|
NAME | DESCRIPTION | SYSTEM MANAGER BOOTUP | BOOTUP IN THE INITIAL RAM DISK (INITRD) | SYSTEM MANAGER SHUTDOWN | SEE ALSO | COLOPHON |
BOOTUP(7) bootup BOOTUP(7)
bootup - System bootup process
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 dracut(8), which looks for the root
file system (possibly using systemd(1) for this). After the root file
system is found and mounted, the initrd hands over control to the
host's system manager (such as systemd(1)) stored on the OS image,
which is then responsible for probing all remaining hardware,
mounting all necessary file systems and spawning all configured
services.
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.
Additional information about the system boot process may be found in
boot(7).
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 systemd(1) systems, this
process is split up in various discrete steps which are exposed as
target units. (See systemd.target(5) 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.
When systemd starts up the system, it will activate all units that
are dependencies of default.target (as well as recursively all
dependencies of these dependencies). Usually, default.target is
simply an alias of graphical.target or multi-user.target, 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
systemd.special(7).
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.
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 rescue.target
timers.target paths.target | sockets.target
| | | |
v \_________________ | ___________________/
\|/
v
basic.target
|
____________________________________/| emergency.service
/ | | |
| | | v
v v v emergency.target
display- (various system (various system
manager.service services services)
| required for |
| graphical UIs) v
| | multi-user.target
| | |
\_________________ | _________________/
\|/
v
graphical.target
Target units that are commonly used as boot targets are emphasized.
These units are good choices as goal targets, for example by passing
them to the systemd.unit= kernel command line option (see systemd(1))
or by symlinking default.target to them.
timers.target is pulled-in by basic.target asynchronously. This
allows timers units to depend on services which become only available
later in boot.
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.
The default target in the initrd is initrd.target. The bootup process
begins identical to the system manager bootup (see above) until it
reaches basic.target. From there, systemd approaches the special
target initrd.target. 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
systemd-hibernate-resume@.service which must be finished before
local-fs-pre.target, so no filesystems can be mounted before the
check is complete. When the root device becomes available,
initd-root-device.target is reached. If the root device can be
mounted at /sysroot, the sysroot.mount unit becomes active and
initrd-root-fs.target is reached. The service
initrd-parse-etc.service scans /sysroot/etc/fstab for a possible /usr
mount point and additional entries marked with the x-initrd.mount
option. All entries found are mounted below /sysroot, and
initrd-fs.target is reached. The service initrd-cleanup.service
isolates to the initrd-switch-root.target, where cleanup services can
run. As the very last step, the initrd-switch-root.service is
activated, which will cause the system to switch its root to
/sysroot.
: (beginning identical to above)
:
v
basic.target
| emergency.service
______________________/| |
/ | v
| initrd-root-device.target emergency.target
| |
| 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
System shutdown with systemd also consists of various target units
with some minimal ordering structure applied:
(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
reboot.target poweroff.target halt.target kexec.target
Commonly used system shutdown targets are emphasized.
systemd(1), boot(7), systemd.special(7), systemd.target(5), dracut(8)
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
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you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
systemd 234 BOOTUP(7)
Pages that refer to this page: systemd(1), boot(7), dracut.bootup(7), systemd.directives(7), systemd.index(7), systemd.special(7)