While building and testing LiveCDs, QEMU and VirtualBox can speed up development cycles… until a bug is encountered: apt-get update hanging on a rename operation. Maybe aufs is at fault? Its author then asks for some data, including what happens when some SysRq keys are pressed. What they are is already quite extensively explained in Documentation/sysrq.txt (Linux sources), but I’d like to point to a nice trick for people using emulation and virtualization tools (ever tried to send Ctrl+Alt+Delete to a virtual machine?):

# echo $letter > /proc/sysrq-trigger

where $letter is any of the supported SysRq keys. As an example:

# echo d > /proc/sysrq-trigger
# dmesg | tail -1
[558193.423427] SysRq : HELP : loglevel0-8 reBoot Crashdump tErm Full kIll
saK showMem Nice powerOff showPc show-all-timers(Q) unRaw Sync showTasks
Unmount shoW-blocked-tasks