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'\" t
.TH "BOOTUP" "7" "" "systemd 241" "bootup"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
bootup \- System bootup process
.SH "DESCRIPTION"
.PP
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
\fBdracut\fR(8), which looks for the root file system (possibly using
\fBsystemd\fR(1)
for this)\&. After the root file system is found and mounted, the initrd hands over control to the host\*(Aqs system manager (such as
\fBsystemd\fR(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\&.
.PP
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\&.
.PP
Additional information about the system boot process may be found in
\fBboot\fR(7)\&.
.SH "SYSTEM MANAGER BOOTUP"
.PP
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
\fBsystemd\fR(1)
systems, this process is split up in various discrete steps which are exposed as target units\&. (See
\fBsystemd.target\fR(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\&.
.PP
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
\fBsystemd.special\fR(7)\&.
.PP
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\&.
.sp
.if n \{\
.RS 4
.\}
.nf
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, \&.\&.\&.)
| | | | |
\e__________________|_________________ | ___________________|____________________/
\e|/
v
sysinit\&.target
|
____________________________________/|\e________________________________________
/ | | | \e
| | | | |
v v | v v
(various (various | (various rescue\&.service
timers\&.\&.\&.) paths\&.\&.\&.) | sockets\&.\&.\&.) |
| | | | v
v v | v \fIrescue\&.target\fR
timers\&.target paths\&.target | sockets\&.target
| | | |
v \e_________________ | ___________________/
\e|/
v
basic\&.target
|
____________________________________/| emergency\&.service
/ | | |
| | | v
v v v \fIemergency\&.target\fR
display\- (various system (various system
manager\&.service services services)
| required for |
| graphical UIs) v
| | \fImulti\-user\&.target\fR
| | |
\e_________________ | _________________/
\e|/
v
\fIgraphical\&.target\fR
.fi
.if n \{\
.RE
.\}
.PP
Target units that are commonly used as boot targets are
\fIemphasized\fR\&. These units are good choices as goal targets, for example by passing them to the
\fIsystemd\&.unit=\fR
kernel command line option (see
\fBsystemd\fR(1)) or by symlinking
default\&.target
to them\&.
.PP
timers\&.target
is pulled\-in by
basic\&.target
asynchronously\&. This allows timers units to depend on services which become only available later in boot\&.
.SH "BOOTUP IN THE INITIAL RAM DISK (INITRD)"
.PP
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\&.
.PP
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,
initd\-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
\fIx\-initrd\&.mount\fR
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\&.
.sp
.if n \{\
.RS 4
.\}
.nf
: (beginning identical to above)
:
v
basic\&.target
| emergency\&.service
______________________/| |
/ | v
| initrd\-root\-device\&.target \fIemergency\&.target\fR
| |
| 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
\e______________________ |
\e|
v
initrd\&.target
|
v
initrd\-cleanup\&.service
isolates to
initrd\-switch\-root\&.target
|
v
______________________/|
/ v
| initrd\-udevadm\-cleanup\-db\&.service
v |
(custom initrd |
services\&.\&.\&.) |
\e______________________ |
\e|
v
initrd\-switch\-root\&.target
|
v
initrd\-switch\-root\&.service
|
v
Transition to Host OS
.fi
.if n \{\
.RE
.\}
.SH "SYSTEM MANAGER SHUTDOWN"
.PP
System shutdown with systemd also consists of various target units with some minimal ordering structure applied:
.sp
.if n \{\
.RS 4
.\}
.nf
(conflicts with (conflicts with
all system all file system
services) mounts, swaps,
| cryptsetup
| devices, \&.\&.\&.)
| |
v v
shutdown\&.target umount\&.target
| |
\e_______ ______/
\e /
v
(various low\-level
services)
|
v
final\&.target
|
_____________________________________/ \e_________________________________
/ | | \e
| | | |
v v v v
systemd\-reboot\&.service systemd\-poweroff\&.service systemd\-halt\&.service systemd\-kexec\&.service
| | | |
v v v v
\fIreboot\&.target\fR \fIpoweroff\&.target\fR \fIhalt\&.target\fR \fIkexec\&.target\fR
.fi
.if n \{\
.RE
.\}
.PP
Commonly used system shutdown targets are
\fIemphasized\fR\&.
.PP
Note that
\fBsystemd-halt.service\fR(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\*(Aqt be stopped or released for some reason during the primary, unit\-based shutdown phase described above\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1),
\fBboot\fR(7),
\fBsystemd.special\fR(7),
\fBsystemd.target\fR(5),
\fBsystemd-halt.service\fR(8),
\fBdracut\fR(8)
|
