Create software RAID1

# create raid1 using /dev/sda1 and /dev/sdb1
mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/sd[ab]1

# List raid1 info
mdadm --detail /dev/md0

# check raid config in /dev/sd[ab]
mdadm -E /dev/sd[ab]

# Then install CentOS on /dev/md0

Refer to: 

1. Setting up RAID 1 (Mirroring) using ‘Two Disks’ in Linux – Part 3