#
# Reboot or Halt a Linux-based system, without any attempt 
# at shutting down *or* at syncing disks.
#
# Used sometime summer 2008, on diagsim, after messing things
# up very much (vmware? cd? disk? forgotten!).
#
# Source:
# (http://blog.air4web.com/linux-force-reboot.html)

# reboot: no sync, no delay, no fuss, no questions!
echo 1 > /proc/sys/kernel/sysrq
echo b > /proc/sysrq-trigger

# shutdown: likewise
echo 1 > /proc/sys/kernel/sysrq
echo o > /proc/sysrq-trigger


It worked...
The better-known  `reboot -f'  had presumably failed 
due to the thrashing disks, which would never have given 
it chance to get a return from `sync'.


Addendum:  the above files under proc will not necessarily
be present: they're dependent on the CONFIG_MAGIC_SYSREQ
kernel option.
See more in the 2009-10 file about magic sysreq.