summaryrefslogtreecommitdiff
path: root/src/grp-system/bootup.xml
blob: 986996398cc4049fe832b7a6eb3da2a67a3d07c4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2012 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

<refentry id="bootup">

  <refentryinfo>
    <title>bootup</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>bootup</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>bootup</refname>
    <refpurpose>System bootup process</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Description</title>

    <para>A number of different components are involved in the system
    boot. Immediately after power-up, the system BIOS will do minimal
    hardware initialization, and hand control over to a boot loader
    stored on a persistent storage device. This boot loader will then
    invoke an OS kernel from disk (or the network). In the Linux case,
    this kernel (optionally) extracts and executes an initial RAM disk
    image (initrd), such as generated by
    <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
    which looks for the root file system (possibly using
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    for this). After the root file system is found and mounted, the
    initrd hands over control to the host's system manager (such as
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
    stored on the OS image, which is then responsible for probing all
    remaining hardware, mounting all necessary file systems and
    spawning all configured services.</para>

    <para>On shutdown, the system manager stops all services, unmounts
    all file systems (detaching the storage technologies backing
    them), and then (optionally) jumps back into the initrd code which
    unmounts/detaches the root file system and the storage it resides
    on. As a last step, the system is powered down.</para>

    <para>Additional information about the system boot process may be
    found in
    <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>System Manager Bootup</title>

    <para>At boot, the system manager on the OS image is responsible
    for initializing the required file systems, services and drivers
    that are necessary for operation of the system. On
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    systems, this process is split up in various discrete steps which
    are exposed as target units. (See
    <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for detailed information about target units.) The boot-up process
    is highly parallelized so that the order in which specific target
    units are reached is not deterministic, but still adheres to a
    limited amount of ordering structure.</para>

    <para>When systemd starts up the system, it will activate all
    units that are dependencies of <filename>default.target</filename>
    (as well as recursively all dependencies of these dependencies).
    Usually, <filename>default.target</filename> is simply an alias of
    <filename>graphical.target</filename> or
    <filename>multi-user.target</filename>, depending on whether the
    system is configured for a graphical UI or only for a text
    console. To enforce minimal ordering between the units pulled in,
    a number of well-known target units are available, as listed on
    <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

    <para>The following chart is a structural overview of these
    well-known units and their position in the boot-up logic. The
    arrows describe which units are pulled in and ordered before which
    other units. Units near the top are started before units nearer to
    the bottom of the chart.</para>

<programlisting>local-fs-pre.target
         |
         v
(various mounts and   (various swap   (various cryptsetup
 fsck services...)     devices...)        devices...)       (various low-level   (various low-level
         |                  |                  |             services: udevd,     API VFS mounts:
         v                  v                  v             tmpfiles, random     mqueue, configfs,
  local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
         |                  |                  |                    |                    |
         \__________________|_________________ | ___________________|____________________/
                                              \|/
                                               v
                                        sysinit.target
                                               |
          ____________________________________/|\________________________________________
         /                  |                  |                    |                    \
         |                  |                  |                    |                    |
         v                  v                  |                    v                    v
     (various           (various               |                (various          rescue.service
    timers...)          paths...)              |               sockets...)               |
         |                  |                  |                    |                    v
         v                  v                  |                    v              <emphasis>rescue.target</emphasis>
   timers.target      paths.target             |             sockets.target
         |                  |                  |                    |
         v                  \_________________ | ___________________/
                                              \|/
                                               v
                                         basic.target
                                               |
          ____________________________________/|                                 emergency.service
         /                  |                  |                                         |
         |                  |                  |                                         v
         v                  v                  v                                 <emphasis>emergency.target</emphasis>
     display-        (various system    (various system
 manager.service         services           services)
         |             required for            |
         |            graphical UIs)           v
         |                  |           <emphasis>multi-user.target</emphasis>
         |                  |                  |
         \_________________ | _________________/
                           \|/
                            v
                  <emphasis>graphical.target</emphasis></programlisting>

