Where is a mysql volume db-data:/var/lib/mysql/data stored?

Question

I'm trying to get my head around docker and confused how the volumes work.

On the official documentation there's a bit of code similar to the docker-compose.yml file I'm working with.

version: "3.7"

services:
  wordpress:
    image: wordpress
    ports:
      - "8080:80"
    networks:
      - overlay
    deploy:
      mode: replicated
      replicas: 2
      endpoint_mode: vip

  mysql:
    image: mysql
    volumes:
       - db-data:/var/lib/mysql/data
    networks:
       - overlay
    deploy:
      mode: replicated
      replicas: 2
      endpoint_mode: dnsrr

volumes:
  db-data:

networks:
  overlay:

I'm confused as to what the db-data:/var/lib/mysql/data refers to and where docker is actually storing the database data, especially as /var/lib/mysql doesn't exist as a directory on the host.

Answer 1

So this line

volumes:
   - db-data:/var/lib/mysql/data

basically mounts the volume db-data from host into /var/lib/mysql/data in the container.

If db-data doesn't exist Docker will create one for you in Host.

You can read more here.