With Red Hat Enterprise Linux 7 released and CentOS version 7 newly unveiled, now is a good time to cover systemd, the replacement for legacy System V (SysV) startup scripts and runlevels.
Red Hat-based distributions are migrating to systemd because it provides more efficient ways of managing services and quicker startup times. With systemd there are fewer files to edit, and all the services are compartmentalized and stand separate from each other. This means that should you screw up one config file, it won’t automatically take out other services. Systemd has been the default system and services manager in Red Hat Fedora since the release of Fedora 15, so it is extensively field-tested. It provides more consistency and troubleshooting ability than SysV – for instance, it will report if a service has failed, is suspended, or is in error. Perhaps the biggest reason for the move to systemd is that it allows multiple services to start up at the same time, in parallel, making machine boot times quicker than they would be with legacy runlevels.