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35 lines
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35 lines
1.3 KiB
Markdown
page_title: Layers
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page_description: Organizing the Docker Root File System
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page_keywords: containers, lxc, concepts, explanation, image, container
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# Layers
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## Introduction
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In a traditional Linux boot, the kernel first mounts the root [*File
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System*](/terms/filesystem) as read-only, checks its
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integrity, and then switches the whole rootfs volume to read-write mode.
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## Layer
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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 read-write
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mode, it takes advantage of a [union
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mount](http://en.wikipedia.org/wiki/Union_mount) to add a read-write
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file system *over* the read-only file system. In fact there may be
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multiple read-only file systems stacked on top of each other. We think
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of each one of these file systems as a **layer**.
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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 something
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needs to update an existing file in a lower layer, then the file gets
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copied to the upper layer and changes go into the copy. The version of
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the file on the lower layer cannot be seen by the applications anymore,
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but it is there, unchanged.
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## Union File System
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We call the union of the read-write layer and all the read-only layers a
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**union file system**.
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