man/systemd-fstab-generator.xml

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   6 <refentry id="systemd-fstab-generator" xmlns:xi="http://www.w3.org/2001/XInclude">
   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 <literal>yes</literal>. If
  74         <literal>no</literal>, causes the generator to ignore any mounts or swap devices configured in
  75         <filename>/etc/fstab</filename>. <varname>rd.fstab=</varname> is honored only in the initrd, while
  76         <varname>fstab=</varname> is honored by both the main system and the initrd.</para>
  77
  78         <xi:include href="version-info.xml" xpointer="v186"/></listitem>
  79       </varlistentry>
  80
  81       <varlistentry>
  82         <term><varname>root=</varname></term>
  83
  84         <listitem><para>Configures the operating system's root filesystem to mount when running in the
  85         initrd. This accepts a device node path (usually <filename>/dev/disk/by-uuid/…</filename> or
  86         <filename>/dev/disk/by-label/…</filename> or similar), or the special values <literal>gpt-auto</literal>,
  87         <literal>fstab</literal>, and <literal>tmpfs</literal>.</para>
  88
  89         <para>Use <literal>gpt-auto</literal> to explicitly request automatic root file system discovery via
  90         <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
  91
  92         <para>Use <literal>fstab</literal> to explicitly request automatic root file system discovery via
  93         the initrd <filename>/etc/fstab</filename> rather than via kernel command line.</para>
  94
  95         <para>Use <literal>tmpfs</literal> in order to mount a <citerefentry
  96         project='man-pages'><refentrytitle>tmpfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
  97         system as root file system of the OS. This is useful in combination with
  98         <varname>mount.usr=</varname> (see below) in order to combine a volatile root file system with a
  99         separate, immutable <filename>/usr/</filename> file system. Also see
 100         <varname>systemd.volatile=</varname> below.</para>
 101
 102         <xi:include href="version-info.xml" xpointer="v217"/></listitem>
 103       </varlistentry>
 104
 105       <varlistentry>
 106         <term><varname>rootfstype=</varname></term>
 107
 108         <listitem><para>Takes the root filesystem type that will be
 109         passed to the mount command. <varname>rootfstype=</varname> is
 110         honored by the initrd.</para>
 111
 112         <xi:include href="version-info.xml" xpointer="v217"/></listitem>
 113       </varlistentry>
 114
 115       <varlistentry>
 116         <term><varname>rootflags=</varname></term>
 117
 118         <listitem><para>Takes the root filesystem mount options to use. <varname>rootflags=</varname> is
 119         honored by the initrd.</para>
 120
 121         <para>Note that unlike most kernel command line options this setting does not override settings made
 122         in configuration files (specifically: the mount option string in
 123         <filename>/etc/fstab</filename>). See
 124         <citerefentry><refentrytitle>systemd-remount-fs.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
 125
 126         <xi:include href="version-info.xml" xpointer="v217"/></listitem>
 127       </varlistentry>
 128
 129       <varlistentry>
 130         <term><varname>mount.usr=</varname></term>
 131
 132         <listitem><para>Takes the <filename>/usr/</filename> filesystem
 133         to be mounted by the initrd. If
 134         <varname>mount.usrfstype=</varname> or
 135         <varname>mount.usrflags=</varname> is set, then
 136         <varname>mount.usr=</varname> will default to the value set in
 137         <varname>root=</varname>.</para>
 138
 139         <para>Otherwise, this parameter defaults to the
 140         <filename>/usr/</filename> entry found in
 141         <filename>/etc/fstab</filename> on the root filesystem.</para>
 142
 143         <para><varname>mount.usr=</varname> is honored by the initrd.