    <para>Target units that are commonly used as boot targets are
    <emphasis>emphasized</emphasis>. These units are good choices as
    goal targets, for example by passing them to the
    <varname>systemd.unit=</varname> kernel command line option (see
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
    or by symlinking <filename>default.target</filename> to them.
    </para>

    <para><filename>timers.target</filename> is pulled-in by
    <filename>basic.target</filename> asynchronously. This allows
    timers units to depend on services which become only available
    later in boot.</para>
  </refsect1>

  <refsect1>
    <title>Bootup in the Initial RAM Disk (initrd)</title>
    <para>The initial RAM disk implementation (initrd) can be set up
    using systemd as well. In this case, boot up inside the initrd
    follows the following structure.</para>

    <para>The default target in the initrd is
    <filename>initrd.target</filename>. The bootup process begins
    identical to the system manager bootup (see above) until it
    reaches <filename>basic.target</filename>. From there, systemd
    approaches the special target <filename>initrd.target</filename>.
    When the root device becomes available,
    <filename>initd-root-device.target</filename> is reached.
    If the root device can be mounted at
    <filename>/sysroot</filename>, the
    <filename>sysroot.mount</filename> unit becomes active and
    <filename>initrd-root-fs.target</filename> is reached. The service
    <filename>initrd-parse-etc.service</filename> scans
    <filename>/sysroot/etc/fstab</filename> for a possible
    <filename>/usr</filename> mount point and additional entries
    marked with the <emphasis>x-initrd.mount</emphasis> option. All
    entries found are mounted below <filename>/sysroot</filename>, and
    <filename>initrd-fs.target</filename> is reached. The service
    <filename>initrd-cleanup.service</filename> isolates to the
    <filename>initrd-switch-root.target</filename>, where cleanup
    services can run. As the very last step, the
    <filename>initrd-switch-root.service</filename> is activated,
    which will cause the system to switch its root to
    <filename>/sysroot</filename>.
    </para>

<programlisting>                                               : (beginning identical to above)
                                               :
                                               v
                                         basic.target
                                               |                                 emergency.service
                        ______________________/|                                         |
                       /                       |                                         v
                       |            initrd-root-device.target                    <emphasis>emergency.target</emphasis>
                       |                       |
                       |                       v
                       |                  sysroot.mount
                       |                       |
                       |                       v
                       |             initrd-root-fs.target
                       |                       |
                       |                       v
                       v            initrd-parse-etc.service
                (custom initrd                 |
                 services...)                  v
                       |            (sysroot-usr.mount and
                       |             various mounts marked
                       |               with fstab option
                       |              x-initrd.mount...)
                       |                       |
                       |                       v
                       |                initrd-fs.target
                       \______________________ |
                                              \|
                                               v
                                          initrd.target
                                               |
                                               v
                                     initrd-cleanup.service
                                          isolates to
                                    initrd-switch-root.target
                                               |
                                               v
                        ______________________/|
                       /                       v
                       |        initrd-udevadm-cleanup-db.service
                       v                       |
                (custom initrd                 |
                 services...)                  |
                       \______________________ |
                                              \|
                                               v
                                   initrd-switch-root.target
                                               |
                                               v
                                   initrd-switch-root.service
                                               |
                                               v
                                     Transition to Host OS</programlisting>
  </refsect1>

  <refsect1>
    <title>System Manager Shutdown</title>

    <para>System shutdown with systemd also consists of various target
    units with some minimal ordering structure applied:</para>

<programlisting>                                  (conflicts with  (conflicts with
                                    all system     all file system
                                     services)     mounts, swaps,
                                         |           cryptsetup
                                         |          devices, ...)
                                         |                |
                                         v                v
                                  shutdown.target    umount.target
                                         |                |
                                         \_______   ______/
                                                 \ /
                                                  v
                                         (various low-level
                                              services)
                                                  |
                                                  v
                                            final.target
                                                  |
            _____________________________________/ \_________________________________
           /                         |                        |                      \
           |                         |                        |                      |
           v                         v                        v                      v
systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
           |                         |                        |                      |
           v                         v                        v                      v
    <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>

    <para>Commonly used system shutdown targets are
    <emphasis>emphasized</emphasis>.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>