SysV is the traditional way to start services in Linux was to place a script in /etc/init.d, and then use the update-rc.d command (or in RedHat based distros, chkconfig) to enable or disable it.

This command uses some mildly complicated logic to create symlinks in /etc/rc#.d, that control the order of starting services. If you run ls /etc/rc2.d you can see the order that services will be killed with a file name like K##xxxx and started with file names S##xxxx. The ## in S##xxxxmeans a “start order” for service xxxx. Conversely, the ## in K##xxxx means the kill order for service xxxx.

One major issue with SysV was that when booting the system, everything had to be done in serial, one thing after another, making system boot times really slow. Attempts were made to parallelize this, but they were haphazard and hard to take full advantage of. This was the main reason that Upstart was created.