How to create Linux system disks with RAID1 hard drive setup

Create partitions on both disks with same configuration.

parted -a opt /dev/sda
mklabel gpt
mkpart primary ext2 1 2mb
mkpart primary ext2 2mb 2000mb
mkpart primary ext2 2000mb 10000mb
mkpart primary ext2 10000mb -1
name 1 grub
name 2 boot
name 3 swap
name 4 root
set 1 bios_grub on
set 2 raid on
set 3 raid on
set 4 raid on

parted /dev/sda set 1 bios_grub on
parted /dev/sdb set 1 bios_grub on

Now create raid:

mdadm --create --verbose /dev/md125 --level=1 --raid-devices=2 /dev/sda2 /dev/sdb2
mdadm --create --verbose /dev/md126 --level=1 --raid-devices=2 /dev/sda3 /dev/sdb3
mdadm --create --verbose /dev/md127 --level=1 --raid-devices=2 /dev/sda4 /dev/sdb4

mkswap /dev/md126 && swapon /dev/md126
mkfs.xfs /dev/md125
mkfs.xfs /dev/md127

Source: How to install Gentoo Linux with RAID1 hard drive setup (VirtualBox tested)