mirror of
https://github.com/moby/moby.git
synced 2022-11-09 12:21:53 -05:00
40 lines
1.4 KiB
ReStructuredText
40 lines
1.4 KiB
ReStructuredText
: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**.
|