I recently played around with Docker, an application container and virtualization technology for Linux. It was pretty cool and I was able to create Docker images and containers within a couple of minutes. Everything was working right out of the box!
At the end of my day I wanted to persist my work. I stumbled over the Docker commands save and export and wondered what their difference is. So I went to StackOverflow and asked a question which was nicely answered by mbarthelemy. Here is what I found out.
How Docker works (in a nutshell)
Docker is based on so called images. These images are comparable to virtual machine images and contain files, configurations and installed programs. And just like virtual machine images you can start instances of them. A running instance of an image is called container. You can make changes to a container (e.g. delete a file), but these changes will not affect the image. However, you can create a new image from a running container (and all it changes) using docker commit <container-id> <image-name>.
Let’s make an example:
# we pull a base Docker image called busybox
just like in the official Hello-World-example
sudo docker pull busybox
let’s check which images we have
we should see the image busybox
sudo docker images
now we make changes to a container of this image
in this case we make a new folder
sudo docker run busybox mkdir /home/test
let’s check which containers we now have
note that the container stops after each command
we should see a busybox container with our command
sudo docker ps -a
now we can commit this changed container
this will create a new image called busybox-1
you see the with the command above
sudo docker commit busybox-1
let’s check which images we have now
we should see the image busybox and busybox-1
sudo docker images
to see the difference between both images we
can use the following check for folders:
sudo docker run busybox [ -d /home/test ] && echo ‘Directory found’ || echo ‘Directory not found’ sudo docker run busybox-1 [ -d /home/test ] && echo ‘Directory found’ || echo ‘Directory not found’
Now we have two different images (busybox and busybox-1) and we have a container made from busybox which also contains the change (the new folder /home/test). Let’s see how we can persist our changes.
Export
Export is used to persist a container (not an image). So we need the container id which we can see like this:
sudo docker ps -a
To export a container we simply do:
sudo docker export > /home/export.tar
The result is a TAR-file which should be around 2.7 MB big (slightly smaller than the one from save).
Save
Save is used to persist an image (not a container). So we need the image name which we can see like this:
sudo docker images
To save an image we simply do:
sudo docker save busybox-1 > /home/save.tar
The result is a TAR-file which should be around 2.8 MB big (slightly bigger than the one from export).
The difference
Now after we created our TAR-files, let’s see what we have. First of all we clean up a little bit - we remove all containers and images we have right now:
# first we see which containers we have sudo docker ps -a
now we remove all of them
sudo docker rm # now we see which images we have sudo docker images
and we remove them too
sudo docker rmi busybox-1 sudo docker rmi busybox
We start with our export we did from the container. We can import it like this:
# import the exported tar ball: cat /home/export.tar | sudo docker import - busybox-1-export:latest
check the available images
sudo docker images
and check if a new container made from this image
contains our folder (it does!)
sudo docker run busybox-1-export [ -d /home/test ] && echo ‘Directory found’ || echo ‘Directory not found’
We can do the same for the saved image:
# import the exported tar ball: docker load < /home/save.tar
check the available images
sudo docker images
and check if a new container made from this image
contains our folder (it does!)
sudo docker run busybox-1 [ -d /home/test ] && echo ‘Directory found’ || echo ‘Directory not found’
So what’s the difference between both? Well, as we saw the exported version is slightly smaller. That is because it is flattened, which means it lost its history and meta-data. We can see this by the following command:
# shows the layers of all images sudo docker images –tree
If we run the command we will see an output like the following. As you can see there, the exported-imported image has lost all of its history whereas the saved-loaded image still have its history and layers. This means that you cannot do any rollback to a previous layer if you export-import it while you can still do this if you save-load the whole (complete) image (you can go back to a previous layer by using docker tag <LAYER ID> <IMAGE NAME>).
vagrant@ubuntu-13:~$ sudo docker images –tree ├─f502877df6a1 Virtual Size: 2.489 MB Tags: busybox-1-export:latest └─511136ea3c5a Virtual Size: 0 B └─bf747efa0e2f Virtual Size: 0 B └─48e5f45168b9 Virtual Size: 2.489 MB └─769b9341d937 Virtual Size: 2.489 MB └─227516d93162 Virtual Size: 2.489 MB Tags: busybox-1:latest
Best regards, Thomas