Volumes

Why We Need Volumes

• By default, all the changes inside the container are lost when the container stops. If we want to keep data between runs, Docker volumes and bind mounts can help.
• Usually used with database systems such as MySQL, MongoDB…

What Are Volumes

• The purpose of using Docker volumes is to persist data outside the container so it can be backed up or shared.
• Multiple containers can mount the same volume.
• There are main 2 ways of doing this
  • Bind mount
    • Bind to local folder Managed by user
  • Named volumes
    • Bind to volume managed by docker

Bind Mounts

• Connection from the container to a directory on the host machine. 
• It allows the host to share its own file system with the container, which can be made read-only or read-write.
• Absolute path is required when using bind volumes with docker run. aka ==pwd== is required
  • However, We can use relative path when using with docker-compose

docker run -it --name nginx -p 8080:80 -v $(pwd)/target:/usr/share/nginx/html nginx

#docker-compose.yml
version: "3.2"
services:
  web:
    image: nginx:latest
    ports:
      - 8080:80
    volumes:
      - ./target:/usr/share/nginx/html  #relative path being used

Named Volumes

• Unlike bind mount, where you can mount any directory from your host, volumes are stored in a single location
  • (most likely /var/lib/docker/volumes/ on unix systems)
• Greatly facilitates managing data (backup, restore, and migration). Docker volumes can safely be shared between several running containers.

docker run -it --name nginx -p 8080:80 -v demo:/usr/share/nginx/html nginx

#if the path{demo} is a name, Docker recognizes it as a named volume and tries to create it if it doesn't exist

• With docker-compose, volumes must be declared at the same level as services. Then you can refer to them by their name.

#docker-compose.yml
version: "3.2"
services:
  web:
    image: nginx:latest
    ports:
      - 8080:80
    volumes:
      - html_files:/usr/share/nginx/html
volumes:
  html_files:

List Volumes

docker volume ls

Create Volume

Volumes are created automatically if they don't exist but can also be explicitly created.

docker volume create --name jupiter-riding

• How to use the above volume

docker run -it --name nginx -p 8080:80 -v jupiter-riding:/usr/share/nginx/html nginx

#need to use external: true when using explicit volumes with docker-compose
version: "3.2"
services:
  web:
    image: nginx:latest
    ports:
      - 8080:80
    volumes:
      - jupiter-riding:/usr/share/nginx/html
 
volumes:
  jupiter-riding:
    external: true  #if the volume doesn't exist docker will throw an error

Remove a Volume

docker volume rm jupiter-riding

Remove Unused Volume

docker volume prune

Volume Detail

Use the docker volume inspect command to view the data volume details.

docker volume inspect [volume_name]