[Nut-upsuser] Doing things before shutdown

[Roger Price]

On Mon, 15 Nov 2021, Kari Lempiainen wrote:

> What would be the preferred way to do some operations just before UPS is about 
> to lose its power? I need to shut down some remote devices which are connected 
> to the same UPS as my Nut server. They are not "NUT compatible" so I need to 
> prepare them for losing power with some scripts. The complete shutdown of 
> these devices takes about 120-150 seconds.
> 
> My first instinct was to call those scripts from the SHUTDOWNCMD. Then I 
> realised that Nut might ask UPS to shut down before that. But this could be 
> modified by offdelay in ups.conf, right?

There are two approaches to shutting down on power failure: 1: wait until the 
UPS has reached LOWBATT status; 2: wait some number of seconds after ONBATT is 
detected.

Option 1 is simpler and is the most common.  However if you go this way, you 
take the risk of battery exhaustion before the remote devices have shut down. 
You can mitigate this by raising battery.charge.low using command upsrw if your 
UPS allows this.

> Second thought was to NOTIFYCMD and upssched.conf with upssched-cmd script. 
> And catch the LOWBATT event there and call my shutdown scripts. Then I read 
> that even that could be interrupted by system (and UPS) shutdown, before 
> letting LOWBATT event to do it's magic.

Option 2 using NOTIFYCMD is more complex to set up but will give you more 
assurance that the remote devices are shutdown correctly, particularly in the 
case of repeated power failures.

> Is it really so that there is no "safe" way to deal with notification event 
> LOWBATTERY?

Even if you use option 2, NUT is still capable of detecting [ob lb] and invoking 
it's built-in shutdown procedure.  However this would only happen after 
excessive repeated power failures.

> Will Nut start the shutdown procedure around same time when it receives "[ob 
> lb]" from UPS and it doesn't wait for LOWBATTERY event to be processed? I know 
> that in my setup UPS will have power for more than 180 seconds after reaching 
> 20% battery level.
> 
> So is the only safe option to use ONBATT event and use START-TIMER method with 
> upssched.conf?

Option 2 is probably "safer" for shutting down remote devices, when shutting 
them down takes a significant part of the UPS survival time, but since it looks 
as if you have a sufficiently large UPS, try option 1, raise battery.charge.low 
and do lots of testing.

Roger