man/bootup.xml

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  10   Copyright 2012 Lennart Poettering
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  25
  26 <refentry id="bootup">
  27
  28   <refentryinfo>
  29     <title>bootup</title>
  30     <productname>systemd</productname>
  31
  32     <authorgroup>
  33       <author>
  34         <contrib>Developer</contrib>
  35         <firstname>Lennart</firstname>
  36         <surname>Poettering</surname>
  37         <email>lennart@poettering.net</email>
  38       </author>
  39     </authorgroup>
  40   </refentryinfo>
  41
  42   <refmeta>
  43     <refentrytitle>bootup</refentrytitle>
  44     <manvolnum>7</manvolnum>
  45   </refmeta>
  46
  47   <refnamediv>
  48     <refname>bootup</refname>
  49     <refpurpose>System bootup process</refpurpose>
  50   </refnamediv>
  51
  52   <refsect1>
  53     <title>Description</title>
  54
  55     <para>A number of different components are involved in the system
  56     boot. Immediately after power-up, the system BIOS will do minimal
  57     hardware initialization, and hand control over to a boot loader
  58     stored on a persistent storage device. This boot loader will then
  59     invoke an OS kernel from disk (or the network). In the Linux case,
  60     this kernel (optionally) extracts and executes an initial RAM disk
  61     image (initrd), such as 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 mounted, the
  66     initrd hands over control to the host's system manager (such as
  67     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
  68     stored on the OS image, which is then responsible for probing all
  69     remaining hardware, mounting all necessary file systems and
  70     spawning all configured services.</para>
  71
  72     <para>On shutdown, the system manager stops all services, unmounts
  73     all file systems (detaching the storage technologies backing
  74     them), and then (optionally) jumps back into the initrd code which
  75     unmounts/detaches the root file system and the storage it resides
  76     on. As a last step, the system is powered down.</para>
  77
  78     <para>Additional information about the system boot process may be
  79     found in
  80     <citerefentry project='man-pages'><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 responsible
  87     for initializing the required file systems, services and drivers
  88     that are necessary for operation of the system. On
  89     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  90     systems, this process is split up in various discrete steps which
  91     are exposed as target units. (See
  92     <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  93     for detailed information about target units.) The boot-up process
  94     is highly parallelized so that the order in which specific target
  95     units are reached is not deterministic, but still adheres to a
  96     limited amount of ordering structure.</para>
  97
  98     <para>When systemd starts up the system, it will activate all
  99     units that are dependencies of <filename>default.target</filename>
 100     (as well as recursively all dependencies of these dependencies).
 101     Usually, <filename>default.target</filename> is simply an alias of
 102     <filename>graphical.target</filename> or
 103     <filename>multi-user.target</filename>, depending on whether the
 104     system is configured for a graphical UI or only for a text
 105     console. To enforce minimal ordering between the units pulled in,
 106     a number of well-known target units are available, as listed on
 107     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 108
 109     <para>The following chart is a structural overview of these
 110     well-known units and their position in the boot-up logic. The
 111     arrows describe which units are pulled in and ordered before which
 112     other units. Units near the top are started before units nearer to
 113     the bottom of the chart.</para>
 114
 115 <programlisting>local-fs-pre.target
 116          |
 117          v
 118 (various mounts and   (various swap   (various cryptsetup
 119  fsck services...)     devices...)        devices...)       (various low-level   (various low-level
 120          |                  |                  |             services: udevd,     API VFS mounts:
 121          v                  v                  v             tmpfiles, random     mqueue, configfs,
 122   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
 123          |                  |                  |                    |                    |
 124          \__________________|_________________ | ___________________|____________________/
 125                                               \|/
 126                                                v
 127                                         sysinit.target
 128                                                |
 129           ____________________________________/|\________________________________________
 130          /                  |                  |                    |                    \
 131          |                  |                  |                    |                    |
 132          v                  v                  |                    v                    v
 133      (various           (various               |                (various          rescue.service
 134     timers...)          paths...)              |               sockets...)               |
 135          |                  |                  |                    |                    v
 136          v                  v                  |                    v              <emphasis>rescue.target</emphasis>
 137    timers.target      paths.target             |             sockets.target
 138          |                  |                  |                    |
 139          v                  \_________________ | ___________________/
 140                                               \|/
 141                                                v
 142                                          basic.target
 143                                                |
 144           ____________________________________/|                                 emergency.service
 145          /                  |                  |                                         |
 146          |                  |                  |                                         v
 147          v                  v                  v                                 <emphasis>emergency.target</emphasis>
 148      display-        (various system    (various system
 149  manager.service         services           services)
 150          |             required for            |
 151          |            graphical UIs)           v
 152          |                  |           <emphasis>multi-user.target</emphasis>
 153          |                  |                  |
 154          \_________________ | _________________/
 155                            \|/
 156                             v
 157                   <emphasis>graphical.target</emphasis></programlisting>
 158
 159     <para>Target units that are commonly used as boot targets are
 160     <emphasis>emphasized</emphasis>. These units are good choices as
 161     goal targets, for example by passing them to the
 162     <varname>systemd.unit=</varname> kernel command line option (see
 163     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
 164     or by symlinking <filename>default.target</filename> to them.
