Re:I want to have an AWS region where everything breaks with...
by kl4m on Tuesday August 11, 2020 @10:59AM
(#24120967)
echo o > /proc/sysrq-trigger
will shutdown a AWS instance immediately. I've used it a couple of times to make real sure my costly spot instance was terminated.
https://www.kernel.org/doc/html/v4.11/admin-guide/sysrq.html...
Re:I want to have an AWS region where everything breaks with...
by derefr on Tuesday August 11, 2020 @11:27AM
(#24121325)
https://elixir.bootlin.com/linux/latest/source/kernel/power/...
Looks like SysRq-o does a clean poweroff, not a dirty/immediate one — it calls kernel_power_off(), not machine_power_off().
This means, importantly, that can actually take some time to happen, as drivers get a chance to run deinitialization code. This also means that a wedged driver that doesn't respond during deinit can prevent the kernel from halting.
Thus, while SysRq-o might be useful for killing a wedged userland, it's not a panacea — especially, it isn't guaranteed to complete a shutdown for unstable kernels, or kernels with badly-written DKMS drivers attached. It's not truly equivalent to a power-cut.
A panicking instance is still "alive" from its hypervisor's perspective (either it'll hang, sleep, or reboot, but it won't usually turn off its vCPU in a way the hypervisor would register as "the instance is now off"); while if a hypervisor box suffers a power cut, the rest of the compute cluster knows that the instances on that node are now very certainly off.