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013d8a39 | 1 | <?xml version='1.0'?> <!--*-nxml-*--> |
3a54a157 | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
0307f791 | 4 | <!-- SPDX-License-Identifier: LGPL-2.1+ --> |
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5 | |
6 | <refentry id="bootup"> | |
7 | ||
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8 | <refentryinfo> |
9 | <title>bootup</title> | |
10 | <productname>systemd</productname> | |
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11 | </refentryinfo> |
12 | ||
13 | <refmeta> | |
14 | <refentrytitle>bootup</refentrytitle> | |
15 | <manvolnum>7</manvolnum> | |
16 | </refmeta> | |
17 | ||
18 | <refnamediv> | |
19 | <refname>bootup</refname> | |
20 | <refpurpose>System bootup process</refpurpose> | |
21 | </refnamediv> | |
22 | ||
23 | <refsect1> | |
24 | <title>Description</title> | |
25 | ||
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26 | <para>A number of different components are involved in the boot of a Linux system. Immediately after |
27 | power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot | |
28 | loader (e.g. | |
29 | <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or | |
30 | <ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This | |
31 | boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types | |
32 | of firmware, this firmware may also load the kernel directly.</para> | |
33 | ||
34 | <para>The kernel (optionally) mounts an in-memory file system, often generated by | |
798d3a52 | 35 | <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
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36 | which looks for the root file system. Nowadays this is usually implemented as an initramfs — a compressed |
37 | archive which is extracted when the kernel boots up into a lightweight in-memory file system based on | |
38 | tmpfs, but in the past normal file systems using an in-memory block device (ramdisk) were used, and the | |
39 | name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads | |
40 | both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file | |
41 | system. <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may | |
42 | be used to manage services in the initrd, similarly to the real system.</para> | |
43 | ||
44 | <para>After the root file system is found and mounted, the initrd hands over control to the host's system | |
45 | manager (such as | |
46 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) stored in | |
47 | the root file system, which is then responsible for probing all remaining hardware, mounting all | |
48 | necessary file systems and spawning all configured services.</para> | |
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49 | |
50 | <para>On shutdown, the system manager stops all services, unmounts | |
51 | all file systems (detaching the storage technologies backing | |
52 | them), and then (optionally) jumps back into the initrd code which | |
53 | unmounts/detaches the root file system and the storage it resides | |
54 | on. As a last step, the system is powered down.</para> | |
55 | ||
56 | <para>Additional information about the system boot process may be | |
57 | found in | |
58 | <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para> | |
59 | </refsect1> | |
60 | ||
61 | <refsect1> | |
62 | <title>System Manager Bootup</title> | |
63 | ||
64 | <para>At boot, the system manager on the OS image is responsible | |
65 | for initializing the required file systems, services and drivers | |
66 | that are necessary for operation of the system. On | |
67 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
68 | systems, this process is split up in various discrete steps which | |
69 | are exposed as target units. (See | |
70 | <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
71 | for detailed information about target units.) The boot-up process | |
72 | is highly parallelized so that the order in which specific target | |
73 | units are reached is not deterministic, but still adheres to a | |
74 | limited amount of ordering structure.</para> | |
75 | ||
76 | <para>When systemd starts up the system, it will activate all | |
77 | units that are dependencies of <filename>default.target</filename> | |
78 | (as well as recursively all dependencies of these dependencies). | |
79 | Usually, <filename>default.target</filename> is simply an alias of | |
80 | <filename>graphical.target</filename> or | |
81 | <filename>multi-user.target</filename>, depending on whether the | |
82 | system is configured for a graphical UI or only for a text | |
83 | console. To enforce minimal ordering between the units pulled in, | |
84 | a number of well-known target units are available, as listed on | |
85 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para> | |
86 | ||
87 | <para>The following chart is a structural overview of these | |
88 | well-known units and their position in the boot-up logic. The | |
89 | arrows describe which units are pulled in and ordered before which | |
