title: function of docker directory structure in /var/lib/docker/
body: <p>when I start a fresh docker daemon, the docker directory is as this:</p>
  
  <pre>
  /var/lib/docker/
  ├── aufs
  │   ├── diff
  │   ├── layers
  │   └── mnt
  ├── containers
  ├── graph
  ├── init
  │   └── dockerinit-0.7.3
  ├── linkgraph.db
  ├── lxc-start-unconfined -> /usr/bin/lxc-start
  ├── repositories-aufs
  └── volumes
  </pre>
  
  <p>as the title say, what is the function of every directory in this structure?</p>