Docker Graphdriver: Devicemapper Overlay

Docker is an open-source platform that allows developers to automate the deployment and management of applications inside containers. One of the key components of Docker is its graphdriver, which is responsible for managing the filesystem of the containers. In this article, we will explore the Devicemapper Overlay graphdriver, its benefits, and how to use it with Docker.

Introduction to Devicemapper

Devicemapper is a Linux kernel subsystem that provides support for creating virtual block devices. It allows Docker to create thin provisioned block devices for each container's filesystem. These devices are stored as files on the host system and are managed by the Devicemapper subsystem.

Devicemapper provides various storage drivers, including loopback, direct-lvm, and overlay. The overlay driver uses the overlay filesystem to create a layered filesystem for containers. Each layer represents a change to the filesystem, and these layers are stacked on top of each other to form the final filesystem.

Benefits of Devicemapper Overlay

The Devicemapper Overlay graphdriver provides several benefits compared to other graphdrivers:

  1. Improved Performance: The overlay filesystem uses the Copy-on-Write (CoW) mechanism to minimize disk space usage and improve performance. Only the changes to the filesystem are stored in the upper layer, reducing the amount of data that needs to be written to the underlying storage.

  2. Space Efficiency: The overlay driver leverages the CoW mechanism to save disk space. Each layer only stores the changes from the previous layer, resulting in efficient disk space utilization.

  3. Fast Container Creation: Devicemapper Overlay enables rapid container creation as it only needs to create a thin provisioned block device and mount the relevant layers. This eliminates the need to copy the entire base filesystem for each container.

Using Devicemapper Overlay with Docker

To use the Devicemapper Overlay graphdriver with Docker, you need to ensure that your Linux kernel has support for overlay filesystems. You can check if the overlay module is loaded by running the following command:

lsmod | grep overlay

If the module is not loaded, you can load it using the modprobe command:

sudo modprobe overlay

Once the overlay module is loaded, you can configure Docker to use the Devicemapper Overlay driver. To do this, edit the Docker daemon configuration file (/etc/docker/daemon.json) and add the following content:

{
  "storage-driver": "devicemapper",
  "storage-opts": [
    "dm.fs=overlay2"
  ]
}

Save the file and restart the Docker daemon for the changes to take effect:

sudo systemctl restart docker

Verification

To verify that the Devicemapper Overlay driver is being used, you can inspect the Docker info output. Run the following command:

docker info

Look for the Storage Driver field, which should be set to devicemapper:

Storage Driver: devicemapper

Conclusion

The Devicemapper Overlay graphdriver is an excellent choice for Docker users who prioritize performance, space efficiency, and fast container creation. By leveraging the overlay filesystem and the Copy-on-Write mechanism, Devicemapper Overlay optimizes storage utilization and provides efficient container management.

In this article, we explored the basics of the Devicemapper Overlay graphdriver, its benefits, and how to use it with Docker. Now you can take advantage of the powerful features offered by the Devicemapper Overlay driver to enhance your Docker workflows.

Table of Contents

  • Introduction to Devicemapper
  • Benefits of Devicemapper Overlay
  • Using Devicemapper Overlay with Docker
  • Verification
  • Conclusion