Comprehensive Overview of Storage Scalability in Docker

“First, a brief backstory on the storage situation for Docker since it was open-sourced in early 2013. At that time, Docker relied on a filesystem called AUFS (advanced multi layered unification filesystem). This Union filesystem provided the necessary features to support several of Docker’s main selling points:
– container creation speed
– copy-on-write image->container
…”

http://developerblog.redhat.com/2014/09/30/overview-storage-scalability-docker/