Docker volume associated to postgres image empty and not persistent - docker

Question

I have a docker-compose file to build a web server with django and a postgres database. It basically looks like that :
version: '3'
services:
server:
build:
context: .
dockerfile: ./docker/server/Dockerfile
image: backend
volumes:
- ./api:/app
ports:
- 8000:8000
depends_on:
- postgres
- redis
environment:
- PYTHONUNBUFFERED=1
postgres:
image: kartoza/postgis:11.0-2.5
volumes:
- pg_data:/var/lib/postgresql/data:rw
environment:
POSTGRES_DB: "gis,backend"
POSTGRES_PORT: "5432"
POSTGRES_USER: "user"
POSTGRES_PASS: "pass"
POSTGRES_MULTIPLE_EXTENSIONS: "postgis,postgis_topology"
ports:
- 5432:5432
redis:
image: "redis:alpine"
volumes:
pg_data:
I'm using a volume to make my data persistent
I managed to run my containers and add data to the database. A volume has successfully been created : docker volume ls
DRIVER VOLUME NAME
local server_pg_data
But this volume is empty as the output of docker system df -v shows:
Local Volumes space usage:
VOLUME NAME LINKS SIZE
server_pg_data 1 0B
Also, if I want or need to build the containers once again using docker-compose down and docker-compose up, data has been purged from my database. Yet, I thought that volumes were used to make data persistent on disk…
I must be missing something in the way I'm using docker and volumes but I don't get what:
why does my volume appears empty while there is some data in my postgres container ?
why does my volume does not persist after doing docker-compose down ?
This thread (How to persist data in a dockerized postgres database using volumes) looked similar but the solution does not seem to apply.

Answer 1

The kartoza/postgis image isn't configured the same way as the standard postgres image. Its documentation notes (under "Cluster Initializations"):
By default, DATADIR will point to /var/lib/postgresql/{major-version}. You can instead mount the parent location like this: -v data-volume:/var/lib/postgresql
If you look at the Dockerfile in GitHub, you will also see that parent directory named as a VOLUME, which has some interesting semantics here.
With the setting you show, the actual data will be stored in /var/lib/postgresql/11.0; you're mounting the named volume on a different directory, /var/lib/postgresql/data, which is why it stays empty. Changing the volume mount to just /var/lib/postgresql should address this:
volumes:
- pg_data:/var/lib/postgresql:rw # not .../data