man/bootup.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   6   This file is part of systemd.
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   8   Copyright 2012 Lennart Poettering
   9
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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 the
  54                 system boot. Immediately after power-up, the system
  55                 BIOS will do minimal hardware initialization, and hand
  56                 control over to a boot loader stored on a persistent
  57                 storage device. This boot loader will then invoke an
  58                 OS kernel from disk (or the network). In the Linux
  59                 case this kernel now (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. After the root
  63                 file system is found and mounted the initrd hands over
  64                 control to the system manager (such as
  65                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
  66                 stored on the OS image which is then responsible for
  67                 probing all remaining hardware, mounting all necessary
  68                 file systems and spawning all configured
  69                 services.</para>
  70
  71                 <para>On shutdown the system manager stops all
  72                 services, unmounts all file systems (detaching the
  73                 storage technologies backing them), and then
  74                 (optionally) jumps back into the initrd code which
  75                 unmounts/detaches the root file system and the storage
  76                 it resides on. As last step the system is powered down.</para>
  77
  78                 <para>Additional information about the system boot
  79                 process may be found in
  80                 <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  81         </refsect1>
  82
  83         <refsect1>
  84                 <title>System Manager Bootup</title>
  85
  86                 <para>At boot, the system manager on the OS image is
  87                 responsible for initializing the required file
  88                 systems, services and drivers that are necessary for
  89                 operation of the system. On
  90                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  91                 systems this process is split up in various discrete
  92                 steps which are exposed as target units. (See
  93                 <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  94                 for detailed information about target units.) The
  95                 boot-up process is highly parallelized so that the
  96                 order in which specific target units are reached is not
  97                 deterministic, but still adheres to a limited amount
  98                 of ordering structure.</para>
  99
 100                 <para>When systemd starts up the system it will
 101                 activate all units that are dependencies of
 102                 <filename>default.target</filename> (as well as
 103                 recursively all dependencies of these
 104                 dependencies). Usually
 105                 <filename>default.target</filename> is simply an alias
 106                 of <filename>graphical.target</filename> or
 107                 <filename>multi-user.target</filename> depending on
 108                 whether the system is configured for a graphical UI or
 109                 only for a text console. To enforce minimal ordering
 110                 between the units pulled in a number of well-known
 111                 target units are available, as listed on
 112                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 113
 114                 <para>The following chart is a structural overview of
 115                 these well-known units and their position in the
 116                 boot-up logic. The arrows describe which units are
 117                 pulled in and ordered before which other units. Units
 118                 near the top are started before units nearer to the
 119                 bottom of the chart.</para>
 120
 121 <programlisting>local-fs-pre.target
 122          |
 123          v
 124 (various mounts and   (various swap   (various cryptsetup
 125  fsck services...)     devices...)        devices...)       (various low-level   (various low-level
 126          |                  |                  |             services: udevd,     API VFS mounts:
 127          v                  v                  v             tmpfiles, random     mqueue, configfs,
 128   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
 129          |                  |                  |                    |                    |
 130          \__________________|_________________ | ___________________|____________________/
 131                                               \|/
 132                                                v
 133                                         sysinit.target
 134                                                |
 135                              _________________/|\___________________
 136                             /                  |                    \
 137                             |                  |                    |
 138                             v                  |                    v
 139                         (various               |              rescue.service
 140                        sockets...)             |                    |
 141                             |                  |                    v
 142                             v                  |              <emphasis>rescue.target</emphasis>
 143                      sockets.target            |
 144                             |                  |
 145                             \_________________ |
 146                                               \|
 147                                                v
 148                                          basic.target
 149                                                |
 150             __________________________________/|                                 emergency.service
 151            /                |                  |                                         |
 152            |                |                  |                                         v
 153            v                v                  v                                 <emphasis>emergency.target</emphasis>
 154        display-      (various system    (various system
 155    manager.service       services           services)
 156            |           required for            |
 157            |          graphical UIs)           v
 158            |                |           <emphasis>multi-user.target</emphasis>
 159            |                |                  |
 160            \_______________ | _________________/
 161                            \|/
 162                             v
 163                     <emphasis>graphical.target</emphasis></programlisting>
 164
 165                 <para>Target units that are commonly used as boot
 166                 targets are <emphasis>emphasized</emphasis>. These
 167                 units are good choices as goal targets, for
 168                 example by passing them to the
 169                 <varname>systemd.unit=</varname> kernel command line
 170                 option (see
 171                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
 172                 or by symlinking <filename>default.target</filename>
 173                 to them.</para>
 174         </refsect1>
 175
 176         <refsect1>
 177                 <title>Initrd System Manager Bootup</title>
 178                 <para>The default target in the initrd is the initrd.target. The bootup process is
 179                 identical to the System Manager Bootup until it reaches basic.target. From there,
 180                 systemd approaches the special initrd.target. If the root device can be mounted at
 181                 /sysroot, the sysroot.mount unit becomes active and the initrd-root-fs.target is
 182                 reached.
