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Really Force Reboot

I've had to do this when the ZFS kernel module has a problem that was preventing shutdown/reboot commands from completing because they try and do so in a tidy way. For those situations there is the following...

When the "reboot" or "shutdown" commands are executed daemons are gracefully stopped and storage volumes unmounted. This is usually accomplished via scripts in the /etc/init.d directory which will wait for each daemon to shut down gracefully before proceeding on to the next one. This is where a situation can develop where your Linux server fails to shutdown cleanly leaving you unable to administer the system until it is inspected locally. This is obviously not ideal so the answer is to force a reboot on the system where you can guarantee that the system will power cycle and come back up. The method will not unmount file systems nor sync delayed disk writes, so use this at your own discretion.

echo 1 > /proc/sys/kernel/sysrq

Then to reboot the machine simply enter the following:

echo b > /proc/sysrq-trigger