SysV init had the ability to restart, allowing upgrades without rebooting. Can systemd do that?
The mechanism was kind of interesting:
SysV init would create socketpair, then fork. The parent (with PID 1, required for correct functionality) would then exec the new SysV init binary with an option specified to make it receive state over the socket. The child would supply that state, then exit.
> Reexecute the systemd manager. This will serialize the manager state, reexecute the process and deserialize the state again. This command is of little use except for debugging and package upgrades. Sometimes, it might be helpful as a heavy-weight daemon-reload. While the daemon is being reexecuted, all sockets systemd listening on behalf of user configuration will stay accessible.
The mechanism was kind of interesting:
SysV init would create socketpair, then fork. The parent (with PID 1, required for correct functionality) would then exec the new SysV init binary with an option specified to make it receive state over the socket. The child would supply that state, then exit.