 165     </para>
 166
 167     <para><filename>timers.target</filename> is pulled-in by
 168     <filename>basic.target</filename> asynchronously. This allows
 169     timers units to depend on services which become only available
 170     later in boot.</para>
 171   </refsect1>
 172
 173   <refsect1>
 174     <title>Bootup in the Initial RAM Disk (initrd)</title>
 175     <para>The initial RAM disk implementation (initrd) can be set up
 176     using systemd as well. In this case, boot up inside the initrd
 177     follows the following structure.</para>
 178
 179     <para>The default target in the initrd is
 180     <filename>initrd.target</filename>. The bootup process begins
 181     identical to the system manager bootup (see above) until it
 182     reaches <filename>basic.target</filename>. From there, systemd
 183     approaches the special target <filename>initrd.target</filename>.
 184
 185     Before any file systems are mounted, it must be determined whether
 186     the system will resume from hibernation or proceed with normal boot.
 187     This is accomplished by <filename>systemd-hibernate-resume@.service</filename>
 188     which must be finished before <filename>local-fs-pre.target</filename>,
 189     so no filesystems can be mounted before the check is complete.
 190
 191     When the root device becomes available,
 192     <filename>initd-root-device.target</filename> is reached.
 193     If the root device can be mounted at
 194     <filename>/sysroot</filename>, the
 195     <filename>sysroot.mount</filename> unit becomes active and
 196     <filename>initrd-root-fs.target</filename> is 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 entries
 200     marked with the <emphasis>x-initrd.mount</emphasis> option. All
 201     entries found are mounted below <filename>/sysroot</filename>, and
 202     <filename>initrd-fs.target</filename> is reached. The service
 203     <filename>initrd-cleanup.service</filename> isolates to the
 204     <filename>initrd-switch-root.target</filename>, where cleanup
 205     services can run. As the very last step, the
 206     <filename>initrd-switch-root.service</filename> is activated,
 207     which will cause the system to switch its root to
 208     <filename>/sysroot</filename>.
 209     </para>
 210
 211 <programlisting>                                               : (beginning identical to above)
 212                                                :
 213                                                v
 214                                          basic.target
 215                                                |                                 emergency.service
 216                         ______________________/|                                         |
 217                        /                       |                                         v
 218                        |            initrd-root-device.target                    <emphasis>emergency.target</emphasis>
 219                        |                       |
 220                        |                       v
 221                        |                  sysroot.mount
 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   <refsect1>
 267     <title>System Manager Shutdown</title>
 268
 269     <para>System shutdown with systemd also consists of various target
 270     units with some minimal ordering structure applied:</para>
 271
 272 <programlisting>                                  (conflicts with  (conflicts with
 273                                     all system     all file system
 274                                      services)     mounts, swaps,
 275                                          |           cryptsetup
 276                                          |          devices, ...)
 277                                          |                |
 278                                          v                v
 279                                   shutdown.target    umount.target
 280                                          |                |
 281                                          \_______   ______/
 282                                                  \ /
 283                                                   v
 284                                          (various low-level
 285                                               services)
 286                                                   |
 287                                                   v
 288                                             final.target
 289                                                   |
 290             _____________________________________/ \_________________________________
 291            /                         |                        |                      \
 292            |                         |                        |                      |
 293            v                         v                        v                      v
 294 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 295            |                         |                        |                      |
 296            v                         v                        v                      v
 297     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 298
 299     <para>Commonly used system shutdown targets are
 300     <emphasis>emphasized</emphasis>.</para>
 301   </refsect1>
 302
 303   <refsect1>
 304     <title>See Also</title>
 305     <para>
 306       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 307       <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 308       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 309       <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 310       <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 311     </para>
 312   </refsect1>
 313
 314 </refentry>