man/bootup.7

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   2 .TH "BOOTUP" "7" "" "systemd 217" "bootup"
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   7 .\" http://bugs.debian.org/507673
   8 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
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  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          |                  |                  |                    |
  84          \e__________________|_________________ | ___________________/
  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\&.
 115 .SH "BOOTUP IN THE INITIAL RAM DISK (INITRD)"
 116 .PP
 117 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\&.
 118 .PP
 119 The default target in the initrd is
 120 initrd\&.target\&. The bootup process begins identical to the system manager bootup (see above) until it reaches
 121 basic\&.target\&. From there, systemd approaches the special target
 122 initrd\&.target\&. If the root device can be mounted at
 123 /sysroot, the
 124 sysroot\&.mount
 125 unit becomes active and
 126 initrd\-root\-fs\&.target
 127 is reached\&. The service
 128 initrd\-parse\-etc\&.service
 129 scans
 130 /sysroot/etc/fstab
 131 for a possible
 132 /usr
 133 mount point and additional entries marked with the
 134 \fIx\-initrd\&.mount\fR
 135 option\&. All entries found are mounted below
 136 /sysroot, and
 137 initrd\-fs\&.target
 138 is reached\&. The service
 139 initrd\-cleanup\&.service
 140 isolates to the
 141 initrd\-switch\-root\&.target, where cleanup services can run\&. As the very last step, the
 142 initrd\-switch\-root\&.service
 143 is activated, which will cause the system to switch its root to
 144 /sysroot\&.
 145 .sp
 146 .if n \{\
 147 .RS 4
 148 .\}
 149 .nf
 150                                                : (beginning identical to above)
 151                                                :
 152                                                v
 153                                          basic\&.target
 154                                                |                                 emergency\&.service
 155                         ______________________/|                                         |
 156                        /                       |                                         v
 157                        |                  sysroot\&.mount                          \fIemergency\&.target\fR
 158                        |                       |
 159                        |                       v
 160                        |             initrd\-root\-fs\&.target
 161                        |                       |
 162                        |                       v
 163                        v            initrd\-parse\-etc\&.service
 164                 (custom initrd                 |
 165                  services\&.\&.\&.)                  v
 166                        |            (sysroot\-usr\&.mount and
 167                        |             various mounts marked
 168                        |               with fstab option
 169                        |              x\-initrd\&.mount\&.\&.\&.)
 170                        |                       |
 171                        |                       v
 172                        |                initrd\-fs\&.target
 173                        \e______________________ |
 174                                               \e|
 175                                                v
 176                                           initrd\&.target
 177                                                |
 178                                                v
 179                                      initrd\-cleanup\&.service
 180                                           isolates to
 181                                     initrd\-switch\-root\&.target
 182                                                |
 183                                                v
 184                         ______________________/|
 185                        /                       v
 186                        |        initrd\-udevadm\-cleanup\-db\&.service
 187                        v                       |
 188                 (custom initrd                 |
 189                  services\&.\&.\&.)                  |
 190                        \e______________________ |
 191                                               \e|
 192                                                v
 193                                    initrd\-switch\-root\&.target
 194                                                |
 195                                                v
 196                                    initrd\-switch\-root\&.service
 197                                                |
 198                                                v
 199                                      Transition to Host OS
 200 .fi
 201 .if n \{\
 202 .RE
 203 .\}
 204 .SH "SYSTEM MANAGER SHUTDOWN"
 205 .PP
 206 System shutdown with systemd also consists of various target units with some minimal ordering structure applied:
 207 .sp
 208 .if n \{\
 209 .RS 4
 210 .\}
 211 .nf
 212                                   (conflicts with  (conflicts with
 213                                     all system     all file system
 214                                      services)     mounts, swaps,
 215                                          |           cryptsetup
 216                                          |          devices, \&.\&.\&.)
 217                                          |                |
 218                                          v                v
 219                                   shutdown\&.target    umount\&.target
 220                                          |                |
 221                                          \e_______   ______/
 222                                                  \e /
 223                                                   v
 224                                          (various low\-level
 225                                               services)
 226                                                   |
 227                                                   v
 228                                             final\&.target
 229                                                   |
 230             _____________________________________/ \e_________________________________
 231            /                         |                        |                      \e
 232            |                         |                        |                      |
 233            v                         v                        v                      v
 234 systemd\-reboot\&.service   systemd\-poweroff\&.service   systemd\-halt\&.service   systemd\-kexec\&.service
 235            |                         |                        |                      |
 236            v                         v                        v                      v
 237     \fIreboot\&.target\fR             \fIpoweroff\&.target\fR            \fIhalt\&.target\fR           \fIkexec\&.target\fR
 238 .fi
 239 .if n \{\
 240 .RE
 241 .\}
 242 .PP
 243 Commonly used system shutdown targets are
 244 \fIemphasized\fR\&.
 245 .SH "SEE ALSO"
 246 .PP
 247 \fBsystemd\fR(1),
 248 \fBboot\fR(7),
 249 \fBsystemd.special\fR(7),
 250 \fBsystemd.target\fR(5),
 251 \fBdracut\fR(8)