How can I run postgres with data dir on /tmp?

Question

I'm running automated integration tests using data stored in a postgres database. I need to start every test run with the same database contents. Instead of always clearing the database and importing a dump, I want to copy the database to /tmp and run it from there. If it works, it is easier and faster. This is what I did:

mkdir -p /tmp/postgres/data
chown -R postgres:postgres /tmp/postgres
rsync -vacHAX /var/lib/postgres/data/ /tmp/postgres/data/

and adapted the postgresql.service file to the new location (including systemctl daemon-reload).

But if I try to start postgres with this setup, it fails and systemctl status postgresql.service says

"/tmp/postgres/data" is missing or empty.

which is just not true.

I double-checked that the postgres user owns and has access to /tmp/pgsql/data/. Free space on /tmp is also sufficient (over 2 times the database size).

I've tried a different location and it worked. So my changes in the postgresql.service file are sufficient to use a non-default locattion. It only fails on /tmp.

What am I missing ?

Answer 1

systemd services can have a private /tmp directory for security reasons. From systemd documentation:

PrivateTmp=

Takes a boolean argument. If true, sets up a new file system namespace for the executed processes and mounts private /tmp/ and /var/tmp/ directories inside it that are not shared by processes outside of the namespace

When you say it's not true that /tmp/postgres/data is empty or non-existing, that's from the point of view of your shell, not the point of view of the postgres service if PrivateTmp=true.