[systemd.git] / man / bootup.7

'\" t
.TH "BOOTUP" "7" "" "systemd 221" "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\&. 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
                       |                  sysroot\&.mount                          \fIemergency\&.target\fR
                       |                       |
                       |                       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\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1),
\fBboot\fR(7),
\fBsystemd.special\fR(7),
\fBsystemd.target\fR(5),
\fBdracut\fR(8)
Commit	Line	Data
14228c0d	1	'\" t
86f210e9	2	.TH "BOOTUP" "7" "" "systemd 221" "bootup"
14228c0d MB	3	.\" -----------------------------------------------------------------
	4	.\" * Define some portability stuff
	5	.\" -----------------------------------------------------------------
	6	.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	7	.\" http://bugs.debian.org/507673
	8	.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
	9	.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	10	.ie \n(.g .ds Aq \(aq
	11	.el .ds Aq '
	12	.\" -----------------------------------------------------------------
	13	.\" * set default formatting
	14	.\" -----------------------------------------------------------------
	15	.\" disable hyphenation
	16	.nh
	17	.\" disable justification (adjust text to left margin only)
	18	.ad l
	19	.\" -----------------------------------------------------------------
	20	.\" * MAIN CONTENT STARTS HERE *
	21	.\" -----------------------------------------------------------------
	22	.SH "NAME"
	23	bootup \- System bootup process
	24	.SH "DESCRIPTION"
	25	.PP
	26	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
	27	\fBdracut\fR(8), which looks for the root file system (possibly using
	28	\fBsystemd\fR(1)
	29	for this)\&. After the root file system is found and mounted, the initrd hands over control to the host\*(Aqs system manager (such as
	30	\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\&.
	31	.PP
	32	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\&.
	33	.PP
	34	Additional information about the system boot process may be found in
	35	\fBboot\fR(7)\&.
	36	.SH "SYSTEM MANAGER BOOTUP"
	37	.PP
	38	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
	39	\fBsystemd\fR(1)
	40	systems, this process is split up in various discrete steps which are exposed as target units\&. (See
	41	\fBsystemd.target\fR(5)
	42	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\&.
	43	.PP
	44	When systemd starts up the system, it will activate all units that are dependencies of
	45	default\&.target
	46	(as well as recursively all dependencies of these dependencies)\&. Usually,
	47	default\&.target
	48	is simply an alias of
	49	graphical\&.target
	50	or
	51	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
	52	\fBsystemd.special\fR(7)\&.
	53	.PP
	54	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\&.
	55	.sp
	56	.if n \{\
	57	.RS 4
	58	.\}
	59	.nf
	60	local\-fs\-pre\&.target
	61	\|
	62	v
	63	(various mounts and (various swap (various cryptsetup
	64	fsck services\&.\&.\&.) devices\&.\&.\&.) devices\&.\&.\&.) (various low\-level (various low\-level
	65	\| \| \| services: udevd, API VFS mounts:
	66	v v v tmpfiles, random mqueue, configfs,
67	local\-fs\&.target swap\&.target cryptsetup\&.target seed, sysctl, \&.\&.\&.) debugfs, \&.\&.\&.)
68	\| \| \| \| \|
69	\e__________________\|_________________ \| ___________________\|____________________/
70	\e\|/
71	v
72	sysinit\&.target
73	\|
74	____________________________________/\|\e________________________________________
75	/ \| \| \| \e
76	\| \| \| \| \|
77	v v \| v v
78	(various (various \| (various rescue\&.service
79	timers\&.\&.\&.) paths\&.\&.\&.) \| sockets\&.\&.\&.) \|
80	\| \| \| \| v
81	v v \| v \fIrescue\&.target\fR
82	timers\&.target paths\&.target \| sockets\&.target
83	\| \| \| \|
e3bff60a	84	v \e_________________ \| ___________________/
14228c0d MB	85	\e\|/
	86	v
	87	basic\&.target
	88	\|
	89	____________________________________/\| emergency\&.service
	90	/ \| \| \|
	91	\| \| \| v
	92	v v v \fIemergency\&.target\fR
	93	display\- (various system (various system
	94	manager\&.service services services)
	95	\| required for \|
	96	\| graphical UIs) v
	97	\| \| \fImulti\-user\&.target\fR
	98	\| \| \|
	99	\e_________________ \| _________________/
	100	\e\|/
	101	v
	102	\fIgraphical\&.target\fR
	103	.fi
	104	.if n \{\
	105	.RE
	106	.\}
	107	.PP
	108	Target units that are commonly used as boot targets are
	109	\fIemphasized\fR\&. These units are good choices as goal targets, for example by passing them to the
	110	\fIsystemd\&.unit=\fR
	111	kernel command line option (see
	112	\fBsystemd\fR(1)) or by symlinking
	113	default\&.target
	114	to them\&.
f47781d8 MP	115	.PP
	116	timers\&.target
	117	is pulled\-in by
	118	basic\&.target
	119	asynchronously\&. This allows timers units to depend on services which become only available later in boot\&.
