<!--[metadata]> +++ draft = true title = "Layers" description = "Organizing the Docker Root File System" keywords = ["containers, lxc, concepts, explanation, image, container"] [menu.main] parent = "mn_use_docker" +++ <![end-metadata]--> # Layers ## Introduction In a traditional Linux boot, the kernel first mounts the root [*File System*](/terms/filesystem) as read-only, checks its integrity, and then switches the whole rootfs volume to read-write mode. ## Layer When Docker mounts the rootfs, it starts read-only, as in a traditional Linux boot, but then, instead of changing the file system to read-write mode, it takes advantage of a [union mount](http://en.wikipedia.org/wiki/Union_mount) to add a read-write file system *over* the read-only file system. In fact there may be multiple read-only file systems stacked on top of each other. We think of each one of these file systems as a **layer**.  At first, the top read-write layer has nothing in it, but any time a process creates a file, this happens in the top layer. And if something needs to update an existing file in a lower layer, then the file gets copied to the upper layer and changes go into the copy. The version of the file on the lower layer cannot be seen by the applications anymore, but it is there, unchanged. ## Union File System We call the union of the read-write layer and all the read-only layers a **union file system**.