One of the fundamental features that make Docker a popular choice is the ability to persist data across container runs. This is possible thanks to Docker volumes. In this article, we’ll explore the concept of Docker volumes comprehensively.

What Are Docker Volumes?

Simply put, Docker volumes are mechanisms that allow data to be shared and persisted between containers and the host where Docker is running. They are especially useful for storing data such as databases, logs, or any other information that needs to “survive” beyond a container’s lifecycle.

Docker volumes offer several advantages, including:

  1. Data Persistence: Data stored in volumes is not lost when a container stops or is removed.

  2. Data Sharing: Volumes can be shared across multiple containers, enabling them to access the same data.

  3. Improved Performance: Compared to copying data directly in and out of containers, using volumes can significantly improve I/O performance.

  4. Simple Backup and Restore: Since data is isolated in volumes, backing up and restoring becomes easier.

Now that we understand why volumes matter, let’s look at how to use them.

Using Docker Volumes Link to heading

Mount Types Link to heading

  • Host Volume Mounts (Bind Mounts): Host volumes, or bind mounts, allow you to mount directories or files from the host’s filesystem directly into a container. This means you can share specific host data with a container. To create a bind mount, use the -v or –volume option followed by the host path and container path.
docker run -v /path/on/host:/path/in/container my_app

This mounts the /path/on/host directory from the host into the container at /path/in/container.

  • Docker Volumes (Managed by Docker): Docker volumes are Docker-specific volumes managed by the Docker daemon. They are created automatically and are generally more flexible regarding where and how data is stored on the host. To create a Docker volume, you can run:
docker volume create my_volume

This creates a volume named my_volume that can be mounted into one or more containers.

Mounting a Volume in a Container Link to heading

Now, let’s create a container and mount our volume to it. Suppose you have an app that needs to store data in a volume.

# -d to run in detached mode (background)
docker run -d -v my_volume:/app/data my_app

This command creates a new container from the image my_app and mounts the volume my_volume to /app/data inside the container. Any data written to /app/data inside the container will be stored in the my_volume volume on the host.

Sharing Volumes Between Containers

One of the benefits of volumes is the ability to share them between containers. This is useful when you want multiple containers to access the same data. To do this, simply mount the same volume in different containers.

docker run -d -v my_volume:/app/data container_1
docker run -d -v my_volume:/app/data container_2

Now, both container_1 and container_2 can access and modify the same data stored in the my_volume volume.

Since we don’t specify permissions, by default all containers have read-write (rw) access.

Viewing Volumes

To list all Docker volumes on your system, use:

docker volume ls

To inspect a specific volume, use:

docker volume inspect my_volume

Examples of Using Docker Volumes Link to heading

Example 1: Storing Data in a Database

Let’s run a MySQL database container and create a volume to store its data:

docker volume create database_data
docker run -d -v database_data:/var/lib/mysql mysql:latest

This ensures the database data persists in the database_data volume.

Example 2: Sharing Logs

Suppose you have a web app and a log server running in separate containers. You can create a volume to share logs between them:

docker volume create logs
docker run -d -v logs:/app/logs my_web_app
docker run -d -v logs:/app/logs log_server

Now both containers can access the same logs in the logs volume.

Conclusion Link to heading

Docker volumes are an essential tool for managing data efficiently and persistently in containers. They allow you to persist data, share information between containers, and improve I/O performance. With the concepts and examples presented here, you’re ready to start using Docker volumes in your own projects. Explore further and experiment with different use cases to sharpen your Docker skills.

Official documentation: https://docs.docker.com/storage/volumes/