mirror of
https://github.com/systemd/systemd.git
synced 2024-12-28 11:21:59 +03:00
725ad3b062
It's useful to be able to combine a regular /usr/ file system with a tmpfs as root, for an OS that boots up in volatile mode on every single boot. Let's add explicit support for this via root=tmpfs. Note the relationship to the existing systemd.volatile= option: 1. The kernel command line "root=/dev/… systemd.volatile=yes" will mount the specified root fs, and then hide everything at the top by overmounting it with a tmpfs, except for the /usr subtree. 2. The kernel command line "root=tmpfs mount.usr=/dev/…" otoh will mount a toot fs at the top (just like the case above), but will then mount the top-level dir of the fs specified in mount.usr= directly below it. Or to say this differently: in the first case /usr/ from the physical storage fs is going to become /usr/ of the hierarchy ultimately booted, while in the second case / from the physical storage fs is going to become /usr of the hierarchy booted. Philosophically I figure systemd.volatile= is more an option for "one-off" boots, while root=tmpfs is something to have as default mode of operation for suitable images. This is currently hard to test reasonably, since Dracut refuses to accept root=tmpfs. This needs to be addressed separately though.
247 lines
12 KiB
XML
247 lines
12 KiB
XML
<?xml version="1.0"?>
|
|
<!--*-nxml-*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
|
|
<refentry id="systemd-fstab-generator">
|
|
|
|
<refentryinfo>
|
|
<title>systemd-fstab-generator</title>
|
|
<productname>systemd</productname>
|
|
</refentryinfo>
|
|
|
|
<refmeta>
|
|
<refentrytitle>systemd-fstab-generator</refentrytitle>
|
|
<manvolnum>8</manvolnum>
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
<refname>systemd-fstab-generator</refname>
|
|
<refpurpose>Unit generator for /etc/fstab</refpurpose>
|
|
</refnamediv>
|
|
|
|
<refsynopsisdiv>
|
|
<para><filename>/usr/lib/systemd/system-generators/systemd-fstab-generator</filename></para>
|
|
</refsynopsisdiv>
|
|
|
|
<refsect1>
|
|
<title>Description</title>
|
|
|
|
<para><filename>systemd-fstab-generator</filename> is a generator
|
|
that translates <filename>/etc/fstab</filename> (see
|
|
<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details) into native systemd units early at boot and when
|
|
configuration of the system manager is reloaded. This will
|
|
instantiate mount and swap units as necessary.</para>
|
|
|
|
<para>The <varname>passno</varname> field is treated like a simple
|
|
boolean, and the ordering information is discarded. However, if
|
|
the root file system is checked, it is checked before all the
|
|
other file systems.</para>
|
|
|
|
<para>See
|
|
<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
and
|
|
<citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for more information about special <filename>/etc/fstab</filename>
|
|
mount options this generator understands.</para>
|
|
|
|
<para>One special topic is handling of symbolic links. Historical init
|
|
implementations supported symlinks in <filename>/etc/fstab</filename>.
|
|
Because mount units will refuse mounts where the target is a symbolic link,
|
|
this generator will resolve any symlinks as far as possible when processing
|
|
<filename>/etc/fstab</filename> in order to enhance backwards compatibility.
|
|
If a symlink target does not exist at the time that this generator runs, it
|
|
is assumed that the symlink target is the final target of the mount.</para>
|
|
|
|
<para><filename>systemd-fstab-generator</filename> implements
|
|
<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Kernel Command Line</title>
|
|
|
|
<para><filename>systemd-fstab-generator</filename> understands the
|
|
following kernel command line parameters:</para>
|
|
|
|
<variablelist class='kernel-commandline-options'>
|
|
|
|
<varlistentry>
|
|
<term><varname>fstab=</varname></term>
|
|
<term><varname>rd.fstab=</varname></term>
|
|
|
|
<listitem><para>Takes a boolean argument. Defaults to
|
|
<literal>yes</literal>. If <literal>no</literal>, causes the
|
|
generator to ignore any mounts or swap devices configured in
|
|
<filename>/etc/fstab</filename>. <varname>rd.fstab=</varname>
|
|
is honored only by the initial RAM disk (initrd) while
|
|
<varname>fstab=</varname> is honored by both the main system
|
|
and the initrd.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>root=</varname></term>
|
|
|
|
<listitem><para>Configures the operating system's root filesystem to mount when running in the
|
|
initrd. This accepts a device node path (usually <filename>/dev/disk/by-uuid/…</filename> or
|
|
<filename>/dev/disk/by-label/…</filename> or similar), or the special values <literal>gpt-auto</literal>
|
|
and <literal>tmpfs</literal>.</para>
|
|
|
|
<para>Use <literal>gpt-auto</literal> to explicitly request automatic root file system discovery via
|
|
<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
|
|
|
|
<para>Use <literal>tmpfs</literal> in order to mount a <citerefentry
|
|
project='man-pages'><refentrytitle>tmpfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
|
|
system as root file system of the OS. This is useful in combination with
|
|
<varname>mount.usr=</varname> (see below) in order to combine a volatile root file system with a
|
|
separate, immutable <filename>/usr/</filename> file system. Also see
|
|
<varname>systemd.volatile=</varname> below.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>rootfstype=</varname></term>
|
|
|
|
<listitem><para>Takes the root filesystem type that will be
|
|
passed to the mount command. <varname>rootfstype=</varname> is
|
|
honored by the initrd.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>rootflags=</varname></term>
|
|
|
|
<listitem><para>Takes the root filesystem mount options to use. <varname>rootflags=</varname> is
|
|
honored by the initrd.</para>
|
|
|
|
<para>Note that unlike most kernel command line options this setting does not override settings made
|
|
in configuration files (specifically: the mount option string in
|
|
<filename>/etc/fstab</filename>). See
|
|
<citerefentry><refentrytitle>systemd-remount-fs.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>mount.usr=</varname></term>
|
|
|
|
<listitem><para>Takes the <filename>/usr/</filename> filesystem
|
|
to be mounted by the initrd. If
|
|
<varname>mount.usrfstype=</varname> or
|
|
<varname>mount.usrflags=</varname> is set, then
|
|
<varname>mount.usr=</varname> will default to the value set in
|
|
<varname>root=</varname>.</para>
|
|
|
|
<para>Otherwise, this parameter defaults to the
|
|
<filename>/usr/</filename> entry found in
|
|
<filename>/etc/fstab</filename> on the root filesystem.</para>
|
|
|
|
<para><varname>mount.usr=</varname> is honored by the initrd.
