☩ Walking in Light with Christ – Faith, Computing, Diary

Being a remote system administrator for many years from now, I’ve experienced many times sudden Linux crashes, mostly caused by system hardware overloads.

This is a common thing especially on busy Web Servers with (Apache/MySQL). Situations like this are really messy and many times in order to bringt the server back online, one has to either physically go to the Data Center or contact the Technical support to request a server reboot.

In this terrible times, data loss might occur if the Server reset button is used to cold reboot it.

Happily in 99% of the cases the data loss which might occur could be prevented with Linux’s kernel capability to support the Magic SysRQ key !

Having the Magic SysRQ key functionality being supported in Linux in past times wasn’t that common, thanksfully these days this has changed andlmost every Linux distrubution supports this handy feature.

Now you might wonder what is the magic with those Magic SysRQ key ?

Let me explain, Magic SysRQ is a kernel level functionality which supports even completely crashed Linux systems with the horrifying:

Kernel Panic
message to be properly shutdown.

Using Magic SysRQ instead of the mostly used indiced cold reboots is really advantageous, as all the opened files by programs on the crashed server which hanged will be properly saved and closed and thus possible data loss caused by the sudden server crash will be minimized.

One other good thing about the Magic SysRQ Key keyboard combination is that initiating the Magic SysRQ could be made blindly (no need for server monitor or display to present any error messages on the failed server).

Now to check if the magic sysrq is enabled on a server one should issue the command:

Here are some Magic SysRQ keyboard combinations one can use in case of server failure:

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 all mounted filesystems;
ALT+SYSRQ+E to terminate processes;
ALT+SYSRQ+I to kill all processes
ALT+SYSRQ+U to try to unmount once again;
ALT+SYSRQ+B to reboot.

I would skip to explain what each of the keyboard combinations will do as I believe the above description explains it well.

One classics of combinations one might want to issue on a failed Linux server supporting the Magic SysRQ would be:

ALT+SYSRQ+R
ALT+SYSRQ+E
ALT+SYSRQ+I
ALT+SYSRQ+S
ALT+SYSRQ+U
ALT+SYSRQ+B

The ALT+SYSRQ+REISUB key combination is very popular among system administrators.
Also Magic SysRQ is heavily used by kernel developers as it includes many handy debugging options.

If you try the Magic SysRQ key on older servers and you realize it doesn’t react you will have to recompile the linux kernel and enable the CONFIG_MAGIC_SYSRQ kernel time compilation option.

The Magic SysRQ can also be initiated remotely on a properly running server 😉 by initiating:

server:~# echo b > /proc/sysrq-trigger

This command would be useful if you want to just have fun and test that magic sysrq works on your system for sure 😉

To sum it up using the Magic SysRQ will guarantee your locked up, server a Safe Reboot and will hopefully save you a lot of time for backups recovery.

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This entry was posted on Monday, June 20th, 2011 at 7:41 pm and is filed under Linux, System Administration. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.