Understanding Docker Volumes and Host‑Container Data Sharing

Docker stores its volumes under /var/lib/docker . When you need to share a host directory with a container—such as mapping MySQL's /var/lib/mysql to the host—you use Docker's volume mechanism.

To share data between the host and a container, simply add the -v flag when starting the container, using the format -v HOST_PATH:CONTAINER_PATH .

For example, to mount the host directory /data to /usr/local/data inside a MySQL container, run:

docker run -v /data:/usr/local/data -it mysql

After the container starts, the files in /usr/local/data inside the container correspond to /data on the host, and the data persists across container restarts.

Docker volumes are implemented by creating a directory for each volume under /var/lib/docker/volumes . Each volume directory contains an _data subdirectory that holds the actual files. The volume bypasses Docker's union file system and directly mounts the host directory into the container.

Example workflow:

1. Create a volume named volume-data :

docker volume create volume-data

2. List the volumes directory to see the new volume:

ls -l /var/lib/docker/volumes

3. Inspect the volume's content:

ls -l /var/lib/docker/volumes/volume-data

You will see an _data folder inside the volume directory.

4. Run a container and mount the volume to /data :

docker run -it --mount source=volume-data,target=/data busybox

5. Inside the container, create a file:

cd /data && touch data.log

6. On the host, verify the file appears in the volume's _data directory:

ls -l /var/lib/docker/volumes/volume-data/_data

The presence of data.log confirms that operations inside the container directly affect the host's storage, providing persistent data.

In summary, Docker volumes are stored under /var/lib/docker/volumes , each with an _data subdirectory. When a container is started with --mount (or -v ), Docker maps the host directory to the container, enabling data persistence across container lifecycles.