If the root filesystem is unmounted, then how can init be accessed to run the last few steps that occur afterwards?
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init is the first process to be executed after the kernel was loaded and the last to "shut down the lights" - figuratively spoken.
The kernel does not need / to be mounted to run init, as it can be run completely from memory.
"Live" distributions are running almost completely from memory and there are even some, whose cd/dvd/usbstick/floppy you can even remove from the pc after the system was loaded into memory.
Now to the question in the title of your question: yes, it is unmounted on most distributions - other remount / read-only.
More information can be found in /etc/inittab (if the system is running with sysvinit) of your system and for example on http://wiki.linuxquestions.org/wiki/Run_Levels
On systemd you can view which objects are evaluated with systemctl list-dependencies --after systemd-halt.service.
answered Jul 25 '16 at 19:10
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And distribtions take care that none of the tasks they want to do after unmounting
/require filesystem access. – Henrik Jul 25 '16 at 19:21 -
RedHat 6, for example, remounts
/readonly (/etc/rc.d/rc0.d/S01halt). – Stephen Harris Jul 25 '16 at 19:27 -
what I was trying to figure out right now: how can one tell what happens with systemd? at least on my ubuntu there is no
/etc/inittab– Phillip -Zyan K Lee- Stockmann Jul 25 '16 at 19:30 -
The
systemd-shutdownmanpage may provide guidance. In particular it mentions: It is necessary to have this code in a separate binary because otherwise rebooting after an upgrade might be broken -- the running PID 1 could still depend on libraries which are not available any more, thus keeping the file system busy, which then cannot be re-mounted read-only. – Stephen Harris Jul 25 '16 at 19:41 -
systemctl list-dependencies --after systemd-halt.servicegot me a tree of units which are run/stopped/umounted after ashutdownwas initiated. The man-page @StephenHarris mentioned led me to the explanation ofsystemd-halt.serviceand its friendssystemd-poweroff.service,systemd-reboot.serviceandsystemd-kexec.service. Which helped me with thelist-dependenciescommand. Thanks for that. – Phillip -Zyan K Lee- Stockmann Jul 25 '16 at 19:44
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PID 1 can be run from a RAM-based filesystem. For example systemd + dracut can pivot_root into a tmpfs during shutdown. dracut-shutdown.service(8) has some more information about this.
Otherwise, you remount the remaining filesystems as read-only. After remounting read-only, the filesystem is clean. It is consistent on-disk; requires no repair operations; has no remaining writes cached or queued up in RAM.