14228c0d MB	120	.SH "BOOTUP IN THE INITIAL RAM DISK (INITRD)"
	121	.PP
	122	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\&.
	123	.PP
	124	The default target in the initrd is
	125	initrd\&.target\&. The bootup process begins identical to the system manager bootup (see above) until it reaches
	126	basic\&.target\&. From there, systemd approaches the special target
	127	initrd\&.target\&. If the root device can be mounted at
	128	/sysroot, the
	129	sysroot\&.mount
	130	unit becomes active and
	131	initrd\-root\-fs\&.target
	132	is reached\&. The service
	133	initrd\-parse\-etc\&.service
	134	scans
	135	/sysroot/etc/fstab
	136	for a possible
	137	/usr
	138	mount point and additional entries marked with the
	139	\fIx\-initrd\&.mount\fR
	140	option\&. All entries found are mounted below
	141	/sysroot, and
	142	initrd\-fs\&.target
	143	is reached\&. The service
	144	initrd\-cleanup\&.service
	145	isolates to the
	146	initrd\-switch\-root\&.target, where cleanup services can run\&. As the very last step, the
	147	initrd\-switch\-root\&.service
	148	is activated, which will cause the system to switch its root to
	149	/sysroot\&.
	150	.sp
	151	.if n \{\
	152	.RS 4
	153	.\}
	154	.nf
	155	: (beginning identical to above)
	156	:
	157	v
	158	basic\&.target
	159	\| emergency\&.service
	160	______________________/\| \|
	161	/ \| v
	162	\| sysroot\&.mount \fIemergency\&.target\fR
	163	\| \|
	164	\| v
	165	\| initrd\-root\-fs\&.target
	166	\| \|
	167	\| v
	168	v initrd\-parse\-etc\&.service
	169	(custom initrd \|
	170	services\&.\&.\&.) v
	171	\| (sysroot\-usr\&.mount and
	172	\| various mounts marked
	173	\| with fstab option
	174	\| x\-initrd\&.mount\&.\&.\&.)
	175	\| \|
	176	\| v
	177	\| initrd\-fs\&.target
	178	\e______________________ \|
	179	\e\|
	180	v
	181	initrd\&.target
	182	\|
	183	v
184	initrd\-cleanup\&.service
185	isolates to
186	initrd\-switch\-root\&.target
187	\|
188	v
189	______________________/\|
190	/ v
191	\| initrd\-udevadm\-cleanup\-db\&.service
192	v \|
193	(custom initrd \|
194	services\&.\&.\&.) \|
195	\e______________________ \|
196	\e\|
197	v
198	initrd\-switch\-root\&.target
199	\|
200	v
201	initrd\-switch\-root\&.service
202	\|
203	v
204	Transition to Host OS
205	.fi
206	.if n \{\
207	.RE
208	.\}
209	.SH "SYSTEM MANAGER SHUTDOWN"
210	.PP
211	System shutdown with systemd also consists of various target units with some minimal ordering structure applied:
212	.sp
213	.if n \{\
214	.RS 4
215	.\}
216	.nf
217	(conflicts with (conflicts with
218	all system all file system
219	services) mounts, swaps,
220	\| cryptsetup
221	\| devices, \&.\&.\&.)
222	\| \|
223	v v
224	shutdown\&.target umount\&.target
225	\| \|
226	\e_______ ______/
227	\e /
228	v
229	(various low\-level
230	services)
231	\|
232	v
233	final\&.target
234	\|
235	_____________________________________/ \e_________________________________
236	/ \| \| \e
237	\| \| \| \|
238	v v v v
239	systemd\-reboot\&.service systemd\-poweroff\&.service systemd\-halt\&.service systemd\-kexec\&.service
240	\| \| \| \|
241	v v v v
242	\fIreboot\&.target\fR \fIpoweroff\&.target\fR \fIhalt\&.target\fR \fIkexec\&.target\fR
243	.fi
244	.if n \{\
245	.RE
246	.\}
247	.PP
248	Commonly used system shutdown targets are
249	\fIemphasized\fR\&.
250	.SH "SEE ALSO"
251	.PP
252	\fBsystemd\fR(1),
253	\fBboot\fR(7),
254	\fBsystemd.special\fR(7),
255	\fBsystemd.target\fR(5),
256	\fBdracut\fR(8)