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- :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**.
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