Unite several directories into a single virtual filesystem.
Note: Aufs3 (Aufs version 3.x) only supports linux-3.0 and later.
USE="kernel-patch" emerge sys-fs/aufs3
You need to rebuild your kernel after instalation
genkernel all
modprove -v aufs
Create sample dirs
mkdir -p dir_one dir_two both_dirs
touch dir_one/file1 dir_two/file2
Mount
mount -t aufs -o dirs=dir_one:dir_two=ro none both_dirs
or
mount -t aufs -o br:dir_one:dir_two=ro none both_dirs
Now both_dirs = dir_two + dir_one
ls -1 both_dirs/
file1
file2
If you touch a file in both_dirs, this file will be show up in dir_one, not in dir_two, because dir_two is read only