47 lines
1.3 KiB
ReStructuredText
47 lines
1.3 KiB
ReStructuredText
:title: Images
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:description: Definition of an image
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:keywords: containers, lxc, concepts, explanation, image, container
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.. _image_def:
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Image
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=====
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.. image:: images/docker-filesystems-debian.png
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In Docker terminology, a read-only :ref:`layer_def` is called an
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**image**. An image never changes.
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Since Docker uses a :ref:`ufs_def`, the processes think the whole file
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system is mounted read-write. But all the changes go to the top-most
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writeable layer, and underneath, the original file in the read-only
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image is unchanged. Since images don't change, images do not have state.
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.. image:: images/docker-filesystems-debianrw.png
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.. _parent_image_def:
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Parent Image
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............
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.. image:: images/docker-filesystems-multilayer.png
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Each image may depend on one more image which forms the layer beneath
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it. We sometimes say that the lower image is the **parent** of the
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upper image.
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.. _base_image_def:
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Base Image
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..........
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An image that has no parent is a **base image**.
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Image IDs
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.........
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All images are identified by a 64 hexadecimal digit string (internally a 256bit
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value). To simplify their use, a short ID of the first 12 characters can be used
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on the command line. There is a small possibility of short id collisions, so the
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docker server will always return the long ID.
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