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