 183                 The initrd-parse-etc.service scans /sysroot/etc/fstab for a possible /usr mountpoint
 184                 and additional entries marked with the <emphasis>x-initrd.mount</emphasis> option.
 185                 All entries found are mounted below /sysroot, and the initrd-fs.target is reached.
 186                 The initrd-cleanup.service isolates to the initrd-switch-root.target, where cleanup
 187                 services can run. As the very last step, the initrd-switch-root.service is activated,
 188                 which will cause the system to switch its root to /sysroot.
 189                 </para>
 190
 191 <programlisting>
 192                              (identical to System Manager Bootup)
 193                                                :
 194                                                :
 195                                                v
 196                                          basic.target
 197                                                |                                 emergency.service
 198                         ______________________/|                                         |
 199                        /                       |                                         v
 200                        |                  sysroot.mount                          <emphasis>emergency.target</emphasis>
 201                        |                       |
 202                        |                       v
 203                        |             initrd-root-fs.target
 204                        |                       |
 205                        |                       v
 206                        |            initrd-parse-etc.service
 207            (custom initrd services)            |
 208                        |                       v
 209                        |            (sysroot-usr.mount and
 210                        |             various mounts marked
 211                        |               with fstab option
 212                        |                x-initrd.mount)
 213                        |                       |
 214                        |                       v
 215                        |                initrd-fs.target
 216                        \______________________ |
 217                                               \|
 218                                                v
 219                                           initrd.target
 220                                                |
 221                                                v
 222                                      initrd-cleanup.service
 223                                           isolates to
 224                                     initrd-switch-root.target
 225                                                |
 226                                                v
 227                         ______________________/|
 228                        /                       |
 229                        |        initrd-udevadm-cleanup-db.service
 230                        |                       |
 231            (custom initrd services)            |
 232                        |                       |
 233                        \______________________ |
 234                                               \|
 235                                                v
 236                                    initrd-switch-root.target
 237                                                |
 238                                                v
 239                                    initrd-switch-root.service
 240                                                |
 241                                                v
 242                                           switch-root
 243 </programlisting>
 244         </refsect1>
 245
 246
 247         <refsect1>
 248                 <title>System Manager Shutdown</title>
 249
 250                 <para>System shutdown also consists of various target
 251                 units with some minimal ordering structure
 252                 applied:</para>
 253
 254
 255
 256
 257 <programlisting>                                  (conflicts with  (conflicts with
 258                                     all system     all file system
 259                                      services)     mounts, swaps,
 260                                          |           cryptsetup
 261                                          |          devices, ...)
 262                                          |                |
 263                                          v                v
 264                                   shutdown.target    umount.target
 265                                          |                |
 266                                          \_______   ______/
 267                                                  \ /
 268                                                   v
 269                                          (various low-level
 270                                               services)
 271                                                   |
 272                                                   v
 273                                             final.target
 274                                                   |
 275             _____________________________________/ \_________________________________
 276            /                         |                        |                      \
 277            |                         |                        |                      |
 278            v                         v                        v                      v
 279 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 280            |                         |                        |                      |
 281            v                         v                        v                      v
 282     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 283
 284                 <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
 285         </refsect1>
 286
 287         <refsect1>
 288                 <title>See Also</title>
 289                 <para>
 290                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 291                         <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 292                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 293                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 294                         <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 295                 </para>
 296         </refsect1>
 297
 298 </refentry>