Demonstrating Docker Volume

Under this page, we will illustrate the concept of volume. We will see how to use volume

• in a Dockerfile

• at runtime with the -v option

• using the volume API

We will also see what is bind-mounting on a simple example.

Data persistency without a volume

We will first illustrate how data is not persisted outside of a container by default. Let’s run an interactive shell within an alpine container named c1.

docker container run --name c1 -ti alpine sh

We will create the /data folder and a dummy hello.txt file in it.

mkdir /data && cd /data && touch hello.txt

We will then check how the read-write layer(container layer) is accessible from the host. Let exit the container first.

exit

Let’s inspect our container in order to get the location of the container’s layer. We can use the inspect command and then scroll into the output until the GraphDriver key, like the following.

docker container inspect c1

Or we can directly use the Go template notation and get the content of the GraphDriver keys right away.

docker container inspect -f "{{ json .GraphDriver }}" c1 | python -m json.tool

You can also use tools like grep too.

You should then get an output like the following (the ID will not be the same though)

{
    "Data": {
        "LowerDir": "/var/lib/docker/overlay2/55922a6b646ba6681c5eca253a19e90270e3872329a239a82877b2f8c505c9a2-init/diff:/var/lib/docker/overlay2/30474f5fc34277d1d9e5ed5b48e2fb979eee9805a61a0b2c4bf33b766ba65a16/diff",
        "MergedDir": "/var/lib/docker/overlay2/55922a6b646ba6681c5eca253a19e90270e3872329a239a82877b2f8c505c9a2/merged",
        "UpperDir": "/var/lib/docker/overlay2/55922a6b646ba6681c5eca253a19e90270e3872329a239a82877b2f8c505c9a2/diff",
        "WorkDir": "/var/lib/docker/overlay2/55922a6b646ba6681c5eca253a19e90270e3872329a239a82877b2f8c505c9a2/work"
    },
    "Name": "overlay2"
}

From our host, if we inspect the folder which path is specified in UpperDir, we can see our /data and the hello.txt file we created are there.

Try the below command, to see the contents of the /data folder:

ls /var/lib/docker/overlay2/[YOUR_ID]/diff/data

What happen if we remove our c1 container now? Let’s try.

docker container rm c1

It seems the folder defined in the UpperDir above does not exist anymore. Do you confirm that? Try running the ls command again and see the results.

This shows that data created in a container is not persisted. It’s removed with the container’s layer when the container is deleted.