Can you explain the principle of OverlayFS and its application in Docker containerization?

OverlayFS is a filesystem in Linux that allows multiple directories (layers) to be combined into a single virtual layer. It operates on a 'layered' principle: there is a lower layer (read-only) and an upper layer (read-write). All changes are written to the upper layer, while reading occurs from the combined view of both layers.

In Docker, OverlayFS is used to implement a copy-on-write mechanism for storing images and containers. Images consist of several layers stacked on top of each other. When a container is started, an upper layer is created where all changes are recorded, while the base layers remain unchanged. This allows efficient use of disk space and quick creation of new containers.

For example, if you have a base image with Ubuntu and then an image with an application, OverlayFS will combine them, and the container will see them as a single filesystem, with changes only saved in the upper layer.