cat /proc/mdstat
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md3 : active raid1 sda4[3] sdb4[2]
2778744127 blocks super 1.2 [2/1] [_U]
[>....................] recovery = 1.3% (38884416/2778744127) finish=534.3min speed=85452K/sec
md2 : active raid1 sda3[3] sdb3[2]
134216632 blocks super 1.2 [2/2] [UU]
md1 : active raid1 sda2[3] sdb2[2]
524276 blocks super 1.2 [2/2] [UU]
md0 : active (auto-read-only) raid1 sda1[3] sdb1[2]
16776120 blocks super 1.2 [2/2] [UU]
unused devices: <none>
mdadm --manage /dev/md0 --fail /dev/sda1
mdadm --manage /dev/md1 --fail /dev/sda2
mdadm --manage /dev/md2 --fail /dev/sda3
mdadm --manage /dev/md3 --fail /dev/sda4
mdadm --manage /dev/md0 --remove /dev/sda1
mdadm --manage /dev/md1 --remove /dev/sda2
mdadm --manage /dev/md2 --remove /dev/sda3
mdadm --manage /dev/md3 --remove /dev/sda4
shutdown and remove failed disk
boot
mdadm --manage /dev/md0 --add /dev/sda1
mdadm --manage /dev/md1 --add /dev/sda2
mdadm --manage /dev/md2 --add /dev/sda3
mdadm --manage /dev/md3 --add /dev/sda4
watch cat /proc/mdstat