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.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.
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 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.
Note that
systemd-halt.service(8),
systemd-reboot.service, systemd-poweroff.service and
systemd-kexec.service will transition the system and server manager (PID 1) into the second
phase of system shutdown (implemented in the systemd-shutdown binary), which will unmount any
remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
robust fashion, without taking any service or unit concept into account anymore. At that point, regular
applications and resources are generally terminated and released already, the second phase hence operates only as
safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
shutdown phase described above.