From ece96e7d6714837b34c097ac4350f67cc1d9a84a Mon Sep 17 00:00:00 2001
From: Chl <chl-dev@bugness.org>
Date: Tue, 8 Aug 2023 22:58:23 +0200
Subject: [PATCH] Adding SystemD unit: postgresqlontmpfs

---
 systemd-units/postgresqlontmpfs@.service | 73 ++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 systemd-units/postgresqlontmpfs@.service

diff --git a/systemd-units/postgresqlontmpfs@.service b/systemd-units/postgresqlontmpfs@.service
new file mode 100644
index 0000000..07a05ba
--- /dev/null
+++ b/systemd-units/postgresqlontmpfs@.service
@@ -0,0 +1,73 @@
+# 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"