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
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  11   under the terms of the GNU Lesser General Public License as published by
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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                 <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  62                 which looks for the root file system (possibly using
  63                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  64                 for this). After the root file system is found and
  65                 mounted the initrd hands over control to the host's
  66                 system manager (such as
  67                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
  68                 stored on the OS image which is then responsible for
  69                 probing all remaining hardware, mounting all necessary
  70                 file systems and spawning all configured
  71                 services.</para>
  72
  73                 <para>On shutdown the system manager stops all
  74                 services, unmounts all file systems (detaching the
  75                 storage technologies backing them), and then
  76                 (optionally) jumps back into the initrd code which
  77                 unmounts/detaches the root file system and the storage
  78                 it resides on. As last step the system is powered down.</para>
  79
  80                 <para>Additional information about the system boot
  81                 process may be found in
  82                 <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  83         </refsect1>
  84
  85         <refsect1>
  86                 <title>System Manager Bootup</title>
  87
  88                 <para>At boot, the system manager on the OS image is
  89                 responsible for initializing the required file
  90                 systems, services and drivers that are necessary for
  91                 operation of the system. On
  92                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  93                 systems this process is split up in various discrete
  94                 steps which are exposed as target units. (See
  95                 <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  96                 for detailed information about target units.) The
  97                 boot-up process is highly parallelized so that the
  98                 order in which specific target units are reached is not
  99                 deterministic, but still adheres to a limited amount
 100                 of ordering structure.</para>
 101
 102                 <para>When systemd starts up the system it will
 103                 activate all units that are dependencies of
 104                 <filename>default.target</filename> (as well as
 105                 recursively all dependencies of these
 106                 dependencies). Usually
 107                 <filename>default.target</filename> is simply an alias
 108                 of <filename>graphical.target</filename> or
 109                 <filename>multi-user.target</filename> depending on
 110                 whether the system is configured for a graphical UI or
 111                 only for a text console. To enforce minimal ordering
 112                 between the units pulled in a number of well-known
 113                 target units are available, as listed on
 114                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 115
 116                 <para>The following chart is a structural overview of
 117                 these well-known units and their position in the
 118                 boot-up logic. The arrows describe which units are
 119                 pulled in and ordered before which other units. Units
 120                 near the top are started before units nearer to the
 121                 bottom of the chart.</para>
 122
 123 <programlisting>local-fs-pre.target
 124          |
 125          v
 126 (various mounts and   (various swap   (various cryptsetup
 127  fsck services...)     devices...)        devices...)       (various low-level   (various low-level
 128          |                  |                  |             services: udevd,     API VFS mounts:
 129          v                  v                  v             tmpfiles, random     mqueue, configfs,
 130   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
 131          |                  |                  |                    |                    |
 132          \__________________|_________________ | ___________________|____________________/
 133                                               \|/
 134                                                v
 135                                         sysinit.target
 136                                                |
 137           ____________________________________/|\________________________________________
 138          /                  |                  |                    |                    \
 139          |                  |                  |                    |                    |
 140          v                  v                  |                    v                    v
 141      (various           (various               |                (various          rescue.service
 142     timers...)          paths...)              |               sockets...)               |
 143          |                  |                  |                    |                    v
 144          v                  v                  |                    v              <emphasis>rescue.target</emphasis>
 145    timers.target      paths.target             |             sockets.target
 146          |                  |                  |                    |
 147          \__________________|_________________ | ___________________/
 148                                               \|/
 149                                                v
 150                                          basic.target
 151                                                |
 152           ____________________________________/|                                 emergency.service
 153          /                  |                  |                                         |
 154          |                  |                  |                                         v
 155          v                  v                  v                                 <emphasis>emergency.target</emphasis>
 156      display-        (various system    (various system
 157  manager.service         services           services)
 158          |             required for            |
 159          |            graphical UIs)           v
 160          |                  |           <emphasis>multi-user.target</emphasis>
 161          |                  |                  |
 162          \_________________ | _________________/
 163                            \|/
 164                             v
 165                   <emphasis>graphical.target</emphasis></programlisting>
 166
 167                 <para>Target units that are commonly used as boot
 168                 targets are <emphasis>emphasized</emphasis>. These
 169                 units are good choices as goal targets, for
 170                 example by passing them to the
 171                 <varname>systemd.unit=</varname> kernel command line
 172                 option (see
 173                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
 174                 or by symlinking <filename>default.target</filename>
 175                 to them.</para>
 176         </refsect1>
 177
 178         <refsect1>
 179                 <title>Bootup in the Initial RAM Disk (initrd)</title>
 180                 <para>The initial RAM disk implementation (initrd) can
