Stop data loss even on system crash? Remember BUSIER

[Richard Daggett]

Howdy

 YOu would need to check your kernel settings to see if this is enabled.

 cat /proc/sys/kernel/sysr1 - RHEL 5 it is not enabled by default

Here are more items you can do with it:

<Alt><SysRq> 
(Non-essential.) This is a group of key combinations implemented at the Linux kernel level (a low level). It means, chances are these key combinations will work most of the time. The combinations are meant for debugging purposes and in an emergency (mostly developers); you should try other, safer solutions first. The key <SysRq> is also knows on PC as <PrintScreen>. The combinations can be enabled/disabled by setting the relevant kernel variable to "1" or "0", e.g. : echo "1" > /proc/sys/kernel/sysrq 
<Alt><SysRq><k> Kill all processes (including X) which are running on the currently active virtual console. This key combination is know as "secure access key" (SAK). 
<Alt><SysRq><e> Send the TERM signal to all running processes except init, asking them to exit. 
<Alt><SysRq><i> Send the KILL signal to all running processes except init. This may be more successful in killing runaway processes than the previous key combination, but it may cause some of them to exit abnormally. 
<Alt><SysRq><l> Send the KILL signal to all processes, including init. The system will not be functional. 
<Alt><SysRq><s> Run an emergency sync (cache write) on all mounted filesystems. This can prevent data loss. 
<Alt><SysRq><u> Remount all mounted filesystems as read-only. This has the same effect as the sync combination above, but with one important benefit: if the operation is successful, fsck won't have to check all filesystems after a computer hardware reset. 
<Alt><SysRq><r> Turn off keyboard raw mode. This can be useful when your X session hangs. After issueing this command you may be able to use <Ctrl><Alt><Del>. 
<Alt><SysRq><b> Reboot immediately without syncing or unmounting your disks. Your will likely end up with filesystem errors. 
<Alt><SysRq><o> Shut the system off (if configured and supported). 
<Alt><SysRq><p> Dump the current registers and flags to your console. 
<Alt><SysRq><t> Dump a list of current tasks and their information to your console. 
<Alt><SysRq><m> Dump memory info to your console. 
<Alt>SysRq><digit> The digit is '0' to '9'. Set the console log level, controlling which kernel messages will be printed to your console. For example, '0' will cause only emergency messages like PANICs or OOPSes displayed on your console. 
<Alt><SysRq><h> Display help. Also, any other unsupported <Alt><SysRq><key> combination will display the same help.

Richard

--- On Mon, 6/29/09, Ryan Rix <phrkonaleash at gmail.com> wrote:

> From: Ryan Rix <phrkonaleash at gmail.com>
> Subject: Stop data loss even on system crash? Remember BUSIER
> To: "Main PLUG discussion list" <plug-discuss at lists.plug.phoenix.az.us>
> Date: Monday, June 29, 2009, 8:40 AM
> Hey,
> 
> Ever wonder what the SysRq key does on your computer? Most
> people have no idea 
> that this can be one of the biggest lifesavers in a sticky
> situation.
> 
> When your system hardlocks, either from a nasty process, a
> bug in X, or for 
> some other reason, you risk losing gobs of data if you have
> to take your 
> system offline. Journaling filesystems alleviate this issue
> to some extent but 
> data loss is always a huge risk when taking a system down
> hard.
> 
> But, one simple word can help you take this risk to
> virtually 0 the next time 
> you must take a system down hard (holding the power button
> for 5 seconds): 
> BUSIER - or backwards REISUB, more accurately
> 
> R - resets the keyboard driver to raw mode
> E - Sends term to all running processes
> I - sends kill signal to all processes
> S - Syncs all mounted partitions
> U - unmount the filesystems and remount as read only
> B - reboot
> 
> Pressing alt-SysRq and the keys above will send a command
> to the kernel (by 
> passing X, applications and other things which may be
> causing the hardlock. 
> This simple method is failsafe and will work in all
> situations except 
> situations in which the kernel is busy, rather than an
> application. In that 
> situation there is little you can do but hope and pray that
> the Journal holds.
> 
> Ryan.
> 
>  
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