Overlay File Systems used by containers

This post is a small diagram discussing overlay file systems used by containers like docker for saving disk space. The “layers” we see as docker runs through every command of Dockerfile is result of docker building another lower directory (possibly reusable) to the cascaded overlay file system used for a container. Since container systems like docker use overlays, re-building an existing image with different tag doesn’t go through the build steps like installing packages again! :)

Show me some code/commands!

• Union one lower directory and an upper directory:

  sudo mount -t overlay overlay -o lowerdir=./lower,upperdir=./upper,workdir=./parent ./parent
  

• Union multiple lower directories and one upper directory:

  sudo mount -t overlay overlay -o lowerdir:./lower_1:./lower_2:./lower_3,upperdir=./upper,workdir=./parent ./parent
  

References/More to read

• Julia Evans’ blog: https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/
• Docker: https://docs.docker.com/storage/storagedriver/