Here's a fun new feature we are working on in systemd: userspace-only reboot. In order to reduce grey-out times on image-based OS updates to next to nothing we are making a reboot happen where kernel stays as it is, but userspace shuts down as usual, then possibly transitions into a new rootfs, and starts up again with an initial transaction as it would on a classic system boot. During the transition selected services can pass along their fds and listening sockets, to pass "live" resources…