BOOTUP(7) bootup BOOTUP(7)
NAME
bootup - System bootup process
DESCRIPTION
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[1]) 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, which looks for
the root file system. Nowadays this is implemented as an "initramfs" — a
compressed CPIO archive that the kernel extracts into a tmpfs. 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
non-busy file systems (detaching the storage technologies backing them),
and then (optionally) jumps into the exitrd. The exitrd is backed by
tmpfs and unmounts/detaches the remaining file systems, including the
real root. 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.
cryptsetup-pre.target veritysetup-pre.target
|
(various low-level v
API VFS mounts: (various cryptsetup/veritysetup 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 | remote-veritysetup.target |
| | services: udevd, fsck services...) | | |
| | tmpfiles, random | | | remote-fs.target
| | 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.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.
USER MANAGER STARTUP
The system manager starts the user@uid.service 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 default.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.target
BOOTUP IN THE INITRD
Systemd can be used in the initrd as well. It detects the initrd
environment by checking for the /etc/initrd-release file. The default
target in the initrd is initrd.target. The bootup process is identical
to the system manager bootup until the target basic.target. After that,
systemd executes the special target initrd.target. Before any file
systems are mounted, the manager will determine whether the system shall
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 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/
| veritysetup
| devices, ...)
| |
v v
shutdown.target umount.target
| |
\_______ ______/
\ /
v
(various low-level
services)
|
v
final.target
|
___________________________/ \_________________________________
/ | | | \
| | | | |
v | | | |
systemd-reboot.service | | | |
| v | | |
| systemd-poweroff.service | | |
v | v | |
reboot.target | systemd-halt.service | |
v | v |
poweroff.target | systemd-kexec.service |
v | |
halt.target | systemd-soft-reboot.service
v |
kexec.target |
v
soft-reboot.target
Commonly 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 could not be stopped or released for some reason during
the primary, unit-based shutdown phase described above.
THE EXITRD
The "exitrd" is a concept symmetrical to the initrd. When the system
manager is shutting down and /run/initramfs/shutdown exists, it will
switch root to /run/initramfs/ and execute /shutdown. This program runs
from the tmpfs mounted on /run/, so it can unmount the old root file
system and perform additional steps, for example dismantle complex
storage or perform additional logging about the shutdown.
SEE ALSO
systemd(1), boot(7), systemd.special(7), systemd.target(5), systemd-
halt.service(8), systemd-soft-reboot.service(8)
NOTES
1. GRUB
https://www.gnu.org/software/grub/
systemd 257.9 BOOTUP(7)
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