Provided by: systemd_229-4ubuntu21.31_amd64 
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. 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 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)