Provided by: systemd_229-4ubuntu4_amd64 bug


       bootup - System bootup process


       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 dracut(8),
       which looks for the root file system (possibly using systemd(1) for this). 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 on the OS image, which is then responsible for probing
       all remaining hardware, mounting all necessary file systems and spawning all configured

       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 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

       When systemd starts up the system, it will activate all units that are dependencies of (as well as recursively all dependencies of these dependencies). Usually, is simply an alias of or, 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

           (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,
       seed, sysctl, ...)      debugfs, ...)
                    |                  |                  |                    |                    |
                    \__________________|_________________ | ___________________|____________________/
                    /                  |                  |                    |                    \
                    |                  |                  |                    |                    |
                    v                  v                  |                    v                    v
                (various           (various               |                (various          rescue.service
               timers...)          paths...)              |               sockets...)               |
                    |                  |                  |                    |                    v
                    v                  v                  |                    v    
                    |                  |                  |                    |
                    v                  \_________________ | ___________________/
                     ____________________________________/|                                 emergency.service
                    /                  |                  |                                         |
                    |                  |                  |                                         v
                    v                  v                  v                       
                display-        (various system    (various system
            manager.service         services           services)
                    |             required for            |
                    |            graphical UIs)           v
                    |                  | 
                    |                  |                  |
                    \_________________ | _________________/

       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 to them. is pulled-in by asynchronously. This allows timers units to
       depend on services which become only available later in boot.


       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.

       The default target in the initrd is The bootup process begins identical to
       the system manager bootup (see above) until it reaches From there, systemd
       approaches the special target If the root device can be mounted at
       /sysroot, the sysroot.mount unit becomes active and 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 is reached. The service
       initrd-cleanup.service isolates to the, 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)
                                                          |                                 emergency.service
                                   ______________________/|                                         |
                                  /                       |                                         v
                                  |                  sysroot.mount                
                                  |                       |
                                  |                       v
                                  |                       |
                                  |                       v
                                  v            initrd-parse-etc.service
                           (custom initrd                 |
                            services...)                  v
                                  |            (sysroot-usr.mount and
                                  |             various mounts marked
                                  |               with fstab option
                                  |              x-initrd.mount...)
                                  |                       |
                                  |                       v
                                  \______________________ |
                                                     isolates to
                                  /                       v
                                  |        initrd-udevadm-cleanup-db.service
                                  v                       |
                           (custom initrd                 |
                            services...)                  |
                                  \______________________ |
                                                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
                                                    |          devices, ...)
                                                    |                |
                                                    v                v
                                                    |                |
                                                    \_______   ______/
                                                            \ /
                                                    (various low-level
                       _____________________________________/ \_________________________________
                      /                         |                        |                      \
                      |                         |                        |                      |
                      v                         v                        v                      v
           systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
                      |                         |                        |                      |
                      v                         v                        v                      v

       Commonly used system shutdown targets are emphasized.


       systemd(1), boot(7), systemd.special(7),, dracut(8)