man/systemd-fstab-generator.xml

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   6 <refentry id="systemd-fstab-generator">
   7
   8   <refentryinfo>
   9     <title>systemd-fstab-generator</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-fstab-generator</refentrytitle>
  15     <manvolnum>8</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd-fstab-generator</refname>
  20     <refpurpose>Unit generator for /etc/fstab</refpurpose>
  21   </refnamediv>
  22
  23   <refsynopsisdiv>
  24     <para><filename>/usr/lib/systemd/system-generators/systemd-fstab-generator</filename></para>
  25   </refsynopsisdiv>
  26
  27   <refsect1>
  28     <title>Description</title>
  29
  30     <para><filename>systemd-fstab-generator</filename> is a generator
  31     that translates <filename>/etc/fstab</filename> (see
  32     <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  33     for details) into native systemd units early at boot and when
  34     configuration of the system manager is reloaded. This will
  35     instantiate mount and swap units as necessary.</para>
  36
  37     <para>The <varname>passno</varname> field is treated like a simple
  38     boolean, and the ordering information is discarded. However, if
  39     the root file system is checked, it is checked before all the
  40     other file systems.</para>
  41
  42     <para>See
  43     <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  44     and
  45     <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  46     for more information about special <filename>/etc/fstab</filename>
  47     mount options this generator understands.</para>
  48
  49     <para>One special topic is handling of symbolic links.  Historical init
  50     implementations supported symlinks in <filename>/etc/fstab</filename>.
  51     Because mount units will refuse mounts where the target is a symbolic link,
  52     this generator will resolve any symlinks as far as possible when processing
  53     <filename>/etc/fstab</filename> in order to enhance backwards compatibility.
  54     If a symlink target does not exist at the time that this generator runs, it
  55     is assumed that the symlink target is the final target of the mount.</para>
  56
  57     <para><filename>systemd-fstab-generator</filename> implements
  58     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  59   </refsect1>
  60
  61   <refsect1>
  62     <title>Kernel Command Line</title>
  63
  64     <para><filename>systemd-fstab-generator</filename> understands the
  65     following kernel command line parameters:</para>
  66
  67     <variablelist class='kernel-commandline-options'>
  68
  69       <varlistentry>
  70         <term><varname>fstab=</varname></term>
  71         <term><varname>rd.fstab=</varname></term>
  72
  73         <listitem><para>Takes a boolean argument. Defaults to
  74         <literal>yes</literal>. If <literal>no</literal>, causes the
  75         generator to ignore any mounts or swap devices configured in
  76         <filename>/etc/fstab</filename>. <varname>rd.fstab=</varname>
  77         is honored only by the initial RAM disk (initrd) while
  78         <varname>fstab=</varname> is honored by both the main system
  79         and the initrd.</para></listitem>
  80       </varlistentry>
  81
  82       <varlistentry>
  83         <term><varname>root=</varname></term>
  84
  85         <listitem><para>Takes the root filesystem to mount in the
  86         initrd. <varname>root=</varname> is honored by the
  87         initrd.</para></listitem>
  88       </varlistentry>
  89
  90       <varlistentry>
  91         <term><varname>rootfstype=</varname></term>
  92
  93         <listitem><para>Takes the root filesystem type that will be
  94         passed to the mount command. <varname>rootfstype=</varname> is
  95         honored by the initrd.</para></listitem>
  96       </varlistentry>
  97
  98       <varlistentry>
  99         <term><varname>rootflags=</varname></term>
 100
 101         <listitem><para>Takes the root filesystem mount options to
 102         use. <varname>rootflags=</varname> is honored by the
 103         initrd.</para></listitem>
 104       </varlistentry>
 105
 106       <varlistentry>
 107         <term><varname>mount.usr=</varname></term>
 108
 109         <listitem><para>Takes the <filename>/usr</filename> filesystem
 110         to be mounted by the initrd. If
 111         <varname>mount.usrfstype=</varname> or
 112         <varname>mount.usrflags=</varname> is set, then
 113         <varname>mount.usr=</varname> will default to the value set in
 114         <varname>root=</varname>.</para>
 115
 116         <para>Otherwise, this parameter defaults to the
 117         <filename>/usr</filename> entry found in
 118         <filename>/etc/fstab</filename> on the root filesystem.</para>
 119
 120         <para><varname>mount.usr=</varname> is honored by the initrd.
