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 now (optionally) extracts and executes an initial RAM disk image (initrd) such as dracut(8) which looks for the root file system. After the root file system is found and mounted the initrd hands over control to the 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 follow 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 (various | rescue.service sockets...) | | | | v v | rescue.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.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.
System shutdown 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.