unionfs/overlayfs fuse on older kernels (e.g. centos 6)

unionfs-fuse.md

build it

sudo yum install gcc fuse-devel wget unzip
wget "https://github.com/rpodgorny/unionfs-fuse/archive/refs/heads/master.zip"
unzip master.zip
cd unionfs-fuse-master/
make
sudo make install

mount it

mkdir  ./overlay
# assume you already have a "lower" and "upper" directory
unionfs -o cow  ./top/=RW:./lower/ ./overlay/