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[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, often generated by
dracut(8), 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 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 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.
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
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
[email protected] 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 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 |
v
kexec.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 couldn't be stopped or
released for some reason during the primary, unit-based shutdown phase
described above.
systemd(1),
boot(7),
systemd.special(7),
systemd.target(5),
systemd-halt.service(8),
dracut(8)
- 1.
- GRUB