man/bootup.xml

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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><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><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          \__________________|_________________ | ___________________/
 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         </refsect1>
 178
 179         <refsect1>
 180                 <title>Bootup in the Initial RAM Disk (initrd)</title>
 181                 <para>The initial RAM disk implementation (initrd) can
 182                 be set up using systemd as well. In this case, boot up
 183                 inside the initrd follows the following
 184                 structure.</para>
 185
 186                 <para>The default target in the initrd is
 187                 <filename>initrd.target</filename>. The bootup process
 188                 begins identical to the system manager bootup (see
 189                 above) until it reaches
 190                 <filename>basic.target</filename>. From there, systemd
 191                 approaches the special target
 192                 <filename>initrd.target</filename>. If the root device
 193                 can be mounted at <filename>/sysroot</filename>, the
 194                 <filename>sysroot.mount</filename> unit becomes active
 195                 and <filename>initrd-root-fs.target</filename> is
 196                 reached.  The service
 197                 <filename>initrd-parse-etc.service</filename> scans
 198                 <filename>/sysroot/etc/fstab</filename> for a possible
 199                 <filename>/usr</filename> mount point and additional
 200                 entries marked with the
 201                 <emphasis>x-initrd.mount</emphasis> option. All
 202                 entries found are mounted below
 203                 <filename>/sysroot</filename>, and
 204                 <filename>initrd-fs.target</filename> is reached. The
 205                 service <filename>initrd-cleanup.service</filename>
 206                 isolates to the
 207                 <filename>initrd-switch-root.target</filename>, where
 208                 cleanup services can run. As the very last step, the
 209                 <filename>initrd-switch-root.service</filename> is
 210                 activated, which will cause the system to switch its
 211                 root to <filename>/sysroot</filename>.
 212                 </para>
 213
 214 <programlisting>                                               : (beginning identical to above)
 215                                                :
 216                                                v
 217                                          basic.target
 218                                                |                                 emergency.service
 219                         ______________________/|                                         |
 220                        /                       |                                         v
 221                        |                  sysroot.mount                          <emphasis>emergency.target</emphasis>
 222                        |                       |
 223                        |                       v
 224                        |             initrd-root-fs.target
 225                        |                       |
 226                        |                       v
 227                        v            initrd-parse-etc.service
 228                 (custom initrd                 |
 229                  services...)                  v
 230                        |            (sysroot-usr.mount and
 231                        |             various mounts marked
 232                        |               with fstab option
 233                        |              x-initrd.mount...)
 234                        |                       |
 235                        |                       v
 236                        |                initrd-fs.target
 237                        \______________________ |
 238                                               \|
 239                                                v
 240                                           initrd.target
 241                                                |
 242                                                v
 243                                      initrd-cleanup.service
 244                                           isolates to
 245                                     initrd-switch-root.target
 246                                                |
 247                                                v
 248                         ______________________/|
 249                        /                       v
 250                        |        initrd-udevadm-cleanup-db.service
 251                        v                       |
 252                 (custom initrd                 |
 253                  services...)                  |
 254                        \______________________ |
 255                                               \|
 256                                                v
 257                                    initrd-switch-root.target
 258                                                |
 259                                                v
 260                                    initrd-switch-root.service
 261                                                |
 262                                                v
 263                                      Transition to Host OS</programlisting>
 264         </refsect1>
 265
 266
 267         <refsect1>
 268                 <title>System Manager Shutdown</title>
 269
 270                 <para>System shutdown with systemd also consists of
 271                 various target units with some minimal ordering
 272                 structure applied:</para>
 273
 274
 275
 276
 277 <programlisting>                                  (conflicts with  (conflicts with
 278                                     all system     all file system
 279                                      services)     mounts, swaps,
 280                                          |           cryptsetup
 281                                          |          devices, ...)
 282                                          |                |
 283                                          v                v
 284                                   shutdown.target    umount.target
 285                                          |                |
 286                                          \_______   ______/
 287                                                  \ /
 288                                                   v
 289                                          (various low-level
 290                                               services)
 291                                                   |
 292                                                   v
 293                                             final.target
 294                                                   |
 295             _____________________________________/ \_________________________________
 296            /                         |                        |                      \
 297            |                         |                        |                      |
 298            v                         v                        v                      v
 299 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 300            |                         |                        |                      |
 301            v                         v                        v                      v
 302     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 303
 304                 <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
 305         </refsect1>
 306
 307         <refsect1>
 308                 <title>See Also</title>
 309                 <para>
 310                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 311                         <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 312                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 313                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 314                         <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 315                 </para>
 316         </refsect1>
 317
 318 </refentry>