Magic SysRq Key

You may come across the situation that your Linux server is hung or freeze and your system is not responding even  for Ctrl+Alt+Del in console itself and you must need to do a hard reboot by pressing reset button. As everyone know, the hard reboots is not good and can crash the File systems. so what to do now?

There is a way in Linux,

Hold down the Right Alt and SysRq keys and press this sequence:

  R E I S U B

This will cleanly unmount the drives, terminate the processes and nicely reboot your machine.

of course, To get this worked, you need to “enable” this feature on the running kernel first !

On 2.6 kernel

echo 1 > /proc/sys/kernel/sysrq

This will do the trick.

In Some distributions, you may have a way to enable this feature at boot time. 

On Fedora and RHEL, edit the file /etc/sysctl.conf, and change the line kernel.sysrq = 0 to kernel.sysrq = 1

 

Detailed document 

The magic SysRq key is a key combination understood by the Linux kernel, which allows the user to perform various low level commands regardless of the system's state. It is often used to recover from freezes, or to reboot a computer without corrupting the file system.

To be able to use this functionality the CONFIG_MAGIC_SYSRQ option has to be enabled at kernel compile time.

Linux is very stable, but sometimes a kernel panic could happen, sometimes this stops the X-server and you cant change to the console. What should be done? Hitting the reset button and risk file system integrity?
NO! There is a possibility to shut down the system cleanly or find out the source of the kernel panic. For this purpose there is a kernel option called "Magic SysRQ Key" in the section kernel hacking.
If this option is enabled you can use a set of keyboard commands.
Alt+SysRq+r takes keyboard and mouse control from the X server. This can be useful if the X-Server crashed, you can change to a console and kill the X-Server or check the error log.
Alt+SysRQ+k kills all processes on the current terminal. Its a bad idea to do this on a console where X is running. The graphic will stop and you cant see what you type.
Alt+SysRQ+b is like a reset: a reboot without umounting or sync.
Alt+SysRQ+o shuts down via APM.
Alt+SysRQ+s writes all data from the disc cache to the harddiscs, its a sync.
Alt+SysRQ+u remounts all mounted filesystem readonly. After using this key, you can reboot the system with Alt+SysRQ+b without harming the system.
Alt+SysRQ+m prints memory information to the console.
Alt+SysRQ+e sends SIGTERM to all processes except init.
Alt+SysRQ+i sends SIGKILL to all processes except init.
Alt+SysRQ+l sends SIGKILL to all processes, inclusive init. (The system is not working after using this.)...
To shut down the system after a really bad kernel panic, do the following:
Alt+SysRQ+e (sends TERM-signal, processes can shutdown properly (e.g. save data))
Alt+SysRQ+u (a sync will be done when unmounting anyway)
Alt+SysRQ+i (for the processes that didn't listen for the TERM signal, this is a kill -9 process)
Alt+SysRQ+b (reboot).

To be able to use the SysRq feature, you need enable

echo "1" > /proc/sys/kernel/sysrq

or add an entry to /etc/sysctl.conf:

kernel.sysrq = 1

By the way, its a good idea to print this out and store near your PC, when it happens, you can't check back here...
Ref: http://en.wikipedia.org/wiki/Magic_SysRq_key