mdadm --misc --detail

[coconnor@grr:~]$ sudo mdadm --misc --detail /dev/md127
/dev/md127:
        Version : 1.2
  Creation Time : Sun Apr 10 13:19:04 2016
     Raid Level : raid1
     Array Size : 1047552 (1023.00 MiB 1072.69 MB)
  Used Dev Size : 1047552 (1023.00 MiB 1072.69 MB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Tue Mar  6 09:17:52 2018
          State : clean 
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

           Name : nixos:md0
           UUID : e8b915ab:a36ac554:3191076c:b905066f
         Events : 476

    Number   Major   Minor   RaidDevice State
       0       8       50        0      active sync   /dev/sdd2
       1       8       66        1      active sync   /dev/sde2
       2       8       82        2      active sync   /dev/sdf2
       3       8        2        3      active sync   /dev/sda2