Unable to successfully change Postgresql data_directory and restart Postgres

Question

My goal is to store my postgresql database data on an external ssd hard drive that I have plugged in to a machine running Linux Ubuntu 20.04 LTS ARM x64. I am running into problems when changing the data directory in the postgresql.conf file and then restarting postgres.

psql: could not connect to server: No such file or directory

Is the server running locally and accepting connections on 
Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

I am aware this error occurs when there is a problem with postgres configuration. I have tried setting permissions and ownership for user postgres on the /mnt/data/postgresql/12/main directory. No luck.

Below are the steps I took:

Create mount directory

sudo mkdir /mnt/data

Mount drive to directory

sudo mount -o defaults /dev/sda1 /mnt/data

Add directory to mounted drive where I want to store postgres data

sudo mkdir -p /mnt/data/postgresql/12/main

Change the data directory to /mnt/data/postgresql/12/main

sudo nano /etc/postgresql/12/main/postgresql.conf

Restart postgresql service

sudo systemctl restart postgresql.service

Answer 1

You should use initdb to create a new data directory as user postgres, e.g.

sudo mkdir -p /mnt/data/postgresql/12/main
sudo chown -R postgres:postgres /mnt/data/postgresql
sudo -u postgres /usr/lib/postgresql/12/bin/initdb -D /mnt/data/postgresql/12/main
sudo service postgresql stop

This also creates (unused) config files postgresql.conf, pg_hba.conf and pg_ident.conf in /mnt/data/postgresql/12/main.

Now change the path to data_directory in /etc/postgresql/12/main/postgresql.conf, start the service and test if the data_directory is set as expected:

sudo service postgresql start
sudo -u postgres psql -c 'show data_directory;'

This should output:

        data_directory        
------------------------------
 /mnt/data/postgresql/12/main
(1 row)