# This systemd unit puts the PostgreSQL storage on tmpfs and (tries to) sync
# it back at shutdown.

# Of course, in case of power failure or any problem preventing this
# unit to stop properly and sync back the data to disk : DATA LOSS.

# To install :
# - apt install rsync
# - copy this file to /etc/systemd/system/postgresqlontmpfs@.service
# - in the following lines, replace NN-xxxx by the version and cluster name targeted, for example: postgresqlontmpfs@15-main
# - systemctl enable postgresqlontmpfs@NN-xxxx
# - systemctl stop postgresql
# - systemctl start postgresqlontmpfs@NN-xxxxx
# - systemctl start postgresql

# This file is free software licensed under the WTFPL (v2)
#         DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#                    Version 2, December 2004
#
# Copyright (C) 2004 Sam Hocevar <sam@hocevar.net>
#
# Everyone is permitted to copy and distribute verbatim or modified
# copies of this license document, and changing it is allowed as long
# as the name is changed.
#
#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
#
#  0. You just DO WHAT THE FUCK YOU WANT TO.

[Unit]
Description=Put /var/lib/postgresql/%I on tmpfs before PostgreSQL starts
Before=postgresql@%i.service
ConditionPathIsDirectory=/var/lib/postgresql/%I
ConditionPathExists=/usr/bin/rsync


[Install]
WantedBy=multi-user.target


[Service]
Type=oneshot
RemainAfterExit=yes
Restart=no

# We double-check that PostgreSQL is not running before doing anything
# Per https://www.freedesktop.org/software/systemd/man/systemd.service.html
# when an ExecCondition= command exits with exit code 1 through 254
# (inclusive), the remaining commands are skipped and the unit is not marked as
# failed. However, if an ExecCondition= command exits with 255 or abnormally
# (e.g. timeout, killed by a signal, etc.), the unit will be considered failed
# (and remaining commands will be skipped).
ExecCondition=/bin/sh -c "systemctl --quiet is-active postgresql@%i && echo 'PostgreSQL still active' && exit 255 || exit 0"

# If the ON-DISK directory doesn't exist, simply rename /var/lib/postgresql/NN/main to
# /var/lib/postgresql/NN/main.ON-DISK and recreate /var/lib/postgresql/NN/main
ExecStart=/bin/sh -c "if [ ! -e /var/lib/postgresql/%I.ON-DISK ] ; then mv -v /var/lib/postgresql/%I /var/lib/postgresql/%I.ON-DISK && mkdir -v /var/lib/postgresql/%I && chown -v postgres:postgres /var/lib/postgresql/%I; fi"
# Mount the tmpfs
# (note: to forcefully limit memory consumption, we can add 'size=1024M' to the mount options)
ExecStart=/bin/mount -v -t tmpfs -o uid=postgres,gid=postgres,mode=0755 none /var/lib/postgresql/%I
# Copy the ON-DISK content to the tmpfs
ExecStart=/usr/bin/rsync -avH --delete /var/lib/postgresql/%I.ON-DISK/ /var/lib/postgresql/%I/

# At stop :
# - Little notice for the logs
ExecStop=/bin/sh -c "echo 'Syncing tmpfs:/var/lib/postgresql/%i to disk'"
# - fetch back the in-memory data to disk
ExecStop=/usr/bin/rsync -avH --delete /var/lib/postgresql/%I/ /var/lib/postgresql/%I.ON-DISK/
# - destroy the tmpfs
ExecStop=/bin/umount -v /var/lib/postgresql/%I
# - put back the on-disk storage in its original path
ExecStop=/bin/sh -c "rmdir -v /var/lib/postgresql/%I && mv -v /var/lib/postgresql/%I.ON-DISK /var/lib/postgresql/%I"