raid

When installing archlinux on raid do not forget update hooks at file
vim /etc/mkinitcpio.conf

update line
HOOKS="base udev autodetect block mdadm_udev filesystems usbinput fsck"

and update initramfs
mkinitcpio -p linux

Scan all arrays and try to fix them
mdadm --assemble --scan

after successful boot save raid configuration
mdadm --detail --scan >> /etc/mdadm/mdadm.conf

And in the end generate new initramfs