[Pkg-sysvinit-devel] Bug#577146: Bug#577146: Is halt the correct action?

[Elliott Mitchell]

From: Henrique de Moraes Holschuh <hmh at debian.org>
> On Wed, 18 May 2011, Elliott Mitchell wrote:
> > When looking at bug #577146, the thought comes to mind; is halt the
> > correct action to do in case of power failure?
> 
> You will be without power soon, but you have no idea how soon.  And you
> don't have any idea for how long.
> 
> Yes, halt (with poweroff) is about the only thing you can do that is
> even remotely safe.  Fastpath shutdown would be safer, but we don't have
> it.

I'm not suggesting you don't want to get the system down, I'm wondering
whether "halt" versus "reboot" is the right target to have everything
setup for.

Two scenarios come to mind to ponder:
Power fails, everything is on UPS power. UPS power gets down to a
critical value, so the shutdown process starts. Moments later power
returns, but the shutdown process is already in progress. In this case,
you're only going to be down for moments (just long enough for the UPS to
charge some), and you've got something more akin to a reboot than a
"halt".

Second case. Power fails, everything is on UPS power, but for the moment
UPS power is holding. Just now though, a scheduled reboot occurs due to
security patches or other condition. Moments after the reboot process has
started, UPS power hits critical; suddenly you're set to be restarting,
but by the time the UPS monitor is up and running there may no longer be
enough battery to bring the system back down. Meanwhile, the scheduled
shutdown might have been a good opportunity to shutdown the UPS anyway.


I'm just pondering the situation, and ending up concluding runlevel 6 is
the right one to use for this, not runlevel 0. I guess my mental model
of "halt" is the system is going down for maintainance and won't be back
up until a *human* turns power back on. Whereas my mental model of
"reboot" is the system is going down "briefly" and will be back up as
fast as hardware (length of time for firmware memory checks, or battery
charge level) allows it to be.


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