 144         </para>
 145
 146         <xi:include href="version-info.xml" xpointer="v217"/></listitem>
 147       </varlistentry>
 148
 149       <varlistentry>
 150         <term><varname>mount.usrfstype=</varname></term>
 151
 152         <listitem><para>Takes the <filename>/usr/</filename> filesystem
 153         type that will be passed to the mount command. If
 154         <varname>mount.usr=</varname> or
 155         <varname>mount.usrflags=</varname> is set, then
 156         <varname>mount.usrfstype=</varname> will default to the value
 157         set in <varname>rootfstype=</varname>.</para>
 158
 159         <para>Otherwise, this value will be read from the
 160         <filename>/usr/</filename> entry in
 161         <filename>/etc/fstab</filename> on the root filesystem.</para>
 162
 163         <para><varname>mount.usrfstype=</varname> is honored by the
 164         initrd.</para>
 165
 166         <xi:include href="version-info.xml" xpointer="v217"/></listitem>
 167       </varlistentry>
 168
 169       <varlistentry>
 170         <term><varname>mount.usrflags=</varname></term>
 171
 172         <listitem><para>Takes the <filename>/usr/</filename> filesystem
 173         mount options to use. If <varname>mount.usr=</varname> or
 174         <varname>mount.usrfstype=</varname> is set, then
 175         <varname>mount.usrflags=</varname> will default to the value
 176         set in <varname>rootflags=</varname>.</para>
 177
 178         <para>Otherwise, this value will be read from the
 179         <filename>/usr/</filename> entry in
 180         <filename>/etc/fstab</filename> on the root filesystem.</para>
 181
 182         <para><varname>mount.usrflags=</varname> is honored by the
 183         initrd.</para>
 184
 185         <xi:include href="version-info.xml" xpointer="v217"/></listitem>
 186       </varlistentry>
 187
 188       <varlistentry>
 189         <term><varname>roothash=</varname></term>
 190         <term><varname>usrhash=</varname></term>
 191
 192         <listitem><para>These options are primarily read by
 193         <citerefentry><refentrytitle>systemd-veritysetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>. When
 194         set this indicates that the root file system (or <filename>/usr/</filename>) shall be mounted from
 195         Verity volumes with the specified hashes. If these kernel command line options are set the root (or
 196         <filename>/usr/</filename>) file system is thus mounted from a device mapper volume
 197         <filename>/dev/mapper/root</filename> (or <filename>/dev/mapper/usr</filename>).</para>
 198
 199         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 200       </varlistentry>
 201
 202       <varlistentry>
 203         <term><varname>systemd.volatile=</varname></term>
 204
 205         <listitem><para>Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the
 206         special value <option>state</option>.</para>
 207
 208         <para>If false (the default), this generator makes no changes to the mount tree and the system is booted up in
 209         normal mode.</para>
 210
 211         <para>If true the generator ensures
 212         <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 213         is run in the initrd. This service changes the mount table before transitioning to the host system,
 214         so that a volatile memory file system (<literal>tmpfs</literal>) is used as root directory, with only
 215         <filename>/usr/</filename> mounted into it from the configured root file system, in read-only mode.
 216         This way the system operates in fully stateless mode, with all configuration and state reset at boot
 217         and lost at shutdown, as <filename>/etc/</filename> and <filename>/var/</filename> will be served
 218         from the (initially unpopulated) volatile memory file system.</para>
 219
 220         <para>If set to <option>state</option> the generator will leave the root directory mount point unaltered,
 221         however will mount a <literal>tmpfs</literal> file system to <filename>/var/</filename>. In this mode the normal
 222         system configuration (i.e. the contents of <literal>/etc/</literal>) is in effect (and may be modified during
 223         system runtime), however the system state (i.e. the contents of <literal>/var/</literal>) is reset at boot and
 224         lost at shutdown.</para>
 225
 226         <para>If this setting is set to <literal>overlay</literal> the root file system is set up as
 227         <literal>overlayfs</literal> mount combining the read-only root directory with a writable
 228         <literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
 229         nonetheless with all changes being lost at reboot.</para>
 230
 231         <para>Note that in none of these modes the root directory, <filename>/etc/</filename>, <filename>/var/</filename>
 232         or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
 233         that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
 234
 235         <para>Note that with the exception of <literal>overlay</literal> mode, enabling this setting will
 236         only work correctly on operating systems that can boot up with only <filename>/usr/</filename>
 237         mounted, and are able to automatically populate <filename>/etc/</filename>, and also
 238         <filename>/var/</filename> in case of <literal>systemd.volatile=yes</literal>.</para>
 239
 240         <para>Also see <varname>root=tmpfs</varname> above, for a method to combine a
 241         <literal>tmpfs</literal> file system with a regular <filename>/usr/</filename> file system (as
 242         configured via <varname>mount.usr=</varname>). The main distinction between
 243         <varname>systemd.volatile=yes</varname>, and <varname>root=tmpfs</varname> in combination
 244         <varname>mount.usr=</varname> is that the former operates on top of a regular root file system and
 245         temporarily obstructs the files and directories above its <filename>/usr/</filename> subdirectory,
 246         while the latter does not hide any files, but simply mounts a unpopulated tmpfs as root file system
 247         and combines it with a user picked <filename>/usr/</filename> file system.</para>
 248
 249         <xi:include href="version-info.xml" xpointer="v233"/></listitem>
 250       </varlistentry>
 251
 252       <varlistentry>
 253         <term><varname>systemd.swap=</varname></term>
 254
 255         <listitem><para>Takes a boolean argument or enables the option if specified
 256         without an argument. If disabled, causes the generator to ignore
 257         any swap devices configured in <filename>/etc/fstab</filename>.
