Hung Linux system: Now what?

Finally, I had remembered to try out Magic SysRq key in real life. One of the XenServer hosts decided to refuse to reboot and hung during shutdown. Magic SysRq is really a keyboard combo that allows user to force kernel to perform certain function, even though machine is unresponsive. In my case I needed to safely reboot the host.

For Magic SysRq to work, it must be enabled in kernel:

[root@db0004 ~]# cat /proc/sys/kernel/sysrq

If output is 1, then you are good to go. Thankfully, XenServer had this enabled. So, while holding down Alt and SysRq keys I pressed the following keys: r e i s u b.

So, what do all the letters mean? I ripped the explanation from Wikipedia. Article is worth checking out, especially the section on how to deal with keyboards other than QWERTY.

The Wikipedia article also mentions couple of mnemonics to help remembering the key sequence.

Posted on October 30, 2013 at 09:04 by somedude · Permalink
In: centos, linux, linux tips, redhat

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