Create partitions on both disks with same configuration.

parted -a opt /dev/sda
mklabel gpt
mkpart primary xfs 1 2mb
mkpart primary xfs 2mb 2000mb
mkpart primary xfs 2000mb 12000mb
mkpart primary xfs 12000mb -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

##################################

1. Boot up with the Gentoo minimal cd.

2. Mount your gentoo partitions…

mount /dev/sda3 /mnt/gentoo

mount /dev/sda1 /mnt/gentoo/boot

mount -t proc none /mnt/gentoo/proc

mount -o bind /dev /mnt/gentoo/dev

mount --rbind /sys /mnt/gentoo/sys

mount --rbind /dev /mnt/gentoo/dev

3. chroot into the install gentoo environment…

cd /mnt/gentoo

chroot /mnt/gentoo /bin/bash

env-update

source /etc/profile

genkernel --lvm --mdadm --install initramfs

If need To install GRUB, need to enter installed Gentoo system: Enter Installed System From Install Cd

and install GRUB2 on both physical disks with RAID support

emerge — ask sys-boot/grub:2
emerge mdadm
grub-install /dev/sda
grub-install /dev/sdb
grub-mkconfig -o /boot/grub/grub.cfg

add RAID support at initial boot sequence add

“domdadm rootfstype=xfs” to /etc/default/grub

(Gento):

GRUB_CMDLINE_LINUX="root=UUID=a5c783a2-2cb5-4480-8e61-860b503a4c05 rootfstype=XFS domdadm  dolvm"

list HDD UUID’s

blkid

add HDD UUID’s to fstab file

nano -w /etc/fstab
UUID=cce19bb0–9ec1–4417-b18d-69b9b9eb610a /boot xfs defaults,noatime 0 2
UUID=eb55e616–2951–42db-be28–880f66f1d185 none swap sw 0 0
UUID=748ec6a3-be66–4b5b-a73b-da5f066baaf4 / xfs noatime 0 1

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

Add records to mdadm.conf file:

mdadm --detail --scan >> /etc/mdadm/mdadm.conf