So, we figured out how to build
Docker images using
Dockerfile in the last article. I hope everyone now knows how to build a
Docker image from scratch.
Before we start this article, let’s get familiar with
Docker image filesystem. How many of you know about the
Docker image filesystem?
Docker images are read-only templates, which could contain an operating system and an application on top of it. It is then used for creating
But most importantly, we should know how does a Docker image work?
Docker image file system is layered. It uses Union file systems to combine these layers to form a single image.
One of the reasons Docker is so lightweight is because of these layers. When you change a Docker image—for example, update an application to a new version— a new layer gets built. Thus, rather than replacing the whole image or entirely rebuilding, as you may do with a virtual machine, only that layer is added or updated. Now you don’t need to distribute a whole new image, just the update, making distributing Docker images faster and simpler.
Source: Official Docker Documentation
Deep Diving to understand Docker Image File System
Let’s do some more digging to understand it practically. I’m using Docker-Machine on mac and I have started the default environment:
…and I’ve removed everything so it should not show me any containers, images or intermediate images.
You can see that, my docker host is clean and no signs of an image or a container.
Did you ever think where are Docker images stored?
The first place to look is in
/var/lib/docker/. We are using
Machine, so I’m going to ssh into my
Docker-Machine and show you:
Right, now nothing is present as we have not pulled or build any images. Let me pull the
Docker RunDeck image from my DockerHub repository and see what happens:
docker pull bhalothia/docker-rundeck:v1.1
Alright, now the RunDeck image is downloaded and we are good to create containers out of it. We can check what all is updated inside
/var/lib/docker folder on the
Before that, let’s do
docker images and
docker images -a:
Now, at this point of time you might be wondering what is the difference between
docker images and
docker images -a?
Docker file system layers are by default stored at
/var/lib/docker/graph, which is called as the graph database. And these layers corresponds to the
When I did
docker pull bhalothia/docker-rundeck:v1.1, the image was downloaded one layer at a time. Layer
ba249489d0b6 was downloaded as
<none>:<none> image and so on. The final image was
bhalothia\docker-rundeck:v1 and all other layers were downloaded as intermediate images.
Docker allows you to see all intermediate images using the flat
Every layer, one after the other, starting from
ba249489d0b6 to the final image
241f5ca9955c have the parent-child hierarchical relationship.
To verify this, you can run this command:
cd /var/lib/docker/graph && more layer_id/json
So, if you are worried about the
<none>:<none> images, which are also called as intermediate images then you can relax.
But, wait…what is Dangling Image?
Hey, you need to know about the evil
<none>:<none> images as well.. those which aren’t intermediate and which shows up when you do
Docker keeps all of the images in its cache that you have used in the disk, even if those are not actively running.
Check all open issues on
Here’s how you can clean up dangling images:
docker rmi $(docker images --quiet --filter "dangling=true")
It will give you an error if there’s no dangling image present. If you don’t want that, then this might work for you:
docker images -qf dangling=true | xargs docker rmi
This command will do the manual garbage collection.
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