Editing Magic SysRq Key
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| − | + | It is a 'magical' key combo you can hit which the kernel will respond to | |
regardless of whatever else it is doing, unless it is completely locked up. | regardless of whatever else it is doing, unless it is completely locked up. | ||
| − | + | When running a kernel with SysRq compiled in (that's right in openVZ case), | |
| − | + | <tt>/proc/sys/kernel/sysrq</tt> controls the functions allowed to be invoked via the SysRq key. | |
| − | When running a kernel with SysRq compiled in ( | ||
| − | <tt>/proc/sys/kernel/sysrq</tt> | ||
Here is the list of possible values in <tt>/proc/sys/kernel/sysrq</tt>:<br> | Here is the list of possible values in <tt>/proc/sys/kernel/sysrq</tt>:<br> | ||
| − | + | <tt>0</tt> - disable sysrq completely<br> | |
| − | + | <tt>1</tt> - enable all functions of sysrq<br> | |
| − | + | ><tt>1</tt> - bitmask of allowed sysrq functions<br> | |
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You can set the value in the file by the following command: | You can set the value in the file by the following command: | ||
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</pre> | </pre> | ||
| − | + | To use SysRq feature you should press the key combo 'ALT-SysRq-<command key>'. | |
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| − | To use SysRq feature you should press the key combo ' | ||
The 'SysRq' key is also known as the 'Print Screen' key. Other way, if you don't have | The 'SysRq' key is also known as the 'Print Screen' key. Other way, if you don't have | ||
physical access to the node, only ssh, you can do | physical access to the node, only ssh, you can do | ||
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Here is the list of most useful command keys: | Here is the list of most useful command keys: | ||
| − | + | <tt>b</tt> - Will immediately reboot the system without syncing or unmounting | |
| − | + | your disks.<br> | |
| − | + | <tt>s</tt> - Will attempt to sync all mounted filesystems.<br> | |
| − | + | <tt>u</tt> - Will attempt to remount all mounted filesystems read-only.<br> | |
| − | + | <tt>p</tt> - Will dump the current registers and flags to your console.<br> | |
| − | + | <tt>t</tt> - Will dump a list of current tasks and their information to your | |
| − | + | console.<br> | |
| − | + | <tt>m</tt> - Will dump current memory info to your console.<br> | |
| − | + | <tt>0</tt>-<tt>9</tt> - Sets the console log level, controlling which kernel messages | |
| − | + | will be printed to your console. ('0', for example would make | |
| − | + | it so that only emergency messages like PANICs or OOPSes would | |
| − | + | make it to your console.)<br> | |
| − | + | <tt>f</tt> - Will call oom_kill to kill a memory hog process<br> | |
| − | + | <tt>e</tt> - Send a SIGTERM to all processes, except for init.<br> | |
| − | + | <tt>i</tt> - Send a SIGKILL to all processes, except for init.<br> | |
| − | + | <tt>h</tt> - Will display help<br> | |
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| − | + | All ouptput will be on console or in <tt>/var/log/messages</tt> | |
| − | Suppose for some strange reason your system is working slow and you want to | + | Example of using SysRq. Suppose for some strange reason your system is working slow and you want to |
ask developers what happens, then don't forget to press | ask developers what happens, then don't forget to press | ||
| − | <tt> | + | <tt>alt-sysrq-p</tt> and <tt>alt-sysrq-t</tt> several times, |
| − | and attach <tt>/var/log/messages</tt> to | + | and attach <tt>/var/log/messages</tt> to request. |
[[Category: Troubleshooting]] | [[Category: Troubleshooting]] | ||
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