man/bootup.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3         "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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   6   This file is part of systemd.
   7
   8   Copyright 2012 Lennart Poettering
   9
  10   systemd is free software; you can redistribute it and/or modify it
  11   under the terms of the GNU Lesser General Public License as published by
  12   the Free Software Foundation; either version 2.1 of the License, or
  13   (at your option) any later version.
  14
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  18   Lesser General Public License for more details.
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  23
  24 <refentry id="bootup">
  25
  26         <refentryinfo>
  27                 <title>bootup</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>bootup</refentrytitle>
  42                 <manvolnum>7</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>bootup</refname>
  47                 <refpurpose>System bootup process</refpurpose>
  48         </refnamediv>
  49
  50         <refsect1>
  51                 <title>Description</title>
  52
  53                 <para>A number of different components are involved in
  54                 the system boot. Immediately after power-up, the
  55                 system BIOS will do minimal hardware initialization,
  56                 and hand control over to a boot loader stored on a
  57                 persistent storage device. This boot loader will then
  58                 invoke an OS kernel from disk (or the network). In the
  59                 Linux case, this kernel (optionally) extracts and
  60                 executes an initial RAM disk image (initrd), such as
  61                 generated by
  62                 <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  63                 which looks for the root file system (possibly using
  64                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  65                 for this). After the root file system is found and
  66                 mounted, the initrd hands over control to the host's
  67                 system manager (such as
  68                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
  69                 stored on the OS image, which is then responsible for
  70                 probing all remaining hardware, mounting all necessary
  71                 file systems and spawning all configured
  72                 services.</para>
  73
  74                 <para>On shutdown, the system manager stops all
  75                 services, unmounts all file systems (detaching the
  76                 storage technologies backing them), and then
  77                 (optionally) jumps back into the initrd code which
  78                 unmounts/detaches the root file system and the storage
  79                 it resides on. As a last step, the system is powered down.</para>
  80
  81                 <para>Additional information about the system boot
  82                 process may be found in
  83                 <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  84         </refsect1>
  85
  86         <refsect1>
  87                 <title>System Manager Bootup</title>
  88
  89                 <para>At boot, the system manager on the OS image is
  90                 responsible for initializing the required file
  91                 systems, services and drivers that are necessary for
  92                 operation of the system. On
  93                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  94                 systems, this process is split up in various discrete
  95                 steps which are exposed as target units. (See
  96                 <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  97                 for detailed information about target units.) The
  98                 boot-up process is highly parallelized so that the
  99                 order in which specific target units are reached is not
 100                 deterministic, but still adheres to a limited amount
 101                 of ordering structure.</para>
 102
 103                 <para>When systemd starts up the system, it will
 104                 activate all units that are dependencies of
 105                 <filename>default.target</filename> (as well as
 106                 recursively all dependencies of these
 107                 dependencies). Usually,
 108                 <filename>default.target</filename> is simply an alias
 109                 of <filename>graphical.target</filename> or
 110                 <filename>multi-user.target</filename>, depending on
 111                 whether the system is configured for a graphical UI or
 112                 only for a text console. To enforce minimal ordering
 113                 between the units pulled in, a number of well-known
 114                 target units are available, as listed on
 115                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 116
 117                 <para>The following chart is a structural overview of
 118                 these well-known units and their position in the
 119                 boot-up logic. The arrows describe which units are
 120                 pulled in and ordered before which other units. Units
 121                 near the top are started before units nearer to the
 122                 bottom of the chart.</para>
 123
 124 <programlisting>local-fs-pre.target
 125          |
 126          v
 127 (various mounts and   (various swap   (various cryptsetup
 128  fsck services...)     devices...)        devices...)       (various low-level   (various low-level
 129          |                  |                  |             services: udevd,     API VFS mounts:
 130          v                  v                  v             tmpfiles, random     mqueue, configfs,
 131   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
 132          |                  |                  |                    |                    |
 133          \__________________|_________________ | ___________________|____________________/
 134                                               \|/
 135                                                v
 136                                         sysinit.target
 137                                                |
 138           ____________________________________/|\________________________________________
 139          /                  |                  |                    |                    \
 140          |                  |                  |                    |                    |
 141          v                  v                  |                    v                    v
 142      (various           (various               |                (various          rescue.service
 143     timers...)          paths...)              |               sockets...)               |
 144          |                  |                  |                    |                    v
 145          v                  v                  |                    v              <emphasis>rescue.target</emphasis>
 146    timers.target      paths.target             |             sockets.target
 147          |                  |                  |                    |
 148          v                  |_________________ | ___________________/
 149                                               \|/
 150                                                v
 151                                          basic.target
 152                                                |
 153           ____________________________________/|                                 emergency.service
 154          /                  |                  |                                         |
 155          |                  |                  |                                         v
 156          v                  v                  v                                 <emphasis>emergency.target</emphasis>
 157      display-        (various system    (various system
 158  manager.service         services           services)
 159          |             required for            |
 160          |            graphical UIs)           v
 161          |                  |           <emphasis>multi-user.target</emphasis>
 162          |                  |                  |
 163          \_________________ | _________________/
 164                            \|/
 165                             v
 166                   <emphasis>graphical.target</emphasis></programlisting>
 167
 168                 <para>Target units that are commonly used as boot
 169                 targets are <emphasis>emphasized</emphasis>. These
 170                 units are good choices as goal targets, for
 171                 example by passing them to the
 172                 <varname>systemd.unit=</varname> kernel command line
 173                 option (see
 174                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
 175                 or by symlinking <filename>default.target</filename>
 176                 to them.</para>
 177
 178                 <para><filename>timers.target</filename> is pulled-in
 179                 by <filename>basic.target</filename> asynchronously.
