Using ALT+SYSRQ Keyboard Sequences

Press ALT+SYSRQ+H (3 keys simultaneously) and check what is printed at the server console, for example:

SysRq: unRaw Boot Sync Unmount showPc showTasks showMem loglevel0-8 tErm kIll killalL Calls Oops

This output shows you what ALT+SYSRQ sequences you may use for performing this or that command. The capital letters in the command names identify the sequence. Thus, if there are any troubles with the machine and you're about to reboot it, please press the following sequences before pressing the Power button:

ALT+SYSRQ+M to dump memory info

ALT+SYSRQ+P to dump processes states

ALT+SYSRQ+S to sync disks

ALT+SYSRQ+U to unmount filesystems

ALT+SYSRQ+L to kill all processes

ALT+SYSRQ+U try to unmount once again

ALT+SYSRQ+B to reboot

If the server is not rebooted after that, you can press the Power button.