 181                 be set up using systemd as well. In this case boot up
 182                 inside the initrd follows the following
 183                 structure.</para>
 184
 185                 <para>The default target in the initrd is
 186                 <filename>initrd.target</filename>. The bootup process
 187                 begins identical to the system manager bootup (see
 188                 above) until it reaches
 189                 <filename>basic.target</filename>. From there, systemd
 190                 approaches the special target
 191                 <filename>initrd.target</filename>. If the root device
 192                 can be mounted at <filename>/sysroot</filename>, the
 193                 <filename>sysroot.mount</filename> unit becomes active
 194                 and <filename>initrd-root-fs.target</filename> is
 195                 reached.  The service
 196                 <filename>initrd-parse-etc.service</filename> scans
 197                 <filename>/sysroot/etc/fstab</filename> for a possible
 198                 <filename>/usr</filename> mount point and additional
 199                 entries marked with the
 200                 <emphasis>x-initrd.mount</emphasis> option. All
 201                 entries found are mounted below
 202                 <filename>/sysroot</filename>, and
 203                 <filename>initrd-fs.target</filename> is reached. The
 204                 service <filename>initrd-cleanup.service</filename>
 205                 isolates to the
 206                 <filename>initrd-switch-root.target</filename>, where
 207                 cleanup services can run. As the very last step, the
 208                 <filename>initrd-switch-root.service</filename> is
 209                 activated, which will cause the system to switch its
 210                 root to <filename>/sysroot</filename>.
 211                 </para>
 212
 213 <programlisting>                                               : (beginning identical to above)
 214                                                :
 215                                                v
 216                                          basic.target
 217                                                |                                 emergency.service
 218                         ______________________/|                                         |
 219                        /                       |                                         v
 220                        |                  sysroot.mount                          <emphasis>emergency.target</emphasis>
 221                        |                       |
 222                        |                       v
 223                        |             initrd-root-fs.target
 224                        |                       |
 225                        |                       v
 226                        v            initrd-parse-etc.service
 227                 (custom initrd                 |
 228                  services...)                  v
 229                        |            (sysroot-usr.mount and
 230                        |             various mounts marked
 231                        |               with fstab option
 232                        |              x-initrd.mount...)
 233                        |                       |
 234                        |                       v
 235                        |                initrd-fs.target
 236                        \______________________ |
 237                                               \|
 238                                                v
 239                                           initrd.target
 240                                                |
 241                                                v
 242                                      initrd-cleanup.service
 243                                           isolates to
 244                                     initrd-switch-root.target
 245                                                |
 246                                                v
 247                         ______________________/|
 248                        /                       v
 249                        |        initrd-udevadm-cleanup-db.service
 250                        v                       |
 251                 (custom initrd                 |
 252                  services...)                  |
 253                        \______________________ |
 254                                               \|
 255                                                v
 256                                    initrd-switch-root.target
 257                                                |
 258                                                v
 259                                    initrd-switch-root.service
 260                                                |
 261                                                v
 262                                      Transition to Host OS</programlisting>
 263         </refsect1>
 264
 265
 266         <refsect1>
 267                 <title>System Manager Shutdown</title>
 268
 269                 <para>System shutdown with systemd also consists of
 270                 various target units with some minimal ordering
 271                 structure applied:</para>
 272
 273
 274
 275
 276 <programlisting>                                  (conflicts with  (conflicts with
 277                                     all system     all file system
 278                                      services)     mounts, swaps,
 279                                          |           cryptsetup
 280                                          |          devices, ...)
 281                                          |                |
 282                                          v                v
 283                                   shutdown.target    umount.target
 284                                          |                |
 285                                          \_______   ______/
 286                                                  \ /
 287                                                   v
 288                                          (various low-level
 289                                               services)
 290                                                   |
 291                                                   v
 292                                             final.target
 293                                                   |
 294             _____________________________________/ \_________________________________
 295            /                         |                        |                      \
 296            |                         |                        |                      |
 297            v                         v                        v                      v
 298 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 299            |                         |                        |                      |
 300            v                         v                        v                      v
 301     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 302
 303                 <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
 304         </refsect1>
 305
 306         <refsect1>
 307                 <title>See Also</title>
 308                 <para>
 309                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 310                         <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 311                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 312                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 313                         <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 314                 </para>
 315         </refsect1>
 316
 317 </refentry>