Docker Volume Type Comparisons

docker_volume_type_comparisons.md

title	subtitle	author	date	source	snippet	gist	notoc
Docker Volume Type Comparisons	Quick reference for comparing Docker Volume Types.	Docker	May 16, 2021	https://github.com/docker/getting-started/blob/master/docs/tutorial/using-bind-mounts/index.md#quick-volume-type-comparisons	https://jonlabelle.com/snippets/view/markdown/docker-volume-type-comparisons	https://gist.github.com/jonlabelle/df5cc879d110137c210a53b115063b98	false

	Named Volumes	Bind Mounts
Host Location	Docker chooses	You control
Mount Example (using -v)	my-volume:/usr/local/data	/path/to/data:/usr/local/data
Populates new volume with container contents	Yes	No
Supports Volume Drivers	Yes	No

Choose the right type of mount

No matter which type of mount you choose to use, the data looks the same from within the container. It is exposed as either a directory or an individual file in the container's filesystem.

An easy way to visualize the difference among volumes, bind mounts, and tmpfs mounts is to think about where the data lives on the Docker host.

• Volumes are stored in a part of the host filesystem which is managed by Docker (/var/lib/docker/volumes/ on Linux). Non-Docker processes should not modify this part of the filesystem. Volumes are the best way to persist data in Docker.

• Bind mounts may be stored anywhere on the host system. They may even be important system files or directories. Non-Docker processes on the Docker host or a Docker container can modify them at any time.

• tmpfs mounts are stored in the host system's memory only, and are never written to the host system's filesystem.

Good use cases for Volumes

Volumes are the preferred way to persist data in Docker containers and services. Some use cases for volumes include:

• Sharing data among multiple running containers. If you don't explicitly create it, a volume is created the first time it is mounted into a container. When that container stops or is removed, the volume still exists. Multiple containers can mount the same volume simultaneously, either read-write or read-only. Volumes are only removed when you explicitly remove them.

• When the Docker host is not guaranteed to have a given directory or file structure. Volumes help you decouple the configuration of the Docker host from the container runtime.

• When you want to store your container's data on a remote host or a cloud provider, rather than locally.

• When you need to back up, restore, or migrate data from one Docker host to another, volumes are a better choice. You can stop containers using the volume, then back up the volume's directory (such as /var/lib/docker/volumes/<volume-name>).

• When your application requires high-performance I/O on Docker Desktop. Volumes are stored in the Linux VM rather than the host, which means that the reads and writes have much lower latency and higher throughput.

• When your application requires fully native file system behavior on Docker Desktop. For example, a database engine requires precise control over disk flushing to guarantee transaction durability. Volumes are stored in the Linux VM and can make these guarantees, whereas bind mounts are remoted to macOS or Windows, where the file systems behave slightly differently.

Good use cases for Bind Mounts

In general, you should use volumes where possible. Bind mounts are appropriate for the following types of use case:

• Sharing configuration files from the host machine to containers. This is how Docker provides DNS resolution to containers by default, by mounting /etc/resolv.conf from the host machine into each container.

• Sharing source code or build artifacts between a development environment on the Docker host and a container. For instance, you may mount a Maven target/ directory into a container, and each time you build the Maven project on the Docker host, the container gets access to the rebuilt artifacts.

  If you use Docker for development this way, your production Dockerfile would copy the production-ready artifacts directly into the image, rather than relying on a bind mount.

• When the file or directory structure of the Docker host is guaranteed to be consistent with the bind mounts the containers require.

Tips for using bind mounts or volumes

If you use either bind mounts or volumes, keep the following in mind:

• If you mount an empty volume into a directory in the container in which files or directories exist, these files or directories are propagated (copied) into the volume. Similarly, if you start a container and specify a volume which does not already exist, an empty volume is created for you. This is a good way to pre-populate data that another container needs.

• If you mount a bind mount or non-empty volume into a directory in the container in which some files or directories exist, these files or directories are obscured by the mount, just as if you saved files into /mnt on a Linux host and then mounted a USB drive into /mnt. The contents of /mnt would be obscured by the contents of the USB drive until the USB drive were unmounted. The obscured files are not removed or altered, but are not accessible while the bind mount or volume is mounted.