cd /tmp
Create the necessary directories.
mkdir -p ~/tmp/lower ~/tmp/upper ~/tmp/overlay ~/tmp/workdir
Lets create a fake block device to hold our "lower" filesystem.
dd if=/dev/zero of=lower-fs.img bs=4096 count=102400
dd if=/dev/zero of=upper-fs.img bs=4096 count=102400
Give this block device an ext4 filesystem.
mkfs -t ext4 lower-fs.img
mkfs -t ext4 upper-fs.img
Mount the filesystem we just created and give it a file.
sudo mount lower-fs.img ~/tmp/lower
sudo chown $USER:$USER ~/tmp/lower
echo "hello world" >> ~/tmp/lower/lower-file.txt
Remount the lower filesystem as read only just for giggles.
sudo mount -o remount,ro lower-fs.img /tmp/lower
Mount the upper filesystem
sudo mount upper-fs.img /tmp/upper
sudo chown $USER:$USER /tmp/upper
Create the workdir in the upper filesystem and the directory in
the upper filesystem that will act as the upper
directory (they both have to be in the same filesystem)
mkdir /tmp/upper/upper
mkdir /tmp/upper/workdir
Create our overlayfs mount
sudo mount -t overlay -o lowerdir=~/tmp/lower,upperdir=~/tmp/upper,workdir=~/tmp/workdir my-overlay-mount merge
Create needed directories.
mkdir -p ~/tmp/top ~/tmp/lower
Mount in user space 2 directories into directory 'merge'
unionfs-fuse -o cow top=RW:lower=RO merge