derrida writes: "Because the traditional System V init daemon (SysVinit) does not deal well with modern hardware, including hotplug devices, USB hard and flash drives, and network-mounted filesystems, Ubuntu replaced it with the upstart init daemon. Several other replacements for SysVinit are also available. One of the most prominent, initng, is available for Debian and runs on Ubuntu. Solaris uses SMF (Service Management Facility) and Mac OS uses launchd. Over time, Ubuntu will likely come to incorporate features of each of these systems into Upstart. Furthermore, heading in a different direction from its main rivals, Ubuntu Linux will use KVM as its primary virtualization software. Red Hat Enterprise Linux and Novell's Suse Linux Enterprise Server both use the Xen virtualization software, a 'hypervisor' layer that lets multiple operating systems run on the same computer. In contrast, the KVM software runs on top of a version of Linux, the 'host' operating system that provides a foundation for other 'guest' operating systems to run in a virtual mode."