Union File System for Containers

As we all know that container are run from image which are layers. The Implementation is based on Union file systems. UnionFS is a file systems that operate by creating layers making them very lightweight and fast. There are multiple implementations of UnionFS like AUFS, btrfs , vfs and device mapper.

Basically union mounting is a way of combining multiple directories into one that appears to contain their combined contents. So a UnionFS takes a existing file system and overlays it on a newer file system. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new virtual filesystem.

Lets see a implementation of union Fs.  

1.   Install aufs-tools on a Ubnutu machine.

2.   Create a dir in home location : mkdir /home/home_dir.

3.   Create a dir in tmp location : mkdir /tmp/tmp_dir

4.   Mount both dir using the aufs as, mount -t aufs -o br=/tmp/tmp_dir:/home/home_dir none /tmp/aufs-root/

We have mounted the /tmp/tmp_dir and /home/home_dir onto a single file system /tmp/aufs-root

Now once we create file in /tmp/home_dir and /home/home_dir directories and check in the /tmp/aufs-root we can see,

          root@work-node2:/tmp/aufs-root# ll

-rw-r--r-- 1 root root    0 Jul 17 01:38 home_dir_file

-rw-r--r-- 1 root root    0 Jul 17 01:38 tmp_dir_file

So we have basically mounted the 2 directories as layers on /tmp/aufs-root. Changes made to these files will also be reflected on the originals. This is basis for docker container technology.