mirror of
https://github.com/systemd/systemd.git
synced 2024-12-19 15:13:36 +08:00
b7a4734551
In cases where we used both die-net and man-pages for the same reference, I switched to use man-pages everywhere.
357 lines
20 KiB
XML
357 lines
20 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<!-- SPDX-License-Identifier: LGPL-2.1+ -->
|
|
|
|
<refentry id="bootup">
|
|
|
|
<refentryinfo>
|
|
<title>bootup</title>
|
|
<productname>systemd</productname>
|
|
</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 boot of a Linux system. Immediately after
|
|
power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot
|
|
loader (e.g.
|
|
<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
|
|
<ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This
|
|
boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types
|
|
of firmware, this firmware may also load the kernel directly.</para>
|
|
|
|
<para>The kernel (optionally) mounts an in-memory file system, often generated by
|
|
<citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
which looks for the root file system. Nowadays this is usually implemented as an initramfs — a compressed
|
|
archive which is extracted when the kernel boots up into a lightweight in-memory file system based on
|
|
tmpfs, but in the past normal file systems using an in-memory block device (ramdisk) were used, and the
|
|
name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads
|
|
both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file
|
|
system. <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may
|
|
be used to manage services in the initrd, similarly to the real system.</para>
|
|
|
|
<para>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 in
|
|
the root file system, 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>
|
|
|
|
<!-- note: do not use unicode ellipsis here, because docbook will replace that
|
|
with three dots anyway, messing up alignment -->
|
|
<programlisting> cryptsetup-pre.target
|
|
|
|
|
(various low-level v
|
|
API VFS mounts: (various cryptsetup devices...)
|
|
mqueue, configfs, | |
|
|
debugfs, ...) v |
|
|
| cryptsetup.target |
|
|
| (various swap | | remote-fs-pre.target
|
|
| devices...) | | | |
|
|
| | | | | v
|
|
| v local-fs-pre.target | | | (network file systems)
|
|
| swap.target | | v v |
|
|
| | v | remote-cryptsetup.target |
|
|
| | (various low-level (various mounts and | | |
|
|
| | services: udevd, fsck services...) | | remote-fs.target
|
|
| | tmpfiles, random | | | /
|
|
| | seed, sysctl, ...) v | | /
|
|
| | | local-fs.target | | /
|
|
| | | | | | /
|
|
\____|______|_______________ ______|___________/ | /
|
|
\ / | /
|
|
v | /
|
|
sysinit.target | /
|
|
| | /
|
|
______________________/|\_____________________ | /
|
|
/ | | | \ | /
|
|
| | | | | | /
|
|
v v | v | | /
|
|
(various (various | (various | |/
|
|
timers...) paths...) | sockets...) | |
|
|
| | | | | |
|
|
v v | v | |
|
|
timers.target paths.target | sockets.target | |
|
|
| | | | v |
|
|
v \_______ | _____/ rescue.service |
|
|
\|/ | |
|
|
v v |
|
|
basic.target <emphasis>rescue.target</emphasis> |
|
|
| |
|
|
________v____________________ |
|
|
/ | \ |
|
|
| | | |
|
|
v v v |
|
|
display- (various system (various system |
|
|
manager.service services services) |
|
|
| required for | |
|
|
| graphical UIs) v v
|
|
| | <emphasis>multi-user.target</emphasis>
|
|
emergency.service | | |
|
|
| \_____________ | _____________/
|
|
v \|/
|
|
<emphasis>emergency.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>User manager startup</title>
|
|
|
|
<para>The system manager starts the <filename>user@<replaceable>uid</replaceable>.service</filename> unit
|
|
for each user, which launches a separate unprivileged instance of <command>systemd</command> for each
|
|
user — the user manager. Similarly to the system manager, the user manager starts units which are pulled
|
|
in by <filename>default.target</filename>. The following chart is a structural overview of the well-known
|
|
user units. For non-graphical sessions, <filename>default.target</filename> is used. Whenever the user
|
|
logs into a graphical session, the login manager will start the
|
|
<filename>graphical-session.target</filename> target that is used to pull in units required for the
|
|
graphical session. A number of targets (shown on the right side) are started when specific hardware is
|
|
available to the user.</para>
|
|
|
|
<programlisting>
|
|
(various (various (various
|
|
timers...) paths...) sockets...) (sound devices)
|
|
| | | |
|
|
v v v v
|
|
timers.target paths.target sockets.target sound.target
|
|
| | |
|
|
\______________ _|_________________/ (bluetooth devices)
|
|
\ / |
|
|
V v
|
|
basic.target bluetooth.target
|
|
|
|
|
__________/ \_______ (smartcard devices)
|
|
/ \ |
|
|
| | v
|
|
| v smartcard.target
|
|
v graphical-session-pre.target
|
|
(various user services) | (printers)
|
|
| v |
|
|
| (services for the graphical session) v
|
|
| | printer.target
|
|
v v
|
|
<emphasis>default.target</emphasis> graphical-session.target</programlisting>
|
|
|
|
</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>systemd detects that it is run within an initrd by checking
|
|
for the file <filename>/etc/initrd-release</filename>.
|
|
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>.
|
|
|
|
Before any file systems are mounted, it must be determined whether
|
|
the system will resume from hibernation or proceed with normal boot.
|
|
This is accomplished by <filename>systemd-hibernate-resume@.service</filename>
|
|
which must be finished before <filename>local-fs-pre.target</filename>,
|
|
so no filesystems can be mounted before the check is complete.
|
|
|
|
When the root device becomes available,
|
|
<filename>initrd-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>
|
|
|
|
<para>Note that
|
|
<citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
|
|
<filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
|
|
phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
|
|
remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
|
|
robust fashion, without taking any service or unit concept into account anymore. At that point, regular
|
|
applications and resources are generally terminated and released already, the second phase hence operates only as
|
|
safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
|
|
shutdown phase described above.</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><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
</para>
|
|
</refsect1>
|
|
|
|
</refentry>
|