man/bootup.xml

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   5
   6 <refentry id="bootup">
   7
   8   <refentryinfo>
   9     <title>bootup</title>
  10     <productname>systemd</productname>
  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
  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
  35     <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  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>
  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>
  92
  93     <!-- note: do not use unicode ellipsis here, because docbook will replace that
  94          with three dots anyway, messing up alignment -->
  95 <programlisting>local-fs-pre.target
  96          |
  97          v
  98 (various mounts and   (various swap   (various cryptsetup
  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,
 102   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
 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          |                  |                  |                    |
 119          v                  \_________________ | ___________________/
 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>
 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>)
 144     or by symlinking <filename>default.target</filename> to them.
 145     </para>
 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
 159     <para>The default target in the initrd is
 160     <filename>initrd.target</filename>. The bootup process begins
 161     identical to the system manager bootup (see above) until it
 162     reaches <filename>basic.target</filename>. From there, systemd
 163     approaches the special target <filename>initrd.target</filename>.
 164
 165     Before any file systems are mounted, it must be determined whether
 166     the system will resume from hibernation or proceed with normal boot.
 167     This is accomplished by <filename>systemd-hibernate-resume@.service</filename>
 168     which must be finished before <filename>local-fs-pre.target</filename>,
 169     so no filesystems can be mounted before the check is complete.
 170
 171     When the root device becomes available,
 172     <filename>initd-root-device.target</filename> is reached.
 173     If the root device can be mounted at
 174     <filename>/sysroot</filename>, the
 175     <filename>sysroot.mount</filename> unit becomes active and
 176     <filename>initrd-root-fs.target</filename> is reached. The service
 177     <filename>initrd-parse-etc.service</filename> scans
 178     <filename>/sysroot/etc/fstab</filename> for a possible
 179     <filename>/usr</filename> mount point and additional entries
 180     marked with the <emphasis>x-initrd.mount</emphasis> option. All
 181     entries found are mounted below <filename>/sysroot</filename>, and
 182     <filename>initrd-fs.target</filename> is reached. The service
 183     <filename>initrd-cleanup.service</filename> isolates to the
 184     <filename>initrd-switch-root.target</filename>, where cleanup
 185     services can run. As the very last step, the
 186     <filename>initrd-switch-root.service</filename> is activated,
 187     which will cause the system to switch its root to
 188     <filename>/sysroot</filename>.
 189     </para>
 190
 191 <programlisting>                                               : (beginning identical to above)
 192                                                :
 193                                                v
 194                                          basic.target
 195                                                |                                 emergency.service
 196                         ______________________/|                                         |
 197                        /                       |                                         v
 198                        |            initrd-root-device.target                    <emphasis>emergency.target</emphasis>
 199                        |                       |
 200                        |                       v
 201                        |                  sysroot.mount
 202                        |                       |
 203                        |                       v
 204                        |             initrd-root-fs.target
 205                        |                       |
 206                        |                       v
 207                        v            initrd-parse-etc.service
 208                 (custom initrd                 |
 209                  services...)                  v
 210                        |            (sysroot-usr.mount and
 211                        |             various mounts marked
 212                        |               with fstab option
 213                        |              x-initrd.mount...)
 214                        |                       |
 215                        |                       v
 216                        |                initrd-fs.target
 217                        \______________________ |
 218                                               \|
 219                                                v
 220                                           initrd.target
 221                                                |
 222                                                v
 223                                      initrd-cleanup.service
 224                                           isolates to
 225                                     initrd-switch-root.target
 226                                                |
 227                                                v
 228                         ______________________/|
 229                        /                       v
 230                        |        initrd-udevadm-cleanup-db.service
 231                        v                       |
 232                 (custom initrd                 |
 233                  services...)                  |
 234                        \______________________ |
 235                                               \|
 236                                                v
 237                                    initrd-switch-root.target
 238                                                |
 239                                                v
 240                                    initrd-switch-root.service
 241                                                |
 242                                                v
 243                                      Transition to Host OS</programlisting>
 244   </refsect1>
 245
 246   <refsect1>
 247     <title>System Manager Shutdown</title>
 248
 249     <para>System shutdown with systemd also consists of various target
 250     units with some minimal ordering structure applied:</para>
 251
 252 <programlisting>                                  (conflicts with  (conflicts with
 253                                     all system     all file system
 254                                      services)     mounts, swaps,
 255                                          |           cryptsetup
 256                                          |          devices, ...)
 257                                          |                |
 258                                          v                v
 259                                   shutdown.target    umount.target
 260                                          |                |
 261                                          \_______   ______/
 262                                                  \ /
 263                                                   v
 264                                          (various low-level
 265                                               services)
 266                                                   |
 267                                                   v
 268                                             final.target
 269                                                   |
 270             _____________________________________/ \_________________________________
 271            /                         |                        |                      \
 272            |                         |                        |                      |
 273            v                         v                        v                      v
 274 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 275            |                         |                        |                      |
 276            v                         v                        v                      v
 277     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 278
 279     <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
 280
 281     <para>Note that
 282     <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 283     <filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
 284     <filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
 285     phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
 286     remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
 287     robust fashion, without taking any service or unit concept into account anymore. At that point, regular
 288     applications and resources are generally terminated and released already, the second phase hence operates only as
 289     safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
 290     shutdown phase described above.</para>
 291   </refsect1>
 292
 293   <refsect1>
 294     <title>See Also</title>
 295     <para>
 296       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 297       <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 298       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 299       <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 301       <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 302     </para>
 303   </refsect1>
 304
 305 </refentry>