Booting from Gentoo Live CD

Using raid 1

mdadm --assemble /dev/md1 /dev/sda1 /dev/sdb1
mdadm --assemble /dev/md2 /dev/sda2 /dev/sdb2
mdadm --assemble /dev/md3 /dev/sda3 /dev/sdb3

swapon /dev/md2

mount /dev/md3 /mnt/gentoo
mount /dev/md1 /mnt/gentoo/boot

mount -t proc none /mnt/gentoo/proc
mount -o bind /dev /mnt/gentoo/dev

chroot /mnt/gentoo /bin/bash
env-update
source /etc/profile
export PS1="(chroot) $PS1"

source: http://gentoo-wiki.com/HOWTO_Gentoo_Install_on_Software_RAID