 121         </para></listitem>
 122       </varlistentry>
 123
 124       <varlistentry>
 125         <term><varname>mount.usrfstype=</varname></term>
 126
 127         <listitem><para>Takes the <filename>/usr</filename> filesystem
 128         type that will be passed to the mount command. If
 129         <varname>mount.usr=</varname> or
 130         <varname>mount.usrflags=</varname> is set, then
 131         <varname>mount.usrfstype=</varname> will default to the value
 132         set in <varname>rootfstype=</varname>.</para>
 133
 134         <para>Otherwise, this value will be read from the
 135         <filename>/usr</filename> entry in
 136         <filename>/etc/fstab</filename> on the root filesystem.</para>
 137
 138         <para><varname>mount.usrfstype=</varname> is honored by the
 139         initrd.</para></listitem>
 140       </varlistentry>
 141
 142       <varlistentry>
 143         <term><varname>mount.usrflags=</varname></term>
 144
 145         <listitem><para>Takes the <filename>/usr</filename> filesystem
 146         mount options to use. If <varname>mount.usr=</varname> or
 147         <varname>mount.usrfstype=</varname> is set, then
 148         <varname>mount.usrflags=</varname> will default to the value
 149         set in <varname>rootflags=</varname>.</para>
 150
 151         <para>Otherwise, this value will be read from the
 152         <filename>/usr</filename> entry in
 153         <filename>/etc/fstab</filename> on the root filesystem.</para>
 154
 155         <para><varname>mount.usrflags=</varname> is honored by the
 156         initrd.</para></listitem>
 157       </varlistentry>
 158
 159       <varlistentry>
 160         <term><varname>systemd.volatile=</varname></term>
 161
 162         <listitem><para>Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the
 163         special value <option>state</option>.</para>
 164
 165         <para>If false (the default), this generator makes no changes to the mount tree and the system is booted up in
 166         normal mode.</para>
 167
 168         <para>If true the generator ensures
 169         <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 170         is run as part of the initial RAM disk ("initrd"). This service changes the mount table before transitioning to
 171         the host system, so that a volatile memory file system (<literal>tmpfs</literal>) is used as root directory,
 172         with only <filename>/usr</filename> mounted into it from the configured root file system, in read-only
 173         mode. This way the system operates in fully stateless mode, with all configuration and state reset at boot and
 174         lost at shutdown, as <filename>/etc</filename> and <filename>/var</filename> will be served from the (initially
 175         unpopulated) volatile memory file system.</para>
 176
 177         <para>If set to <option>state</option> the generator will leave the root directory mount point unaltered,
 178         however will mount a <literal>tmpfs</literal> file system to <filename>/var</filename>. In this mode the normal
 179         system configuration (i.e. the contents of <literal>/etc</literal>) is in effect (and may be modified during
 180         system runtime), however the system state (i.e. the contents of <literal>/var</literal>) is reset at boot and
 181         lost at shutdown.</para>
 182
 183         <para>If this setting is set to <literal>overlay</literal> the root file system is set up as
 184         <literal>overlayfs</literal> mount combining the read-only root directory with a writable
 185         <literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
 186         nonetheless with all changes being lost at reboot.</para>
 187
 188         <para>Note that in none of these modes the root directory, <filename>/etc</filename>, <filename>/var</filename>
 189         or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
 190         that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
 191
 192         <para>Note that with the exception of <literal>overlay</literal> mode, enabling this setting will only work
 193         correctly on operating systems that can boot up with only <filename>/usr</filename> mounted, and are able to
 194         automatically populate <filename>/etc</filename>, and also <filename>/var</filename> in case of
 195         <literal>systemd.volatile=yes</literal>.</para></listitem>
 196       </varlistentry>
 197     </variablelist>
 198   </refsect1>
 199
 200   <refsect1>
 201     <title>See Also</title>
 202     <para>
 203       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 204       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 205       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 206       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 207       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 208       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 209     </para>
 210   </refsect1>
 211
 212 </refentry>