man/bootup.html

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  22   <a href="../libudev/index.html">gudev </a><span style="float:right">systemd 217</span><hr><div class="refentry"><a name="bootup"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>bootup — System bootup process</p></div><div class="refsect1"><a name="idm214198228736"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p>A number of different components are involved in
  23                 the system boot. Immediately after power-up, the
  24                 system BIOS will do minimal hardware initialization,
  25                 and hand control over to a boot loader stored on a
  26                 persistent storage device. This boot loader will then
  27                 invoke an OS kernel from disk (or the network). In the
  28                 Linux case, this kernel (optionally) extracts and
  29                 executes an initial RAM disk image (initrd), such as
  30                 generated by
  31                 <a href="http://linux.die.net/man/8/dracut"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>,
  32                 which looks for the root file system (possibly using
  33                 <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
  34                 for this). After the root file system is found and
  35                 mounted, the initrd hands over control to the host's
  36                 system manager (such as
  37                 <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
  38                 stored on the OS image, which is then responsible for
  39                 probing all remaining hardware, mounting all necessary
  40                 file systems and spawning all configured
  41                 services.</p><p>On shutdown, the system manager stops all
  42                 services, unmounts all file systems (detaching the
  43                 storage technologies backing them), and then
  44                 (optionally) jumps back into the initrd code which
  45                 unmounts/detaches the root file system and the storage
  46                 it resides on. As a last step, the system is powered down.</p><p>Additional information about the system boot
  47                 process may be found in
  48                 <a href="http://man7.org/linux/man-pages/man7/boot.7.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>.</p></div><div class="refsect1"><a name="idm214194438320"></a><h2 id="System Manager Bootup">System Manager Bootup<a class="headerlink" title="Permalink to this headline" href="#System%20Manager%20Bootup">¶</a></h2><p>At boot, the system manager on the OS image is
  49                 responsible for initializing the required file
  50                 systems, services and drivers that are necessary for
  51                 operation of the system. On
  52                 <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
  53                 systems, this process is split up in various discrete
  54                 steps which are exposed as target units. (See
  55                 <a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>
  56                 for detailed information about target units.) The
  57                 boot-up process is highly parallelized so that the
  58                 order in which specific target units are reached is not
  59                 deterministic, but still adheres to a limited amount
  60                 of ordering structure.</p><p>When systemd starts up the system, it will
  61                 activate all units that are dependencies of
  62                 <code class="filename">default.target</code> (as well as
  63                 recursively all dependencies of these
  64                 dependencies). Usually,
  65                 <code class="filename">default.target</code> is simply an alias
  66                 of <code class="filename">graphical.target</code> or
  67                 <code class="filename">multi-user.target</code>, depending on
  68                 whether the system is configured for a graphical UI or
  69                 only for a text console. To enforce minimal ordering
  70                 between the units pulled in, a number of well-known
  71                 target units are available, as listed on
  72                 <a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>.</p><p>The following chart is a structural overview of
  73                 these well-known units and their position in the
  74                 boot-up logic. The arrows describe which units are
  75                 pulled in and ordered before which other units. Units
  76                 near the top are started before units nearer to the
  77                 bottom of the chart.</p><pre class="programlisting">local-fs-pre.target
  78          |
  79          v
  80 (various mounts and   (various swap   (various cryptsetup
  81  fsck services...)     devices...)        devices...)       (various low-level   (various low-level
  82          |                  |                  |             services: udevd,     API VFS mounts:
  83          v                  v                  v             tmpfiles, random     mqueue, configfs,
  84   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
  85          |                  |                  |                    |                    |
  86          \__________________|_________________ | ___________________|____________________/
  87                                               \|/
  88                                                v
  89                                         sysinit.target
  90                                                |
  91           ____________________________________/|\________________________________________
  92          /                  |                  |                    |                    \
  93          |                  |                  |                    |                    |
  94          v                  v                  |                    v                    v
  95      (various           (various               |                (various          rescue.service
  96     timers...)          paths...)              |               sockets...)               |
  97          |                  |                  |                    |                    v
  98          v                  v                  |                    v              <span class="emphasis"><em>rescue.target</em></span>
  99    timers.target      paths.target             |             sockets.target
 100          |                  |                  |                    |
 101          \__________________|_________________ | ___________________/
 102                                               \|/
 103                                                v
 104                                          basic.target
 105                                                |
 106           ____________________________________/|                                 emergency.service
 107          /                  |                  |                                         |
 108          |                  |                  |                                         v
 109          v                  v                  v                                 <span class="emphasis"><em>emergency.target</em></span>
 110      display-        (various system    (various system
 111  manager.service         services           services)
 112          |             required for            |
 113          |            graphical UIs)           v
 114          |                  |           <span class="emphasis"><em>multi-user.target</em></span>
 115          |                  |                  |
 116          \_________________ | _________________/
 117                            \|/
 118                             v
 119                   <span class="emphasis"><em>graphical.target</em></span></pre><p>Target units that are commonly used as boot
 120                 targets are <span class="emphasis"><em>emphasized</em></span>. These
 121                 units are good choices as goal targets, for
 122                 example by passing them to the
 123                 <code class="varname">systemd.unit=</code> kernel command line
 124                 option (see
 125                 <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
 126                 or by symlinking <code class="filename">default.target</code>
