This service unpacks the initramfs image to /run/initramfs. systemd pivots into /run/initramfs at shutdown, so the root filesystem can be safely unmounted.
The following steps are executed during a shutdown:
- systemd switches to the shutdown.target
- systemd starts /lib/systemd/system/shutdown.target.wants/dracut-shutdown.service
- dracut-shutdown.service executes /usr/lib/dracut/dracut-initramfs-restore which unpacks the initramfs to /run/initramfs
- systemd finishes shutdown.target
- systemd kills all processes
- systemd tries to unmount everything and mounts the remaining read-only
- systemd checks, if there is a /run/initramfs/shutdown executable
- if yes, it does a pivot_root to /run/initramfs and executes ./shutdown. The old root is then mounted on /oldroot. /usr/lib/dracut/modules.d/99shutdown/shutdown.sh is the shutdown executable.
- shutdown will try to umount every /oldroot mount and calls the various shutdown hooks from the dracut modules
This ensures, that all devices are disassembled and unmounted cleanly.
To debug the shutdown process, you can get a shell in the shutdown procedure by injecting "rd.break=pre-shutdown rd.shell" or "rd.break=shutdown rd.shell".
# mkdir -p /run/initramfs/etc/cmdline.d # echo "rd.break=pre-shutdown rd.shell" > /run/initramfs/etc/cmdline.d/debug.conf # touch /run/initramfs/.need_shutdown