Volumes

Volumes

The advantage to a volume is that it can be easily wrapped up with a container and be portable along with an image, meaning that one could wrap up everything together, the image and the volume, and ship or share it.

To create a volume, we need to define it. We do that by either creating it on its own, starting a container with an attached volume, or starting a service with an attached volume. The easiest way is to create it.

$ docker volume create example

Now if we look at what volumes are available, using docker volume ls we’ll see the volume example listed, however this doesn’t do a lot other than reserve some storage on your host for the volume. What we really want to do is have a volume mounted to a container. We do this using the docker run --mount command.

NOTE: The --mount flag and the -v flags will achieve the same things, but with different syntax. The --mount flag is easier to use and has a clearer syntax, so that’s what we’ll be using for these examples.

To start a container with a volume, we’ll use the following command:

$ docker run -i -t \
  --name exampleContainer \
  --mount source=exampleVolume,target=/app \
  ubuntu /bin/bash

Let’s go over this line by line:

docker run -i -t \

This tells docker that we’re going to run a container, we want it to be interactive -i, and we want to be able to use the terminal to interact with it -t.

  --name exampleContainer \

We’re naming our container exampleContainer

  --mount source=exampleVolume,target=/app \

We’re using the volume exampleVolume for this container and mounting it to the /app folder within the container.

  ubuntu /bin/bash

Finally, we’re using the default ubuntu image and using the Bash shell for interacting with it.

If this works, then we should see something that looks like this:

root@8bf1f5964b3d:/#

Where the string after the root@ will vary based on your hash.

Now let’s try an experiment, shall we? While we’re inside this container we just created, we’re going to create two files. One in the /app folder, and one in the user folder, in this case /root.

Run the following commands:

$ echo 'Test volume file' >> /app/test

$ echo 'Test container file' >> ~/container-test

We can check that these files exist by running cat /app/test and cat ~/container-test which should display the expected text.

NOTE: the ~ in bash is a shortcut to the current user’s home folder. For root, this is /root, but for other users it is usually /Users/<my user name>.

Now, let’s exit our container by typing exit. Since we don’t have anything running in the container this will cause it to stop.

Run docker container rm exampleContainer to remove the container.

Finally, recreate the container using the command from above:

$ docker run -i -t \
  --name exampleContainer \
  --mount source=exampleVolume,target=/app \
  ubuntu /bin/bash

This will get us back into our exampleContainer. Notice that we have a new hash in the command line prompt.

Go to your /root folder using cd /root or cd ~ and run ls -al to see what files are there. You should see output similar to

root@af040d25a903:~# ls -al
total 16
drwx------ 2 root root 4096 Oct 10 21:07 .
drwxr-xr-x 1 root root 4096 Oct 26 15:43 ..
-rw-r--r-- 1 root root 3106 Apr  9  2018 .bashrc
-rw-r--r-- 1 root root  148 Aug 17  2015 .profile

Our test container file is gone forever. This happened because we removed the container and everything in it disappeared.

Now, go to your /app folder using cd /app and run ls -al to see what files are there. You should see output similar to

root@af040d25a903:/app# ls -al
total 12
drwxr-xr-x 2 root root 4096 Oct 26 15:32 ./
drwxr-xr-x 1 root root 4096 Oct 26 15:43 ../
-rw-r--r-- 1 root root   17 Oct 26 15:32 test

Run cat test and you’ll see our Test volume file text from before. Why is this? It’s because we mounted a volume to the container in the /app folder. The data from the volume is accessible to the container, but it isn’t tied to the container. In fact, it lives on our host machine! We could visit it without even spinning up a container if we wanted to.

Let’s exit this container and remove it. Run exit from the command line of the container and once in your host terminal run docker container rm exampleContainer. We’ll also remove the example volume by using docker volume rm exampleVolume.

Next, we’ll learn about the other primary method of persistent storage in Docker: Bind mounts.

For further reading on volumes, check out https://docs.docker.com/storage/volumes/