bootup
systemd
Developer
Lennart
Poettering
lennart@poettering.net
bootup
7
bootup
System bootup process
Description
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 now (optionally) extracts and
executes an initial RAM disk image (initrd) such as
dracut8
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
systemd1)
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 last step the system is powered down.
Additional information about the system boot
process may be found in
boot7.
System Manager Bootup
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
systemd1
systems this process is split up in various discrete
steps which are exposed as target units. (See
systemd.target5
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.special7.
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
(various | rescue.service
sockets...) | |
| | v
v | rescue.target
sockets.target |
| |
\_________________ |
\|
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
systemd1)
or by symlinking default.target
to them.
System Manager Shutdown
System shutdown 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.
See Also
systemd1,
boot7,
systemd.special7,
systemd.target5