The Mac Mini doesn’t save this information in its BIOS (or equivalent), so you need to set it on each boot. I use my distribution’s /etc/rc.local file for this.
Either way, these are the commands to run (copy, and uncomment the correct line for your Mini):
## PPC Mac Mini #echo server_mode=1 > /proc/pmu/options ## Intel Mac Mini #setpci -s 0:1f.0 0xa4.b=0 ## nVidia Mac Mini #setpci -s 00:03.0 0x7b.b=0x19 ## Unibody Mac Mini #setpci -s 0:3.0 -0x7b=20