36 lines
1.3 KiB
Markdown
36 lines
1.3 KiB
Markdown
page_title: Layers
|
|
page_description: Organizing the Docker Root File System
|
|
page_keywords: containers, lxc, concepts, explanation, image, container
|
|
|
|
# Layers
|
|
|
|
## Introduction
|
|
|
|
In a traditional Linux boot, the kernel first mounts the root [*File
|
|
System*](/terms/filesystem) as read-only, checks its
|
|
integrity, and then switches the whole rootfs volume to read-write mode.
|
|
|
|
## 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**.
|
|
|
|
![](/terms/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.
|
|
|
|
## Union File System
|
|
|
|
We call the union of the read-write layer and all the read-only layers a
|
|
**union file system**.
|