One thing that always confused me in Ubuntu was how system services are started. I know that Ubuntu uses Upstart and supports SysV, but which one is used to start the services? This matters when you want a "manual" start for a service.

For example, on my system i have files for the following services either in /etc/init.d/<service> (Upstart) and /etc/init/<service>.conf (SysV):

acpid, mysql, networking, qemu-kvm, ufw, libvirt-bin

So if i want to disable MySQL execution at startup, i must use the Upstart way or the SysV way to disable it? Also, how can i tell which of those is really used to start a generic service?

Edit

The really doubt here is not how disable/enable services using SysV/Upstart. What really confuses me is that some services seem to be defined (and enabled) in SysV and Upstart at the same time. Is there any precedence between them (like if mysql is enabled in both launch it using SysV)? Or can it be the case that one tool uses the other in background?