41 lines
1.4 KiB
ReStructuredText
41 lines
1.4 KiB
ReStructuredText
:title: Layers
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:description: Organizing the Docker Root File System
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:keywords: containers, lxc, concepts, explanation, image, container
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Layers
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======
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In a traditional Linux boot, the kernel first mounts the root
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:ref:`filesystem_def` as read-only, checks its integrity, and then
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switches the whole rootfs volume to read-write mode.
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.. _layer_def:
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Layer
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.....
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When Docker mounts the rootfs, it starts read-only, as in a traditional
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Linux boot, but then, instead of changing the file system to
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read-write mode, it takes advantage of a `union mount
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<http://en.wikipedia.org/wiki/Union_mount>`_ to add a read-write file
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system *over* the read-only file system. In fact there may be multiple
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read-only file systems stacked on top of each other. We think of each
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one of these file systems as a **layer**.
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.. image:: images/docker-filesystems-multilayer.png
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At first, the top read-write layer has nothing in it, but any time a
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process creates a file, this happens in the top layer. And if
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something needs to update an existing file in a lower layer, then the
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file gets copied to the upper layer and changes go into the copy. The
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version of the file on the lower layer cannot be seen by the
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applications anymore, but it is there, unchanged.
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.. _ufs_def:
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Union File System
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.................
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We call the union of the read-write layer and all the read-only layers
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a **union file system**.
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