90 | other units. Units near the top are started before units nearer to | |
91 | the bottom of the chart.</para> | |
013d8a39 | 92 | |
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93 | <!-- note: do not use unicode ellipsis here, because docbook will replace that |
94 | with three dots anyway, messing up alignment --> | |
013d8a39 | 95 | <programlisting>local-fs-pre.target |
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96 | | |
97 | v | |
013d8a39 | 98 | (various mounts and (various swap (various cryptsetup |
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99 | fsck services...) devices...) devices...) (various low-level (various low-level |
100 | | | | services: udevd, API VFS mounts: | |
101 | v v v tmpfiles, random mqueue, configfs, | |
013d8a39 | 102 | local-fs.target swap.target cryptsetup.target seed, sysctl, ...) debugfs, ...) |
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103 | | | | | | |
104 | \__________________|_________________ | ___________________|____________________/ | |
105 | \|/ | |
106 | v | |
107 | sysinit.target | |
108 | | | |
109 | ____________________________________/|\________________________________________ | |
110 | / | | | \ | |
111 | | | | | | | |
112 | v v | v v | |
113 | (various (various | (various rescue.service | |
114 | timers...) paths...) | sockets...) | | |
115 | | | | | v | |
116 | v v | v <emphasis>rescue.target</emphasis> | |
117 | timers.target paths.target | sockets.target | |
118 | | | | | | |
b44787bd | 119 | v \_________________ | ___________________/ |
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120 | \|/ |
121 | v | |
122 | basic.target | |
123 | | | |
124 | ____________________________________/| emergency.service | |
125 | / | | | | |
126 | | | | v | |
127 | v v v <emphasis>emergency.target</emphasis> | |
128 | display- (various system (various system | |
129 | manager.service services services) | |
130 | | required for | | |
131 | | graphical UIs) v | |
132 | | | <emphasis>multi-user.target</emphasis> | |
133 | | | | | |
134 | \_________________ | _________________/ | |
135 | \|/ | |
136 | v | |
137 | <emphasis>graphical.target</emphasis></programlisting> | |
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138 | |
139 | <para>Target units that are commonly used as boot targets are | |
140 | <emphasis>emphasized</emphasis>. These units are good choices as | |
141 | goal targets, for example by passing them to the | |
142 | <varname>systemd.unit=</varname> kernel command line option (see | |
143 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) | |
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144 | or by symlinking <filename>default.target</filename> to them. |
145 | </para> | |
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146 | |
147 | <para><filename>timers.target</filename> is pulled-in by | |
148 | <filename>basic.target</filename> asynchronously. This allows | |
149 | timers units to depend on services which become only available | |
150 | later in boot.</para> | |
151 | </refsect1> | |
152 | ||
153 | <refsect1> | |
154 | <title>Bootup in the Initial RAM Disk (initrd)</title> | |
155 | <para>The initial RAM disk implementation (initrd) can be set up | |
156 | using systemd as well. In this case, boot up inside the initrd | |
157 | follows the following structure.</para> | |
158 | ||
6e9d2465 JR |
159 | <para>systemd detects that it is run within an initrd by checking |
160 | for the file <filename>/etc/initrd-release</filename>. | |
161 | The default target in the initrd is | |
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162 | <filename>initrd.target</filename>. The bootup process begins |
163 | identical to the system manager bootup (see above) until it | |
164 | reaches <filename>basic.target</filename>. From there, systemd | |
165 | approaches the special target <filename>initrd.target</filename>. | |
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166 | |
167 | Before any file systems are mounted, it must be determined whether | |
168 | the system will resume from hibernation or proceed with normal boot. | |
169 | This is accomplished by <filename>systemd-hibernate-resume@.service</filename> | |
170 | which must be finished before <filename>local-fs-pre.target</filename>, | |
171 | so no filesystems can be mounted before the check is complete. | |
172 | ||
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173 | When the root device becomes available, |
174 | <filename>initd-root-device.target</filename> is reached. | |
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175 | If the root device can be mounted at |
176 | <filename>/sysroot</filename>, the | |
177 | <filename>sysroot.mount</filename> unit becomes active and | |
178 | <filename>initrd-root-fs.target</filename> is reached. The service | |
179 | <filename>initrd-parse-etc.service</filename> scans | |
180 | <filename>/sysroot/etc/fstab</filename> for a possible | |
181 | <filename>/usr</filename> mount point and additional entries | |
