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, 23 insertions, 0 deletions

diff --git a/man/kernel-command-line.xml b/man/kernel-command-line.xml
index 78e45e66a..7e1d408de 100644
--- a/man/kernel-command-line.xml
+++ b/man/kernel-command-line.xml
@@ -125,6 +125,28 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>systemd.volatile=</varname></term>
+        <listitem>
+          <para>This parameter controls whether the system shall boot up in volatile mode. Takes a boolean argument, or
+          the special value <literal>state</literal>. If false (the default), normal boot mode is selected, the root
+          directory and <filename>/var</filename> are mounted as specified on the kernel command line or
+          <filename>/etc/fstab</filename>, or otherwise configured. If true, full state-less boot mode is selected. In
+          this case the root directory is mounted as volatile memory file system (<literal>tmpfs</literal>), and only
+          <filename>/usr</filename> is mounted from the file system configured as root device, in read-only mode. This
+          enables fully state-less boots were the vendor-supplied OS is used as shipped, with only default
+          configuration and no stored state in effect, as <filename>/etc</filename> and <filename>/var</filename> (as
+          well as all other resources shipped in the root file system) are reset at boot and lost on shutdown. If this
+          setting is set to <literal>state</literal> the root file system is mounted as usual, however
+          <filename>/var</filename> is mounted as a volatile memory file system (<literal>tmpfs</literal>), so that the
+          system boots up with the normal configuration applied, but all state reset at boot and lost at shutdown. For details,
+          see
+          <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+          and
+          <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>quiet</varname></term>
         <listitem>
           <para>Parameter understood by both the kernel and the system
@@ -382,6 +404,7 @@
         <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-modules-load.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-backlight@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-rfkill.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,