Docker Volumes - Persistence

Container uses and generates data folders that are available for the container but not to the host machine. When the container gets restarted or crashed the data generated inside is deleted permanently. Docker volumes is a process of saving the data to the host machine from the container.

Docker Volume COPY | Docker data one time copy

Get the folder copied from container to host machine, this option useful for one-time copy, but not recommended for the databases which need the stateful behavior. Using docker cp we can copy the docker container volume to the host machine.

docker cp <CONTAINER_ID>:<PATH_OF_CONTAINER_DIR_TO_COPY> <DESTINATION_PATH_HOST_MACHINE>

example:

docker cp /etc/nginx/nginx.conf $PWD/`

NOTE: The issue with the above approach is, it will not persists the volume or file or dir, as you remove the container it will be lost. This is not recommended for volume persistency dynamically.

Volume Mounting | Docker Volume

Docker volume is a process of mounting the container volume to the host directory. This can be done in three ways.

Anonymous Volumes
Named Volumes
Named Bind Volumes Or Host Volumes

1. Anonymous Volumes

The location of the data mounted is managed by docker. It is very difficult to refer to the same volume. We can identify the volumes used by containers by using the container inspection. To create an anonymous docker volume run

docker run -v /app nginx:1.11.8-alpine

2. Named Volumes

Named volumes are defined by the user but the location of the data is managed by docker. Referring to named volumes is very easy compared to anonymous volumes.

We can do this in two ways creating a docker volume and referring to it while running the container with the docker run command.

docker volume create jinnabalu_data
docker run -v jinnabalu_data:/usr/share/app nginx:1.11.8-alpine

We can achieve both above commands within one docker-compose.yml

version: '3'
services:
  nginx:
    image: nginx:1.11.8-alpine
    ports:
      - "8081:80"
    volumes:
      - jinnabalu_data:/usr/share/app
volumes:
  jinnabalu_data:

Note : Deleting the named volumes will delete the data as the data location is managed by docker.

3. Named Bind Volumes Or Host Volumes

Named volumes binding or mounting to the custom directory path of the host machine. Docker volume location is defined by the user.

version: '3'
services:
  nginx:
    image: nginx:1.11.8-alpine
    ports:
      - "8081:80"
    volumes:
      - jinnabalu_data:/usr/share/app
volumes:
  jinnabalu_data:
    driver: local
    driver_opts:
       o: bind
       type: none
       device: /home/jinnabalu/data/app

Volumes help save data across restarts of your docker containers. If you need to use a volume, consider the difference between the various kinds before starting development.

26 Dec 2020