When working with Docker, you'll often come across the need to save or export images and containers. Although these terms might seem interchangeable at first glance, they actually refer to different operations with distinct use cases. Understanding the difference between "save" and "export" in Docker is crucial for effectively managing Docker images and containers.

Docker Basics: Images and Containers

Before diving into the differences between saving and exporting, it's important to clarify what Docker images and containers are:

• Docker Image: A lightweight, standalone, and executable package that includes everything needed to run a piece of software. It comprises the application code, libraries, environment variables, and runtime requirements.
• Docker Container: An instance of a Docker image. When an image is run, it becomes a container, offering an isolated environment to execute applications.

Save vs. Export: Understanding the Differences

Docker save

The docker save command is used primarily for exporting Docker images. It packages the image and all associated layers into a tarball (.tar file). This command is particularly useful when you need to transfer an image between different Docker engines or environments that cannot access the Docker registry.

Technical Explanation

• Command Syntax:

  docker save -o <output_file.tar> <image_name:tag>

• What It Includes:
  • Image layers
  • Image metadata, such as labels and history
  • Essential information to reconstruct the same image in another environment
• Usage Scenario:
  • Useful for backup purposes
  • Transferring images to a system without direct access to the Docker registry

Docker export

In contrast, the docker export command is used for exporting Docker containers. When you export a container, it strips away the image layers and only captures the filesystem of the container’s current state into a tarball. This does not include the image metadata and thereby is more lightweight but less comprehensive compared to docker save.

Technical Explanation

• Command Syntax:

  docker export -o <output_file.tar> <container_id>

• What It Includes:
  • The filesystem contents of the container
  • Running state of the container at the time of export (without metadata)
• Usage Scenario:
  • Migrating a container's filesystem to another environment where you want to restore the state
  • Creating a snapshot of the current container state

Key Differences: save vs. export

The table below summarizes the key differences between docker save and docker export:

Additional Considerations

When deciding between docker save and docker export, consider the following:

• Reconstruction Efficiency: If you use docker save, the restoration process (docker load) retains the image with all its layers and metadata, making deployment on new systems straightforward. However, with docker export, you'll need to create a new image manually from the filesystem snapshot using docker import.
• Complexity and Overhead: docker save produces larger files because it retains all image layers and metadata. In contrast, docker export results in smaller files, but you lose the capability to rebuild the image exactly as the original.
• Portability and Flexibility: For environments where the Docker Hub is not accessible, docker save is invaluable. However, if you are interested in capturing and transferring only the changes done after the container started, docker export is the right choice.

Conclusion

In summary, docker save is intended for backing up and sharing Docker images along with their metadata, providing you an exact replica of the original image. On the other hand, docker export focuses on the state of a running container, capturing only the filesystem and omitting layers and metadata. Choosing the appropriate command depends on your specific needs in terms of transferring, backing up, or deploying Docker assets. Understanding these distinctions will help you tailor your Docker management strategy effectively.