Adding an md device to a LVM VG containing the root filesystem

Debian:

When adding an md device to a LVM VG containing the root filesystem, like so:
vgextend vg_with_root /dev/md99

Don't forget to run the following:

/usr/share/mdadm/mkconf > /etc/mdadm/mdadm.conf
dpkg-reconfigure mdadm

The first one is to update the mdadm.conf, and the second one is to reconfigure the initrd image the new md device will also be assembled prior to activating the VG. Otherwise, activating the VG will fail because it cannot find a required PV (the new md device).