Apple Xserve (Early 2009) - Manual (Page 16)

For information about the shutdown command and other command-line shutdown
options, enter man shutdown in Terminal or see Mac OS X Server: Command-Line
Administration at www.apple.com/server/resources.
Forcing Shutdown from the Front Panel
If you can’t shut down the Xserve using the standard methods, you can force it to
shut down by pressing the on/standby button on the front panel until the power light
turns off (approximately five seconds).
16 Chapter 2 Starting Up the Xserve