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
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 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.
(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 v | v rescue.target
timers.target paths.target | sockets.target
| | | |
v \_________________ | ___________________/
/ | | |
| | | v
v v v emergency.target
display- (various system (various system
manager.service services services)
| required for |
| graphical UIs) v
| | multi-user.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
BOOTUP IN THE INITIAL RAM DISK (INITRD)
The initial RAM disk implementation (initrd) can be set up using
systemd as well. In this case, boot up inside the initrd follows the
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)
| (sysroot-usr.mount and
| various mounts marked
| with fstab option
(custom initrd |
Transition to Host OS
SYSTEM MANAGER SHUTDOWN
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,
| devices, ...)
Commonly used system shutdown targets are emphasized.
systemd(1), boot(7), systemd.special(7), systemd.target(5), dracut(8)
systemd 229 BOOTUP(7)
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