note
ph713
The Linux kernel has a compile-time option called "The Magic SysRq key" that makes SysRq actually do what it sounds like - request certain functions directly from the "system" (which is the kernel in this case).<p>
For example:
<p>
<ul>
<li>Alt-SysRq-S asks the kernel to sync all filesystems.</li>
<li>Alt-SysRq-U unmounts all filesystems (actually, remounts them all readonly)</li>
<li>Alt-SysRq-B reboots the system immediately</li>
</ul>
<p>
Many vendor-compiled kernels have the compile-time flag on, but disabled at runtime via /proc/sys. Re-enable it with "<code>echo 1 > /proc/sys/kernel/sysrq</code>", or put "<code>kernel.sysrq = 1</code>" in /etc/sysctl.conf.
<p>
Alt-SysRq-S,U,B (hold down the Alt-SysRq while you sequence through S, U, B) is a safe reboot with no fileystem damage, in cases where the system appears to be locked up, but the kernel is in fact still alive enough to see your SysRq requests.
<p>
See here: <a href="http://lxr.linux.no/source/Documentation/sysrq.txt">Linux Sysrq docs</a> for more details and all of the other sysrq functions available.
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