|
|
</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>mount.usrfstype=</varname></term>
|
|
|
|
<listitem><para>Takes the <filename>/usr/</filename> filesystem
|
|
type that will be passed to the mount command. If
|
|
<varname>mount.usr=</varname> or
|
|
<varname>mount.usrflags=</varname> is set, then
|
|
<varname>mount.usrfstype=</varname> will default to the value
|
|
set in <varname>rootfstype=</varname>.</para>
|
|
|
|
<para>Otherwise, this value will be read from the
|
|
<filename>/usr/</filename> entry in
|
|
<filename>/etc/fstab</filename> on the root filesystem.</para>
|
|
|
|
<para><varname>mount.usrfstype=</varname> is honored by the
|
|
initrd.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>mount.usrflags=</varname></term>
|
|
|
|
<listitem><para>Takes the <filename>/usr/</filename> filesystem
|
|
mount options to use. If <varname>mount.usr=</varname> or
|
|
<varname>mount.usrfstype=</varname> is set, then
|
|
<varname>mount.usrflags=</varname> will default to the value
|
|
set in <varname>rootflags=</varname>.</para>
|
|
|
|
<para>Otherwise, this value will be read from the
|
|
<filename>/usr/</filename> entry in
|
|
<filename>/etc/fstab</filename> on the root filesystem.</para>
|
|
|
|
<para><varname>mount.usrflags=</varname> is honored by the
|
|
initrd.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>systemd.volatile=</varname></term>
|
|
|
|
<listitem><para>Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the
|
|
special value <option>state</option>.</para>
|
|
|
|
<para>If false (the default), this generator makes no changes to the mount tree and the system is booted up in
|
|
normal mode.</para>
|
|
|
|
<para>If true the generator ensures
|
|
<citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
is run as part of the initial RAM disk ("initrd"). This service changes the mount table before transitioning to
|
|
the host system, so that a volatile memory file system (<literal>tmpfs</literal>) is used as root directory,
|
|
with only <filename>/usr/</filename> mounted into it from the configured root file system, in read-only
|
|
mode. This way the system operates in fully stateless mode, with all configuration and state reset at boot and
|
|
lost at shutdown, as <filename>/etc/</filename> and <filename>/var/</filename> will be served from the (initially
|
|
unpopulated) volatile memory file system.</para>
|
|
|
|
<para>If set to <option>state</option> the generator will leave the root directory mount point unaltered,
|
|
however will mount a <literal>tmpfs</literal> file system to <filename>/var/</filename>. In this mode the normal
|
|
system configuration (i.e. the contents of <literal>/etc/</literal>) is in effect (and may be modified during
|
|
system runtime), however the system state (i.e. the contents of <literal>/var/</literal>) is reset at boot and
|
|
lost at shutdown.</para>
|
|
|
|
<para>If this setting is set to <literal>overlay</literal> the root file system is set up as
|
|
<literal>overlayfs</literal> mount combining the read-only root directory with a writable
|
|
<literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
|
|
nonetheless with all changes being lost at reboot.</para>
|
|
|
|
<para>Note that in none of these modes the root directory, <filename>/etc/</filename>, <filename>/var/</filename>
|
|
or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
|
|
that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
|
|
|
|
<para>Note that with the exception of <literal>overlay</literal> mode, enabling this setting will
|
|
only work correctly on operating systems that can boot up with only <filename>/usr/</filename>
|
|
mounted, and are able to automatically populate <filename>/etc/</filename>, and also
|
|
<filename>/var/</filename> in case of <literal>systemd.volatile=yes</literal>.</para>
|
|
|
|
<para>Also see <varname>root=tmpfs</varname> above, for a method to combine a
|
|
<literal>tmpfs</literal> file system with a regular <filename>/usr/</filename> file system (as
|
|
configured via <varname>mount.usr=</varname>). The main distinction between
|
|
<varname>systemd.volatile=yes</varname>, and <varname>root=tmpfs</varname> in combination
|
|
<varname>mount.usr=</varname> is that the former operates on top of a regular root file system and
|
|
temporarily obstructs the files and directories above its <filename>/usr/</filename> subdirectory,
|
|
while the latter does not hide any files, but simply mounts a unpopulated tmpfs as root file system
|
|
and combines it with a user picked <filename>/usr/</filename> file system.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>systemd.swap</varname></term>
|
|
|
|
<listitem><para>Takes a boolean argument or enables the option if specified
|
|
without an argument. If disabled, causes the generator to ignore
|
|
any swap devices configured in <filename>/etc/fstab</filename>.
|
|
Defaults to enabled.</para></listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>See Also</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>
|
|
</para>
|
|
</refsect1>
|
|
|
|
</refentry>
|