Lennart Pottering talked about preparations for adding a soft-reboot mode ("systemctl soft-reboot") to the systemd system manager, which causes only user-space components to be restarted without touching the Linux kernel. Compared to a normal reboot, a soft reboot is expected to reduce downtime when updating environments that use prebuilt system images.
The new mode will allow you to shut down all processes in user space, then replace the root FS image with a new version and start the system initialization process without rebooting the kernel. In addition, saving the state of the running kernel when replacing the user environment will make it possible to update some services in live mode by organizing the transfer of file descriptors and listening network sockets for these services from the old environment to the new one. Thus, it will be possible to significantly reduce the time to replace one version of the system with another and ensure a seamless transfer of resources to the most important services that will continue to work without interruption.
Restart speed is achieved by eliminating such relatively long stages as hardware initialization, bootloader operation, kernel startup, driver initialization, firmware loading, and initrd processing. To update the kernel in combination with a soft reboot, it is suggested to use the livepatch mechanism to patch a running Linux kernel without a full reboot and without shutting down applications.
Source: opennet.ru