How OverlayFS Powers Docker's Image Layering System

Docker containers are not virtual machines but isolated Linux processes using kernel features such as namespaces and cgroups. At the core of Docker's storage model is OverlayFS, a union filesystem that stacks multiple read-only directories to present a single unified view. When a file exists in more than one layer, the topmost layer takes precedence, resolving conflicts without physically merging data. Any changes or new files written inside a running container are directed to a writable layer via a copy-on-write mechanism, leaving the underlying image layers untouched. This layered architecture allows Docker images to be lightweight, reusable, and efficiently shared across containers.