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dockerdocker-composedocker-volume

Why additional anonymous created by docker-compose?

I created a volume named testVol using following command in docker:

docker volume create testVol

Then I tried running a container which uses the same volume.

Here is the docker-compose.yml file:

version: "3"
services:
  webapp:
    image: 'redis:latest'
    volumes:
      - testVol:/app/myfolder 

volumes:
  testVol:
    external: true

When I run docker compose up command, I can see that volume testVol is being attached to the container. But along with it, i can see another anonymous volume created which is also attached to the same container.

enter image description here

What is this anonymous volume and why is it attached to the container?

Solution

  • If you look at the redis image's Dockerfile, it contains the line

    VOLUME /data

    This tells Docker that there always must be some sort of volume mounted on /data. If the operator doesn't explicitly mount something else there, Docker will create an anonymous volume and mount it.

    In your setup you're mounting a volume on to /app/myfolder but nothing into /data, so the extra anonymous volume you're seeing is the volume Docker creates as a result of this VOLUME statement.