:title: Layers
:description: Organizing the Docker Root File System
:keywords: containers, lxc, concepts, explanation, image, container

Layers
======

In a traditional Linux boot, the kernel first mounts the root
:ref:`filesystem_def` as read-only, checks its integrity, and then
switches the whole rootfs volume to read-write mode.

.. _layer_def:

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**.

.. image:: images/docker-filesystems-multilayer.png

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.

.. _ufs_def:

Union File System
.................

We call the union of the read-write layer and all the read-only layers
a **union file system**.
