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Docker Anonymous Volumes

To manage my stacks, containers, images, networks, and volumes, I use Portainer Community Edition.

I was surprised when I found three volumes with randomly generated names mixed in with my carefully organized ones. Initially, I thought it was a bug, but I soon realized they were Anonymous Volumes.

What Are Anonymous Volumes?

A Docker Anonymous Volume is a type of storage volume that Docker automatically creates when a container starts.

These volumes are not explicitly named by the user, which is why they’re called anonymous. Docker uses them to persist data generated or required by the container, especially when a volume mount point is specified in the container's Dockerfile or docker run command without a named volume being provided.

Key Characteristics

  1. Automatic Creation: Docker automatically creates anonymous volumes when a container specifies a mount point for data storage (via the VOLUME directive in the Dockerfile or the -v option in docker run) without assigning a specific name to the volume.
  2. Naming Convention: These volumes are given unique, random names by Docker, making them harder to identify and manage.
  3. Purpose: Anonymous volumes are typically used to persist data that a container needs to retain between runs, such as database files or logs, without requiring user management.
  4. Temporary Nature: If a container is removed, the anonymous volume remains on the system unless manually deleted. Over time, these volumes can accumulate and consume disk space if not properly managed.

Organizing Anonymous Volumes

While anonymous volumes can be useful in certain situations, I find them a bit untidy for production environments, so I decided to trace their origins.

Example Scenario

Let’s consider the following scenario:

  • You run a Redis container using the redis:latest image.

If you run the container without specifying a volume name:

docker run redis:latest

Docker will create an anonymous volume with a random name.

To uncover more details about the volume configuration, you can use the docker image inspect command to query the image’s metadata. For example, with the redis:latest image, the command would be:

docker image inspect redis:latest --format {{.Config.Volumes}}

This command will show the path inside the container where the volume is mounted:

map[/data:{}]

In this case, the path is /data.

This occurs because the Redis Dockerfile includes a VOLUME /data directive, instructing Docker to store Redis data in /data inside the container.

To keep things organized, I added a named volume to my compose.yaml file:

volumes:
    - volume_name:/data

With this adjustment, the problem is solved, and everything is neatly organized once again.

Happy me! 🌱

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