadm --assemble --scan /dev/md0
mdadm: /dev/md0 has been started with 11 drives and 1 spare


testsvr:~ # mdadm --detail /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Feb 18 11:22:35 2021
     Raid Level : raid5