 180                 This allows timers units to depend on services which
 181                 become only available later in boot.</para>
 182         </refsect1>
 183
 184         <refsect1>
 185                 <title>Bootup in the Initial RAM Disk (initrd)</title>
 186                 <para>The initial RAM disk implementation (initrd) can
 187                 be set up using systemd as well. In this case, boot up
 188                 inside the initrd follows the following
 189                 structure.</para>
 190
 191                 <para>The default target in the initrd is
 192                 <filename>initrd.target</filename>. The bootup process
 193                 begins identical to the system manager bootup (see
 194                 above) until it reaches
 195                 <filename>basic.target</filename>. From there, systemd
 196                 approaches the special target
 197                 <filename>initrd.target</filename>. If the root device
 198                 can be mounted at <filename>/sysroot</filename>, the
 199                 <filename>sysroot.mount</filename> unit becomes active
 200                 and <filename>initrd-root-fs.target</filename> is
 201                 reached.  The service
 202                 <filename>initrd-parse-etc.service</filename> scans
 203                 <filename>/sysroot/etc/fstab</filename> for a possible
 204                 <filename>/usr</filename> mount point and additional
 205                 entries marked with the
 206                 <emphasis>x-initrd.mount</emphasis> option. All
 207                 entries found are mounted below
 208                 <filename>/sysroot</filename>, and
 209                 <filename>initrd-fs.target</filename> is reached. The
 210                 service <filename>initrd-cleanup.service</filename>
 211                 isolates to the
 212                 <filename>initrd-switch-root.target</filename>, where
 213                 cleanup services can run. As the very last step, the
 214                 <filename>initrd-switch-root.service</filename> is
 215                 activated, which will cause the system to switch its
 216                 root to <filename>/sysroot</filename>.
 217                 </para>
 218
 219 <programlisting>                                               : (beginning identical to above)
 220                                                :
 221                                                v
 222                                          basic.target
 223                                                |                                 emergency.service
 224                         ______________________/|                                         |
 225                        /                       |                                         v
 226                        |                  sysroot.mount                          <emphasis>emergency.target</emphasis>
 227                        |                       |
 228                        |                       v
 229                        |             initrd-root-fs.target
 230                        |                       |
 231                        |                       v
 232                        v            initrd-parse-etc.service
 233                 (custom initrd                 |
 234                  services...)                  v
 235                        |            (sysroot-usr.mount and
 236                        |             various mounts marked
 237                        |               with fstab option
 238                        |              x-initrd.mount...)
 239                        |                       |
 240                        |                       v
 241                        |                initrd-fs.target
 242                        \______________________ |
 243                                               \|
 244                                                v
 245                                           initrd.target
 246                                                |
 247                                                v
 248                                      initrd-cleanup.service
 249                                           isolates to
 250                                     initrd-switch-root.target
 251                                                |
 252                                                v
 253                         ______________________/|
 254                        /                       v
 255                        |        initrd-udevadm-cleanup-db.service
 256                        v                       |
 257                 (custom initrd                 |
 258                  services...)                  |
 259                        \______________________ |
 260                                               \|
 261                                                v
 262                                    initrd-switch-root.target
 263                                                |
 264                                                v
 265                                    initrd-switch-root.service
 266                                                |
 267                                                v
 268                                      Transition to Host OS</programlisting>
 269         </refsect1>
 270
 271
 272         <refsect1>
 273                 <title>System Manager Shutdown</title>
 274
 275                 <para>System shutdown with systemd also consists of
 276                 various target units with some minimal ordering
 277                 structure applied:</para>
 278
 279
 280
 281
 282 <programlisting>                                  (conflicts with  (conflicts with
 283                                     all system     all file system
 284                                      services)     mounts, swaps,
 285                                          |           cryptsetup
 286                                          |          devices, ...)
 287                                          |                |
 288                                          v                v
 289                                   shutdown.target    umount.target
 290                                          |                |
 291                                          \_______   ______/
 292                                                  \ /
 293                                                   v
 294                                          (various low-level
 295                                               services)
 296                                                   |
 297                                                   v
 298                                             final.target
 299                                                   |
 300             _____________________________________/ \_________________________________
 301            /                         |                        |                      \
 302            |                         |                        |                      |
 303            v                         v                        v                      v
 304 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 305            |                         |                        |                      |
 306            v                         v                        v                      v
 307     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 308
 309                 <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
 310         </refsect1>
 311
 312         <refsect1>
 313                 <title>See Also</title>
 314                 <para>
 315                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 316                         <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 317                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 318                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 319                         <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 320                 </para>
 321         </refsect1>
 322
 323 </refentry>