doc: fix for 'could not open shared memory segment "/PostgreSQL.xxxxxxxxxx": No such file or directory'

[coolaj86]

Re: https://github.com/bnnanet/To-Dos/issues/43, bnnanet/serviceman#5

This happens when postgres is run

• with systemd
• as a system service
• as a user that can login

could not open shared memory segment "/PostgreSQL.2818577498": No such file or directory

For example, when adding postgres with serviceman, running as the app user.

Solutions vs Workarounds

• use openrc (Alpine, Devuan) rather than systemd (Debian, CentOS)
• create a non-login postgres user (TODO: document this)

Option 1: systemd fix

IMPORTANT: This does not work in practice, in my experience. systemd still wipes out the shared memory file periodically.

Disable systemd from clearing a user's /dev/shm during logind lifecycle events:

/etc/systemd/logind.conf:

RemoveIPC=no

sudo systemctl restart systemd-logind.service

Option 1: use a non login user

Create a user that does not have login access to run the postgres daemon.

Option 2: set postgres ipc to sysv or mmap

Switch postgres to use mmap (less efficient)

~/.local/share/postgres/var/postgresql.conf:

dynamic_shared_memory_type = mmap

sudo killall postgres

Triggers

Login via ssh does trigger loginds lifecycle management.

sudo loginctl list-sessions