Docker Anonymous Volumes

Introduction

When working with Docker containers, data persistence is a common challenge. By default, any data created inside a container is lost when the container is removed. Docker provides several solutions for this problem, and one of them is anonymous volumes.

Anonymous volumes are Docker-managed volumes that provide a way to persist data without having to specify a location on the host filesystem. They're called "anonymous" because Docker generates a random name for them, and you don't need to track these names directly.

In this guide, we'll explore:

• What anonymous volumes are
• How they differ from named volumes and bind mounts
• When to use anonymous volumes
• How to create and manage them
• Best practices and limitations

Understanding Docker Storage Options

Before diving into anonymous volumes, let's understand Docker's storage options:

• Volumes: Docker-managed storage in the host filesystem
• Bind Mounts: Direct mapping to a host filesystem path
• tmpfs Mounts: Stored in host memory only

What Are Anonymous Volumes?

Anonymous volumes are Docker-managed volumes that:

1. Are created and managed by Docker
2. Have randomly generated IDs instead of human-readable names
3. Persist beyond the container lifecycle (by default)
4. Are not easily reused across containers

When Docker creates an anonymous volume, it:

• Allocates space in the Docker storage directory (usually /var/lib/docker/volumes/ on Linux)
• Assigns a random UUID as the volume identifier
• Mounts the volume to the specified directory in the container

Creating Anonymous Volumes

Method 1: Using Dockerfile VOLUME Instruction

You can define anonymous volumes in a Dockerfile using the VOLUME instruction:

dockerfile

FROM ubuntu:latest
RUN mkdir -p /app/data
VOLUME /app/data

When you build and run a container from this image, Docker automatically creates an anonymous volume mounted to /app/data.

Method 2: Using the -v Flag with docker run

You can also create anonymous volumes when running a container:

bash

docker run -v /app/data ubuntu:latest

This command creates a container with an anonymous volume mounted at /app/data.

Inspecting Anonymous Volumes

Let's see anonymous volumes in action:

bash

# Create a container with an anonymous volume
docker run --name test-container -v /app/data ubuntu:latest touch /app/data/testfile

# Inspect the container to find the volume
docker inspect test-container

The output will include a "Mounts" section like this:

json

"Mounts": [
    {
        "Type": "volume",
        "Name": "2a3f5766b8c5af8e624581d868f21c9a6a5a873f70999339d51d8a7e8f4b6123",
        "Source": "/var/lib/docker/volumes/2a3f5766b8c5af8e624581d868f21c9a6a5a873f70999339d51d8a7e8f4b6123/_data",
        "Destination": "/app/data",
        "Driver": "local",
        "Mode": "",
        "RW": true,
        "Propagation": ""
    }
]

Notice the randomly generated name - this is characteristic of anonymous volumes.

Listing Anonymous Volumes

You can list all volumes (including anonymous ones) with:

bash

docker volume ls

You'll see output like:

DRIVER    VOLUME NAME
local     2a3f5766b8c5af8e624581d868f21c9a6a5a873f70999339d51d8a7e8f4b6123

Example: Using Anonymous Volumes for a Database Container

Let's create a practical example using a MongoDB container with an anonymous volume:

bash

# Run MongoDB with an anonymous volume for data
docker run -d --name mongo-db -v /data/db mongo:latest

# Create some data
docker exec mongo-db sh -c 'echo "db.test.insert({name:\"example\"})" | mongo'

# Verify the data exists
docker exec mongo-db sh -c 'echo "db.test.find()" | mongo'

Now, let's remove the container but keep the volume:

bash

# Remove the container
docker rm -f mongo-db

# Create a new container with the same anonymous volume
docker run -d --name new-mongo-db --volumes-from mongo-db mongo:latest

# Verify the data still exists
docker exec new-mongo-db sh -c 'echo "db.test.find()" | mongo'

You should see your data is preserved!

Lifecycle of Anonymous Volumes

By default, anonymous volumes are not automatically removed when you remove a container. To remove the container and its anonymous volumes, use:

bash

docker rm -v container_name

When to Use Anonymous Volumes

Anonymous volumes are useful when:

1. You need temporary persistence - Data should outlive the container but doesn't need a permanent name
2. The Dockerfile defines storage needs - The image creator determines where volumes should be mounted
3. You want to protect container directories from being overwritten by bind mounts
4. You don't need to share volumes between multiple containers

Anonymous vs. Named Volumes

Let's compare anonymous volumes with their close relative, named volumes:

Feature	Anonymous Volumes	Named Volumes
Creation	Auto-generated name	User-specified name
Reusability	Harder to reuse	Easy to reference by name
Lifecycle management	More complex	Simpler to manage
Use case	Short-term persistence	Long-term data storage
Creation command	docker run -v /container/path	docker run -v name:/container/path

Best Practices

1. Use named volumes for important data - Anonymous volumes are harder to track and manage
2. Use the -v flag when removing containers to clean up anonymous volumes
3. Use anonymous volumes to protect container directories from being accidentally overwritten
4. Periodically clean up unused volumes with docker volume prune

Limitations and Considerations

• Anonymous volumes are harder to back up and restore
• They can consume disk space if not properly managed
• Difficult to share between multiple containers
• Not ideal for production environments where data tracking is important

Practical Example: Web Server Cache

Let's create a simple web server that uses an anonymous volume for caching:

dockerfile

FROM nginx:alpine
# Create a cache directory and make it a volume
RUN mkdir -p /var/cache/nginx
VOLUME /var/cache/nginx
COPY ./default.conf /etc/nginx/conf.d/default.conf

Build and run the container:

bash

docker build -t cache-example .
docker run -d --name web-server -p 8080:80 cache-example

After some usage, the cache will populate in the anonymous volume, improving performance while allowing the container to be replaced without losing the cache.

Summary

Anonymous volumes provide a simple way to persist data in Docker containers without having to manage volume names. They're useful for:

• Protecting container directories from being overwritten
• Temporary data persistence
• Cases where the Dockerfile should define storage needs

While they're convenient for development and simple use cases, named volumes are generally a better choice for important data in production environments where tracking and management are crucial.

Additional Resources

• Docker Volumes Documentation
• Docker Storage Options

Exercises

1. Create a Dockerfile that uses an anonymous volume to store application logs
2. Run a database container with an anonymous volume, add data, then remove and recreate the container
3. Practice using the docker inspect command to find information about anonymous volumes
4. Compare the performance of a web application with and without an anonymous volume for caching
5. Write a shell script to clean up anonymous volumes older than 7 days