Manage data in Docker

It is possible to store data within the writable layer of a container, but there are some downsides:

• The data won’t persist when that container is no longer running, and it can be difficult to get the data out of the container if another process needs it.
• A container’s writable layer is tightly coupled to the host machine where the container is running. You can’t easily move the data somewhere else.
• Writing into a container’s writable layer requires a storage driver to manage the filesystem. The storage driver provides a union filesystem, using the Linux kernel. This extra abstraction reduces performance as compared to using data volumes, which write directly to the host filesystem.

Docker offers three different ways to mount data into a container from the Docker host: volumes, bind mounts, or tmpfs volumes. When in doubt, volumes are almost always the right choice. Keep reading for more information about each mechanism for mounting data into containers.

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.

More details about mount types

• Volumes: Created and managed by Docker. You can create a volume explicitly using the docker volume create command, or Docker can create a volume during container or service creation.

  When you create a volume, it is stored within a directory on the Docker host. When you mount the volume into a container, this directory is what is mounted into the container. This is similar to the way that bind mounts work, except that volumes are managed by Docker and are isolated from the core functionality of the host machine.

  A given volume can be mounted into multiple containers simultaneously. When no running container is using a volume, the volume is still available to Docker and is not removed automatically. You can remove unused volumes using docker volume prune.

  When you mount a volume, it may be named or anonymous. Anonymous volumes are not given an explicit name when they are first mounted into a container, so Docker gives them a random name that is guaranteed to be unique within a given Docker host. Besides the name, named and anonymous volumes behave in the same ways.

  Volumes also support the use of volume drivers, which allow you to store your data on remote hosts or cloud providers, among other possibilities.

• Bind mounts: Available since the early days of Docker. Bind mounts have limited functionality compared to volumes. When you use a bind mount, a file or directory on the host machine is mounted into a container. The file or directory is referenced by its full path on the host machine. The file or directory does not need to exist on the Docker host already. It is created on demand if it does not yet exist. Bind mounts are very performant, but they rely on the host machine’s filesystem having a specific directory structure available. If you are developing new Docker applications, consider using named volumes instead. You can’t use Docker CLI commands to directly manage bind mounts.

  Warning: One side effect of using bind mounts, for better or for worse, is that you can change the host filesystem via processes running in a container, including creating, modifying, or deleting important system files or directories. This is a powerful ability which can have security implications, including impacting non-Docker processes on the host system.

• tmpfs mounts: A tmpfs mount is not persisted on disk, either on the Docker host or within a container. It can be used by a container during the lifetime of the container, to store non-persistent state or sensitive information. For instance, internally, swarm services use tmpfs mounts to mount secrets into a service’s containers.

Bind mounts and volumes can both mounted into containers using the -v or --volume flag, but the syntax for each is slightly different. For tmpfs mounts, you can use the --tmpfs flag. However, in Docker 17.06 and higher, we recommend using the --mount flag for both containers and services, for bind mounts, volumes, or tmpfs mounts, as the syntax is more clear.

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 be able 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>).

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.

Good use cases for tmpfs mounts

tmpfs mounts are best used for cases when you do not want the data to persist either on the host machine or within the container. This may be for security reasons or to protect the performance of the container when your application needs to write a large volume of non-persistent state data.

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 will be 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.

Next steps

• Learn more about volumes.
• Learn more about bind mounts.
• Learn more about tmpfs mounts.
• Learn more about storage drivers, which are not related to bind mounts or volumes, but allow you to store data in a container’s writable layer.