[Nut-upsuser] shutdown.return from a custom client

Jim Klimov jimklimov+nut at gmail.com
Sun Jan 1 17:25:13 GMT 2023

I wonder if you refer to
discussion from a decade ago...

Essentially, with the command you tell the UPS to cut power in X seconds
from now (some may not support it at all, some may have effective minimal
units e.g. some CPS use whole 30 or 60 second chunks - so asking to go down
in 59 sec often means immediately in fact).

This is a risky thing to request while the OS just begins a shutdown and
would take indeterminate time to stop all services and unmount filesystems.

Traditionally NUT "killpower" handling was injected into the end of
(init/rc-script driven) shutdown routine. While the drivers and upsd were
stopped as any other services, another copy of the driver is spawned in the
final seconds to tell the ups to cut power (on the primary server in upsmon

Often this part of code also deals with a "power race condition" (on all
clients) - as your systems take time to go down, wall power may return and
some UPS models may refuse to continue emergency power off. Power consumers
should wait "a lot" (more time than the UPS provides normally) and reboot.

Nowadays systemd allows shutdown hooks for similar effect, other frameworks
(e.g. SMF) might not. Anyhow these tricks may be time-limited by the
framework ("we were asked to go down, so `killall -9` after X sec").

Still, wondering what was "too convoluted" with standard upsmon? What could
be done better? Note recent ML discussions about `upssched` use that may
allow for finer-grained reaction (but that is tangentially related and may
be "convoluted" indeed).

Hope this helps,
Jim Klimov

On Sun, Jan 1, 2023, 10:41 Gennadiy Poryev via Nut-upsuser <
nut-upsuser at alioth-lists.debian.net> wrote:

> Hi all and happy new year!
> I have a server minifarm at home but it's Kyiv/Ukraine so wall power
> goes on and off unexpectedly quite a few times a day. What I want is for
> servers to gracefully start when power appears and gracefully shut down
> when it disappears.
> To that end I've got some APC Back-UPS RS 1000, set up an usbhid-ups
> driver and upsd. upsmon configuration turned out to be too convoluted so
> I decided to write my own custom solution, since the protocol is fairly
> simple.
> So the daemon I wrote connects to upsd and monitors input.voltage and
> ups.status. BTW had to override pollinterval = 1 and pollfreq = 1 in
> ups.conf to make input.voltage report input voltage in more or less
> real-time instead of cached.
> The code logic is such that as soon as input.voltage goes below
> input.transfer.low and ups.status goes from OL to OB the farm shutdown
> is initiated and ups is issued INSTCMD load.off.delay command and is
> smart enough to shut itself down too.
> So far this part of the project works OK -- the farm turns itself off
> nicely and unattended.
> BUT.
> There seem to be lack of facility to do shutdown.return though. Still
> have to to that manually each time.
> I've attached upsc/upscmd/upsrw outputs but so far haven't figured out a
> combination that might do the trick. Provided my UPS can do it, of
> course, but why shouldn't it?
>  From what I've read in the certain discussion on this maillist that
> occurred 12 years ago and from nut documentation I suspect the hope is
> not lost and it is possible to somehow hack in proper shutdown.return
> But my expertise ends here. Should anyone help me run all the debug mode
> magic I've read of and make good use of it, my thankfullness will have
> no bounds.
> Best regards,
> G.
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