 127                 to them.</p></div><div class="refsect1"><a name="idm214198321168"></a><h2 id="Bootup in the Initial RAM Disk (initrd)">Bootup in the Initial RAM Disk (initrd)<a class="headerlink" title="Permalink to this headline" href="#Bootup%20in%20the%20Initial%20RAM%20Disk%20(initrd)">¶</a></h2><p>The initial RAM disk implementation (initrd) can
 128                 be set up using systemd as well. In this case, boot up
 129                 inside the initrd follows the following
 130                 structure.</p><p>The default target in the initrd is
 131                 <code class="filename">initrd.target</code>. The bootup process
 132                 begins identical to the system manager bootup (see
 133                 above) until it reaches
 134                 <code class="filename">basic.target</code>. From there, systemd
 135                 approaches the special target
 136                 <code class="filename">initrd.target</code>. If the root device
 137                 can be mounted at <code class="filename">/sysroot</code>, the
 138                 <code class="filename">sysroot.mount</code> unit becomes active
 139                 and <code class="filename">initrd-root-fs.target</code> is
 140                 reached.  The service
 141                 <code class="filename">initrd-parse-etc.service</code> scans
 142                 <code class="filename">/sysroot/etc/fstab</code> for a possible
 143                 <code class="filename">/usr</code> mount point and additional
 144                 entries marked with the
 145                 <span class="emphasis"><em>x-initrd.mount</em></span> option. All
 146                 entries found are mounted below
 147                 <code class="filename">/sysroot</code>, and
 148                 <code class="filename">initrd-fs.target</code> is reached. The
 149                 service <code class="filename">initrd-cleanup.service</code>
 150                 isolates to the
 151                 <code class="filename">initrd-switch-root.target</code>, where
 152                 cleanup services can run. As the very last step, the
 153                 <code class="filename">initrd-switch-root.service</code> is
 154                 activated, which will cause the system to switch its
 155                 root to <code class="filename">/sysroot</code>.
 156                 </p><pre class="programlisting">                                               : (beginning identical to above)
 157                                                :
 158                                                v
 159                                          basic.target
 160                                                |                                 emergency.service
 161                         ______________________/|                                         |
 162                        /                       |                                         v
 163                        |                  sysroot.mount                          <span class="emphasis"><em>emergency.target</em></span>
 164                        |                       |
 165                        |                       v
 166                        |             initrd-root-fs.target
 167                        |                       |
 168                        |                       v
 169                        v            initrd-parse-etc.service
 170                 (custom initrd                 |
 171                  services...)                  v
 172                        |            (sysroot-usr.mount and
 173                        |             various mounts marked
 174                        |               with fstab option
 175                        |              x-initrd.mount...)
 176                        |                       |
 177                        |                       v
 178                        |                initrd-fs.target
 179                        \______________________ |
 180                                               \|
 181                                                v
 182                                           initrd.target
 183                                                |
 184                                                v
 185                                      initrd-cleanup.service
 186                                           isolates to
 187                                     initrd-switch-root.target
 188                                                |
 189                                                v
 190                         ______________________/|
 191                        /                       v
 192                        |        initrd-udevadm-cleanup-db.service
 193                        v                       |
 194                 (custom initrd                 |
 195                  services...)                  |
 196                        \______________________ |
 197                                               \|
 198                                                v
 199                                    initrd-switch-root.target
 200                                                |
 201                                                v
 202                                    initrd-switch-root.service
 203                                                |
 204                                                v
 205                                      Transition to Host OS</pre></div><div class="refsect1"><a name="idm214198296176"></a><h2 id="System Manager Shutdown">System Manager Shutdown<a class="headerlink" title="Permalink to this headline" href="#System%20Manager%20Shutdown">¶</a></h2><p>System shutdown with systemd also consists of
 206                 various target units with some minimal ordering
 207                 structure applied:</p><pre class="programlisting">                                  (conflicts with  (conflicts with
 208                                     all system     all file system
 209                                      services)     mounts, swaps,
 210                                          |           cryptsetup
 211                                          |          devices, ...)
 212                                          |                |
 213                                          v                v
 214                                   shutdown.target    umount.target
 215                                          |                |
 216                                          \_______   ______/
 217                                                  \ /
 218                                                   v
 219                                          (various low-level
 220                                               services)
 221                                                   |
 222                                                   v
 223                                             final.target
 224                                                   |
 225             _____________________________________/ \_________________________________
 226            /                         |                        |                      \
 227            |                         |                        |                      |
 228            v                         v                        v                      v
 229 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 230            |                         |                        |                      |
 231            v                         v                        v                      v
 232     <span class="emphasis"><em>reboot.target</em></span>             <span class="emphasis"><em>poweroff.target</em></span>            <span class="emphasis"><em>halt.target</em></span>           <span class="emphasis"><em>kexec.target</em></span></pre><p>Commonly used system shutdown targets are <span class="emphasis"><em>emphasized</em></span>.</p></div><div class="refsect1"><a name="idm214198288240"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
 233                         <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
 234                         <a href="http://man7.org/linux/man-pages/man7/boot.7.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>,
 235                         <a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>,
 236                         <a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>,
 237                         <a href="http://linux.die.net/man/8/dracut"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>
 238                 </p></div></div></body></html>