fstab-generator: add support for volatile boots

This adds support for a new kernel command line option "systemd.volatile=" that
provides the same functionality that systemd-nspawn's --volatile= switch
provides, but for host systems (i.e. systems booting with a kernel).

It takes the same parameter and has the same effect.

In order to implement systemd.volatile=yes a new service
systemd-volatile-root.service is introduced that only runs in the initrd and
rearranges the root directory as needed to become a tmpfs instance. Note that
systemd.volatile=state is implemented different: it simply generates a
var.mount unit file that is part of the normal boot and has no effect on the
initrd execution.

The way this is implemented ensures that other explicit configuration for /var
can always override the effect of these options.  Specifically, the var.mount
unit is generated in the "late" generator directory, so that it only is in
effect if nothing else overrides it.

1 files changed, 79 insertions, 0 deletions

diff --git a/man/systemd-volatile-root.service.xml b/man/systemd-volatile-root.service.xml
new file mode 100644
index 0000000000..b90a3261fa
--- /dev/null
+++ b/man/systemd-volatile-root.service.xml
@@ -0,0 +1,79 @@
+<?xml version="1.0"?>
+<!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!--
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+<refentry id="systemd-volatile-root.service">
+
+  <refentryinfo>
+    <title>systemd-volatile-root.service</title>
+    <productname>systemd</productname>
+
+    <authorgroup>
+      <author>
+        <contrib>Developer</contrib>
+        <firstname>Lennart</firstname>
+        <surname>Poettering</surname>
+        <email>lennart@poettering.net</email>
+      </author>
+    </authorgroup>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd-volatile-root.service</refentrytitle>
+    <manvolnum>8</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd-volatile-root.service</refname>
+    <refname>systemd-volatile-root</refname>
+    <refpurpose>Make the root file system volatile</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <para><filename>systemd-volatile-root.service</filename></para>
+    <para><filename>/usr/lib/systemd/systemd-volatile-root</filename></para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><filename>systemd-volatile-root.service</filename> is a service that replaces the root directory with a
+    volatile memory file system (<literal>tmpfs</literal>), mounting the original (non-volatile)
+    <filename>/usr</filename> inside it read-only. This way, vendor data from <filename>/usr</filename> is available as
+    usual, but all configuration data in <filename>/etc</filename>, all state data in <filename>/var</filename> and all
+    other resources stored directly under the root directory are reset on boot and lost at shutdown, enabling fully
+    stateless systems.</para>
+
+    <para>This service is only enabled if full volatile mode is selected, for example by specifying
+    <literal>systemd.volatile=yes</literal> on the kernel command line. This service runs only in the initial RAM disk
+    ("initrd"), before the system transitions to the host's root directory. Note that this service is not used if
+    <literal>systemd.volatile=state</literal> is used, as in that mode the root directory is non-volatile.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>