[Nut-upsdev] snmp-ups shutdowns

[Jim Klimov]

Hello all,

I was asked about a proper way to ensure powerfail shutdown of a blade
server fed off an APC SmartUPS, which is monitored by all hosts over
SNMP. In particular, they'd like to avoid the "untimely" return of
utility power - when the OSes are already shutting down, and would
stay down because the UPS is well-fed now and thus won't power off
and later power on to boot the servers.

On smaller systems directly connected to an UPS with a ser/usb cable,
I remember sending an ups-poweroff signal in the end of the shutdown
procedure, which effectively rebooted the UPS and its load (if the
wall power is already back). This required picking an ups model that
did not stay down in such case, of course.

Now, with the blade situation - each server might need unpredictably
long to shut down. It would be inappropriate for the first one to yank
power off those which are still going down.

Also, according to the manpage, the snmp-ups driver might not implement
the shutdown commands at all because "it is run at the time when the
networking might already be unconfigured" (strange reasoning, not all
OSes even do that unconfiguration).

So, I wonder if the list members can suggest a way to signal the
APC SmartUPS to go down and return if the power comes back or if
it is already present at the time of UPS poweroff (thus acting
as a reboot)?

Also, is it possible to request a delay in such poweroff, i.e. each
host going down would ask the UPS to poweroff in, say, 5 minutes
(so that other hosts have ample time to shut down properly) with
each such signal "restarting" the timer, or perhaps to explicitly
ask the UPS to cancel pending shutdown and then ask it to do it
again?

Can it be done with SNMP or rather some telnet/expect scripting?
There are no serial ports to connect the UPS directly to one of
the blade servers...

As an alternative, I know it is possible to hack shutdown scripts
of some OSes so that in case of an UPS-initiated shutdown the
system would not "halt" but rather sleep for a while (10-20 minutes)
and reboot, so that if the utility power is back - the servers just
restart without the UPS ever going down. At least, this is what was
suggested in the docs a decade ago and what I did on some systems.

One downside is that if there is no wall-power, the sleeping loop
just depletes the UPS battery which may be not a good thing (i.e.
if the UPS is configured to come back up after it has accumulated
some threshold value of battery charge).

Are there any new ideas in this direction? :)

Thanks for any suggestions, ideas, code/config snippets, etc. :)
//Jim Klimov