Docker volumes consuming a lot of space

This week I was cleaning my home directory from all the things that were not useful anymore. When I went to see the disk usage, I had a surprise: There was only 20% of free space in my SSD. I don’t have a lot of programs installed, neither did I remember downloading any huge files. So something was taking a lot of space without my knowledge.

The search for the root of the problem quickly led me to the Docker folder. The volumes folder was taking more than 30GB of disk space, 50% of the total partition size. I wasn’t making any huge project that would have big volumes. So what was the problem?

Docker doesn’t remove volumes by default

When you create a container that requires a volume, like MongoDB, the volume is not deleted after the container is removed. Even though when you run docker run again a new volume is created, the previous one is not deleted from disk.

So what was happening is that I was running docker run mongo every time I needed to create a fresh database while developing an application. When I needed to wipe out everything, I would just remove the container and create a new one.

What I was unaware of is that deleting the container does not delete the created volume. So every time a MongoDB container was created, a 300MB volume was created alongside it, and never removed. To fix it, the first thing I did was to prune the unused volumes:

$ docker volume prune

Which cleaned more than 30GB of unused volumes.

To stop docker run to keep the volumes while I am developing something, I now run it with the --rm flag, which deletes the volume when the container is removed:

$ docker run --rm mongo

Now I don’t have to be worried about my SSD being consumed by unused volumes.