NAME¶
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 (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).
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.
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.
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 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.
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 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,
| 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¶
systemd(1),
boot(7),
systemd.special(7),
systemd.target(5),
dracut(8)