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diff --git a/man/bootup.xml b/man/bootup.xml new file mode 100644 index 0000000..5f63fe7 --- /dev/null +++ b/man/bootup.xml @@ -0,0 +1,301 @@ +<?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"> + +<!-- + 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 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> + + <!-- note: do not use unicode ellipsis here, because docbook will replace that + with three dots anyway, messing up alignment --> +<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>. + + 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>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> + + <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='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry> + </para> + </refsect1> + +</refentry> |