bootup — System bootup process
A number of different components are involved in the boot of a Linux system. Immediately after power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot loader (e.g. systemd-boot(7) or GRUB) stored on a persistent storage device. This boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types of firmware, this firmware may also load the kernel directly.
The kernel (optionally) mounts an in-memory file system, often generated by dracut(8), which looks for the root file system. Nowadays this is usually implemented as an initramfs — a compressed archive which is extracted when the kernel boots up into a lightweight in-memory file system based on tmpfs, but in the past normal file systems using an in-memory block device (ramdisk) were used, and the name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file system. systemd(1) may be used to manage services in the initrd, similarly to the real system.
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 in the root file system, 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.
cryptsetup-pre.target
|
(various low-level v
API VFS mounts: (various cryptsetup devices...)
mqueue, configfs, | |
debugfs, ...) v |
| cryptsetup.target |
| (various swap | | remote-fs-pre.target
| devices...) | | | |
| | | | | v
| v local-fs-pre.target | | | (network file systems)
| swap.target | | v v |
| | v | remote-cryptsetup.target |
| | (various low-level (various mounts and | | |
| | services: udevd, fsck services...) | | remote-fs.target
| | tmpfiles, random | | | /
| | seed, sysctl, ...) v | | /
| | | local-fs.target | | /
| | | | | | /
\____|______|_______________ ______|___________/ | /
\ / | /
v | /
sysinit.target | /
| | /
______________________/|\_____________________ | /
/ | | | \ | /
| | | | | | /
v v | v | | /
(various (various | (various | |/
timers...) paths...) | sockets...) | |
| | | | | |
v v | v | |
timers.target paths.target | sockets.target | |
| | | | v |
v \_______ | _____/ rescue.service |
\|/ | |
v v |
basic.target rescue.target |
| |
________v____________________ |
/ | \ |
| | | |
v v v |
display- (various system (various system |
manager.service services services) |
| required for | |
| graphical UIs) v v
| | multi-user.target
emergency.service | | |
| \_____________ | _____________/
v \|/
emergency.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 system manager starts the user@ unit
for each user, which launches a separate unprivileged instance of systemd for each
user — the user manager. Similarly to the system manager, the user manager starts units which are pulled
in by uid.servicedefault.target. The following chart is a structural overview of the well-known
user units. For non-graphical sessions, default.target is used. Whenever the user
logs into a graphical session, the login manager will start the
graphical-session.target target that is used to pull in units required for the
graphical session. A number of targets (shown on the right side) are started when specific hardware is
available to the user.
(various (various (various
timers...) paths...) sockets...) (sound devices)
| | | |
v v v v
timers.target paths.target sockets.target sound.target
| | |
\______________ _|_________________/ (bluetooth devices)
\ / |
V v
basic.target bluetooth.target
|
__________/ \_______ (smartcard devices)
/ \ |
| | v
| v smartcard.target
v graphical-session-pre.target
(various user services) | (printers)
| v |
| (services for the graphical session) v
| | printer.target
v v
default.target graphical-session.targetThe 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.
systemd detects that it is run within an initrd by checking
for the file /etc/initrd-release.
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,
initrd-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.