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Update storage docs for content-addressability 

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
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Tonis Tiigi 2015-11-20 11:58:13 -08:00
parent 61773e5cbf
commit 909821f514
1 changed files with 6 additions and 21 deletions
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docs/userguide/storagedriver/imagesandcontainers.md
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@ -17,17 +17,11 @@ stores images. Then, you need an understanding of how these images are used in
## Images and containers rely on layers
Docker images are a series of read-only layers that are stacked
on top of each other to form a single unified view. The first image in the stack
is called a *base image* and all the other layers are stacked on top of this
layer. The diagram below shows the Ubuntu 15:04 image comprising 4 stacked image layers.
Each Docker image references a list of read-only layers that represent filesystem differences. Layers are stacked on top of each other to form a base for a container's root filesystem. The diagram below shows the Ubuntu 15.04 image comprising 4 stacked image layers.
![](images/image-layers.jpg)
When you make a change inside a container by, for example, adding a new file to the Ubuntu 15.04 image, you add a new layer on top of the underlying image stack. This change creates a new image layer containing the newly added file. Each image layer has its own universal unique identifier (UUID) and each successive image layer builds on top of the image layer below it.
Containers (in the storage context) are a combination of a Docker image with a
thin writable layer added to the top known as the *container layer*. The diagram below shows a container running the Ubuntu 15.04 image.
When you make a change inside a container by, for example, adding a new file to a container created from Ubuntu 15.04 image, you add a new layer on top of the underlying stack. This change creates a new writable layer containing the newly added file on top of the image layers. Each image layer is stored by a cryptographic hash over its contents and multiple images can share the same layers. The diagram below shows a container running the Ubuntu 15.04 image.
![](images/container-layers.jpg)
@ -46,7 +40,7 @@ Sharing is a good way to optimize resources. People do this instinctively in
daily life. For example, twins Jane and Joseph taking an Algebra class at
different times from different teachers can share the same exercise book by
passing it between each other. Now, suppose Jane gets an assignment to complete
the homework on page 11 in the book. At that point, Jane copy page 11, complete the homework, and hand in her copy. The original exercise book is unchanged and only Jane has a copy of the changed page 11.
the homework on page 11 in the book. At that point, Jane copies page 11, completes the homework, and hands in her copy. The original exercise book is unchanged and only Jane has a copy of the changed page 11.
Copy-on-write is a similar strategy of sharing and copying. In this strategy,
system processes that need the same data share the same instance of that data
@ -138,9 +132,9 @@ One way to do this is using a Dockerfile and the `docker build` command.
IMAGE CREATED CREATED BY SIZE COMMENT
03b964f68d06 About a minute ago /bin/sh -c echo "Hello world" > /tmp/newfile 12 B
013f3d01d247 6 weeks ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0 B
2bd276ed39d5 6 weeks ago /bin/sh -c sed -i 's/^#\s*\(deb.*universe\)$/ 1.879 kB
13c0c663a321 6 weeks ago /bin/sh -c echo '#!/bin/sh' > /usr/sbin/polic 701 B
6e6a100fa147 6 weeks ago /bin/sh -c #(nop) ADD file:49710b44e2ae0edef4 131.4 MB
<missing> 6 weeks ago /bin/sh -c sed -i 's/^#\s*\(deb.*universe\)$/ 1.879 kB
<missing> 6 weeks ago /bin/sh -c echo '#!/bin/sh' > /usr/sbin/polic 701 B
<missing> 6 weeks ago /bin/sh -c #(nop) ADD file:49710b44e2ae0edef4 131.4 MB
The `docker history` output shows the new `03b964f68d06` image layer at the
top. You know that the `03b964f68d06` layer was added because it was created
@ -148,15 +142,6 @@ One way to do this is using a Dockerfile and the `docker build` command.
The 4 image layers below it are the exact same image layers the make up the
ubuntu:15.04 image as their UUIDs match.
5. List the contents of the local storage area to further confirm.
$ sudo ls /var/lib/docker/aufs/layers
013f3d01d24738964bb7101fa83a926181d600ebecca7206dced59669e6e6778 2bd276ed39d5fcfd3d00ce0a190beeea508332f5aec3c6a125cc619a3fdbade6
03b964f68d06a373933bd6d61d37610a34a355c168b08dfc604f57b20647e073 6e6a100fa147e6db53b684c8516e3e2588b160fd4898b6265545d5d4edb6796d
13c0c663a321cd83a97f4ce1ecbaf17c2ba166527c3b06daaefe30695c5fcb8c
Where before you had four layers stored, you now have 5.
Notice the new `changed-ubuntu` image does not have its own copies of every layer. As can be seen in the diagram below, the new image is sharing it's four underlying layers with the `ubuntu:15.04` image.
![](images/saving-space.jpg)