Docker volumes or bind mounts: where your data lives and how to back it up

What a container writes inside itself disappears when it is deleted. For data to survive, you connect it to somewhere outside the container, in two ways: a volume (managed by Docker) or a bind mount (a server folder you choose). The short answer: for a database, use a volume; for code or configuration files you want to edit by hand on the server, a bind mount.

Which to choose

Volume Bind mount
What it is An area Docker manages (under /var/lib/docker). A folder on the server, at the path you choose.
Written in Compose as db_data:/var/lib/postgresql/data ./config:/app/config
Good for Databases and application data. Configuration and files you edit on the server.
The catch You do not see it in a folder by hand; down -v deletes it. File owners and permissions have to suit the container’s user.

The full set-up of an application with its database is in Docker Compose: an application with its database.

Seeing where the data is, and making a copy

1 List the volumes and see where one is: docker volume ls
docker volume inspect volume-nameCompose prefixes the project name to the volume name (project_db_data). That is the full name.
2 For a database, make a logical copy rather than copying the files while it runs: pg_dump or mysqldump inside the container. The files of a database that is writing can end up inconsistent. See backing up with mysqldump and pg_dump.
3 For other data, copy the volume to a file with a temporary container:docker run --rm -v project_data:/data -v "$PWD":/backup alpine tar czf /backup/data.tar.gz -C /data .This creates data.tar.gz in the folder you are in. To restore, the same with tar xzf onto an empty volume.
4 Keep the copy off the server. A copy on the same disk does not protect you from the disk failing: backing up a VPS.
Watch the owners on a bind mount. If the container runs as a user that does not own the server folder, writing fails with “permission denied” even though everything is “mounted”. Set the folder’s owner to the container’s user, instead of putting everything at 777. And docker compose down -v deletes the volumes, meaning the data: without -v it only removes the containers.
Give volumes clear names in Compose and write down what each holds. Six months from now nobody remembers which volume is the database, and deleting the wrong one is the expensive mistake: clearing the disk without losing data.

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SEE ALSO

Docker Compose: an application with its database

Updating a Docker application without losing data

Docker filling the disk: images, volumes and logs

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