<refnamediv>
<refname>bootup</refname>
- <refpurpose>The System Bootup Process</refpurpose>
+ <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 persistant
- storage device. This boot loader will then invoke an
- OS kernel from disk (or the network). In the Linux
- case this kernel now (optionally) extracts and
+ <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
<citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
- which looks for the root file system. After the root
- file system is found and mounted the initrd hands over
- control to the system manager (such as
+ 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
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 last step the system powered down.</para>
+ it resides on. As last step the system is powered down.</para>
<para>Additional information about the system boot
process may be found in
target units are available, as listed on
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
- <para>The follow chart is a structural overview of
+ <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
v
sysinit.target
|
- _________________/|\___________________
- / | \
- | | |
- v | v
- (various | rescue.service
- sockets...) | |
- | | v
- v | <emphasis>rescue.target</emphasis>
- sockets.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
basic.target
|
- _________________/| emergency.service
- / | |
- | | v
- v v <emphasis>emergency.target</emphasis>
- (various system (various system
- services services)
- required for |
- graphical UIs) v
- | <emphasis>multi-user.target</emphasis>
- | |
- | _________________/
- |/
+ ____________________________________/| 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>
+ <emphasis>graphical.target</emphasis></programlisting>
<para>Target units that are commonly used as boot
targets are <emphasis>emphasized</emphasis>. These
to them.</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>. 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
+ | sysroot.mount <emphasis>emergency.target</emphasis>
+ | |
+ | 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 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
-reboot.service poweroff.service halt.service kexec.service
- | | | |
- v v v v
-<emphasis>reboot.target</emphasis> <emphasis>poweroff.target</emphasis> <emphasis>halt.target</emphasis> <emphasis>kexec.target</emphasis></programlisting>
+ <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>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><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.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>