182 | marked with the <emphasis>x-initrd.mount</emphasis> option. All | |
183 | entries found are mounted below <filename>/sysroot</filename>, and | |
184 | <filename>initrd-fs.target</filename> is reached. The service | |
185 | <filename>initrd-cleanup.service</filename> isolates to the | |
186 | <filename>initrd-switch-root.target</filename>, where cleanup | |
187 | services can run. As the very last step, the | |
188 | <filename>initrd-switch-root.service</filename> is activated, | |
189 | which will cause the system to switch its root to | |
190 | <filename>/sysroot</filename>. | |
191 | </para> | |
192 | ||
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193 | <programlisting> : (beginning identical to above) |
194 | : | |
195 | v | |
196 | basic.target | |
197 | | emergency.service | |
198 | ______________________/| | | |
199 | / | v | |
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200 | | initrd-root-device.target <emphasis>emergency.target</emphasis> |
201 | | | | |
202 | | v | |
203 | | sysroot.mount | |
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204 | | | |
205 | | v | |
206 | | initrd-root-fs.target | |
207 | | | | |
208 | | v | |
209 | v initrd-parse-etc.service | |
210 | (custom initrd | | |
211 | services...) v | |
212 | | (sysroot-usr.mount and | |
213 | | various mounts marked | |
214 | | with fstab option | |
215 | | x-initrd.mount...) | |
216 | | | | |
217 | | v | |
218 | | initrd-fs.target | |
219 | \______________________ | | |
220 | \| | |
221 | v | |
222 | initrd.target | |
223 | | | |
224 | v | |
225 | initrd-cleanup.service | |
226 | isolates to | |
227 | initrd-switch-root.target | |
228 | | | |
229 | v | |
230 | ______________________/| | |
231 | / v | |
232 | | initrd-udevadm-cleanup-db.service | |
233 | v | | |
234 | (custom initrd | | |
235 | services...) | | |
236 | \______________________ | | |
237 | \| | |
238 | v | |
239 | initrd-switch-root.target | |
240 | | | |
241 | v | |
242 | initrd-switch-root.service | |
243 | | | |
244 | v | |
245 | Transition to Host OS</programlisting> | |
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246 | </refsect1> |
247 | ||
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248 | <refsect1> |
249 | <title>System Manager Shutdown</title> | |
250 | ||
251 | <para>System shutdown with systemd also consists of various target | |
252 | units with some minimal ordering structure applied:</para> | |
253 | ||
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254 | <programlisting> (conflicts with (conflicts with |
255 | all system all file system | |
256 | services) mounts, swaps, | |
257 | | cryptsetup | |
258 | | devices, ...) | |
259 | | | | |
260 | v v | |
261 | shutdown.target umount.target | |
262 | | | | |
263 | \_______ ______/ | |
264 | \ / | |
265 | v | |
266 | (various low-level | |
267 | services) | |
268 | | | |
269 | v | |
270 | final.target | |
271 | | | |
272 | _____________________________________/ \_________________________________ | |
273 | / | | \ | |
274 | | | | | | |
275 | v v v v | |
0e0320e0 | 276 | systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service |
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277 | | | | | |
278 | v v v v | |
279 | <emphasis>reboot.target</emphasis> <emphasis>poweroff.target</emphasis> <emphasis>halt.target</emphasis> <emphasis>kexec.target</emphasis></programlisting> | |
798d3a52 | 280 | |
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281 | <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para> |
282 | ||
283 | <para>Note that | |
39e4f5bc | 284 | <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
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285 | <filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and |
286 | <filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second | |
287 | phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any | |
288 | remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and | |
289 | robust fashion, without taking any service or unit concept into account anymore. At that point, regular | |
290 | applications and resources are generally terminated and released already, the second phase hence operates only as | |
291 | safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based | |
292 | shutdown phase described above.</para> | |
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293 | </refsect1> |
294 | ||
295 | <refsect1> | |
296 | <title>See Also</title> | |
297 | <para> | |
298 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
299 | <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, | |
300 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>, | |
301 | <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
e799056e | 302 | <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
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303 | <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
304 | </para> | |
305 | </refsect1> | |
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306 | |
307 | </refentry> |