Mount Postgres data to Windows host directory - postgresql

Question

I want to ensure my Postgres data (using Linux based image) persist, even after my Windows host machine restart.
I tried to follow the steps How to persist data in a dockerized postgres database using volumes
docker-compose.yml
volumes:
- ./postgres_data:/var/lib/postgresql/data
However, I'm getting error
waiting for server to start....FATAL: data directory "/var/lib/postgresql/data/pgdata" has wrong ownership
HINT: The server must be started by the user that owns the data directory.
stopped waiting
pg_ctl: could not start server
Then, I tried to follow step in https://forums.docker.com/t/trying-to-get-postgres-to-work-on-persistent-windows-mount-two-issues/12456/5
The suggested method are
docker volume create --name postgres_data --driver local
docker-compose.yml
services:
postgres:
volumes:
- postgres_data:/var/lib/postgresql/data
volumes:
postgres_data:
external: true
However, I'm confused on the command docker volume create --name postgres_data --driver local. As, it doesn't mention the exact path of Windows host machine.
I tried
C:\celery-hello-world>docker volume create postgres_data
postgres_data
C:\celery-hello-world>docker volume inspect postgres_data
[
{
"CreatedAt": "2018-02-06T14:54:48Z",
"Driver": "local",
"Labels": {},
"Mountpoint": "/var/lib/docker/volumes/postgres_data/_data",
"Name": "postgres_data",
"Options": {},
"Scope": "local"
}
]
May I know where is the Windows directory location, which VOLUME postgres_data mount to?

Answer 1

Today i was asking myself the same question but i have figured it out.
First to note is that inside postgres container the path is:
/var/lib/postgresql/data
Now the path you tried to track down is another path:
C:\celery-hello-world>docker volume inspect postgres_data [
{
"CreatedAt": "2018-02-06T14:54:48Z",
"Driver": "local",
"Labels": {},
"Mountpoint": "/var/lib/docker/volumes/postgres_data/_data",
"Name": "postgres_data",
"Options": {},
"Scope": "local"
} ]
/var/lib/docker/volumes/postgres_data/_data
I am also using Docker for windows and Unix containers.
As you can check normally you will not see this docker machine:
docker-machine ls
NAME ACTIVE DRIVER STATE URL SWARM DOCKER ERRORS
But there's a trick to access it regarding Unix containers:
docker run -it --rm --privileged --pid=host justincormack/nsenter1
Just run this from your CLI and it'll drop you in a container with
full permissions on the Moby VM. Only works for Moby Linux VM (doesn't
work for Windows Containers). Note this also works on Docker for Mac.
Reference: https://www.bretfisher.com/getting-a-shell-in-the-docker-for-windows-vm/
From there you can navigate to this folder (note that i named my volume postgresql-volume):
/var/lib/docker/volumes/postgresql-volume/_data ls
PG_VERSION pg_dynshmem pg_notify pg_stat_tmp postgresql.auto.conf
base pg_hba.conf pg_replslot pg_subtrans postgresql.conf
global pg_ident.conf pg_serial pg_tblspc postmaster.opts
pg_clog pg_logical pg_snapshots pg_twophase postmaster.pid
pg_commit_ts pg_multixact pg_stat pg_xlog
So to be clear it's inside of the VM provided by Docker for Windows in the directory above.
Docker for windows needs Hyper-V enabled for virtualization.
You can also find image location in Docker for windows go to Settings->Advanced->Disk image location in my case it's:
"C:\ProgramData\DockerDesktop\vm-data\DockerDesktop.vhdx"
Also note that the same could be found in Hyper-V Manager.