 258         Defaults to enabled.</para>
 259
 260         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 261       </varlistentry>
 262
 263       <varlistentry>
 264         <term><varname>systemd.mount-extra=<replaceable>WHAT</replaceable>:<replaceable>WHERE</replaceable>[:<replaceable>FSTYPE</replaceable>[:<replaceable>OPTIONS</replaceable>]]</varname></term>
 265         <term><varname>rd.systemd.mount-extra=<replaceable>WHAT</replaceable>:<replaceable>WHERE</replaceable>[:<replaceable>FSTYPE</replaceable>[:<replaceable>OPTIONS</replaceable>]]</varname></term>
 266
 267         <listitem>
 268           <para>Specifies the mount unit. Takes at least two and at most four fields separated with a colon
 269           (<literal>:</literal>). Each field is handled as the corresponding fstab field. This option can be
 270           specified multiple times. <varname>rd.systemd.mount-extra=</varname> is honored only in the initrd,
 271           while <varname>systemd.mount-extra=</varname> is honored by both the main system and the initrd.
 272           In the initrd, the mount point (and also source path if the mount is bind mount) specified in
 273           <varname>systemd.mount-extra=</varname> is prefixed with <filename>/sysroot/</filename>.</para>
 274           <para>Example:
 275           <programlisting>
 276 systemd.mount-extra=/dev/sda1:/mount-point:ext4:rw,noatime</programlisting>
 277           </para>
 278
 279           <xi:include href="version-info.xml" xpointer="v254"/>
 280         </listitem>
 281       </varlistentry>
 282
 283       <varlistentry>
 284         <term><varname>systemd.swap-extra=<replaceable>WHAT</replaceable>[:<replaceable>OPTIONS</replaceable>]</varname></term>
 285         <term><varname>rd.systemd.swap-extra=<replaceable>WHAT</replaceable>[:<replaceable>OPTIONS</replaceable>]</varname></term>
 286
 287         <listitem>
 288           <para>Specifies the swap unit. Takes the block device to be used as a swap device, and optionally
 289           takes mount options followed by a colon (<literal>:</literal>). This option can be specified
 290           multiple times. <varname>rd.systemd.swap-extra=</varname> is honored only in the initrd, while
 291           <varname>systemd.swap-extra=</varname> is honored by both the main system and the initrd.</para>
 292           <para>Example:
 293           <programlisting>
 294 systemd.swap=/dev/sda2:x-systemd.makefs</programlisting>
 295           </para>
 296
 297           <xi:include href="version-info.xml" xpointer="v254"/>
 298         </listitem>
 299       </varlistentry>
 300     </variablelist>
 301   </refsect1>
 302
 303   <refsect1>
 304     <title>System Credentials</title>
 305
 306     <variablelist class='system-credentials'>
 307       <varlistentry>
 308         <term><varname>fstab.extra</varname></term>
 309
 310         <listitem><para>This credential may contain addition mounts to establish, in the same format as
 311         <citerefentry
 312         project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>, with
 313         one mount per line. It is read in addition to <filename>/etc/fstab</filename>.</para>
 314
 315         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 316       </varlistentry>
 317     </variablelist>
 318   </refsect1>
 319
 320   <refsect1>
 321     <title>See Also</title>
 322     <para>
 323       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 324       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 325       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 326       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 327       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 328       <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 329       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 330       <ulink url="https://systemd.io/ENVIRONMENT/">Known Environment Variables</ulink>
 331     </para>
 332   </refsect1>
 333 </refentry>