OverlayFS

Make sure the kernel is newer than 3.18

# for Ubuntu 14.04 (upgrade kernel)
apt-get install linux-generic-lts-vivid linux-headers-generic-lts-vivid

# install kernel module
modprobe overlay
lsmod | grep overlay

# lower layer
low=/home/lower

# upper layer
upp=/home/upper

# tmp directory
wod=/home/work

# overlayfs mount
mount overlay -t overlay -o lowerdir="$low",upperdir="$upp",workdir="$wod" "$upp"