sudo mdadm --detail /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Wed Apr 15 10:26:31 2015
     Raid Level : raid10
     Array Size : 7813772288 (7451.79 GiB 8001.30 GB)
  Used Dev Size : 3906886144 (3725.90 GiB 4000.65 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Fri Jun 10 12:28:33 2016
          State : clean, degraded, recovering
 Active Devices : 3
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 1

         Layout : near=2
     Chunk Size : 512K

 Rebuild Status : 10% complete

           Name : ark:0
           UUID : d6e25e98:9d484eeb:21cfa8eb:304e23a8
         Events : 52443

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       4       8       33        1      active sync   /dev/sdc1
       5       8       49        2      spare rebuilding   /dev/sdd1
       3       8       65        3      active sync   /dev/sde1