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 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 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
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.targetTarget 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.
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. 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
| sysroot.mount emergency.target
| |
| 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 OSSystem 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.targetCommonly used system shutdown targets are emphasized.