So if you follow up with Docker, you might have found out that the default storage that is provided with a normal Docker installation, is backed by the mighty aufs, created by the awesome Junjiro Okajima.
But out there there is another file system that is faster, with smaller footprint, etc. It's named OverlayFS, and was merged in the main kernel. Sounds to good to be true? Well read about it.
TL;DR Docker With OverlayFS on Ubuntu 14.04:
Now, this file system is available only from kernel version 3.18 and up. If you just installed Ubuntu 14.04, as of this date (10th of November, 2015), you should have 3.19 already installed. If you have an older existing Ubuntu 14.04 you need to update your kernel manually.