[mythtv-users] Power outages and UPSs

[David Brodbeck]

On Jul 27, 2007, at 6:43 AM, Preston Crow wrote:
> Mostly, how do you insure that the system will power back up once the
> power is restored?  You can configure the BIOS to power on when  
> power is
> applied, but that's not good enough.
>
> *) What if the system is in the shutdown process when the power comes
> back on?  It will complete the shutdown, and it will stay off.

Most UPSs have a way for the computer to command the UPS to shut off  
the power.  NUT and APC's own UPS software both take advantage of  
this.  A good UPS will honor this even if line power has come back  
on, by turning off its output for a few seconds and then powering  
back on.  This usually eliminates the problem.  The rack-mount APC  
SmartUPS units I use at work are set up this way.

With a cheap UPS, things get trickier.  One option is to make the  
computer kill all processes and sync its disks, wait for several  
minutes, then reboot.  (If power doesn't come back, it never gets to  
the "...then reboot" part.)  NUT can be configured this way.  It can  
cause problems with some hardware RAID arrays, though, so be careful.

>
> *) What if the battery is nearly dead, the power comes back on, but  
> goes
> back out after the system has started booting?  You might not have
> enough power left in the battery to power down completely.

High-end UPSs usually have a feature that will cause the UPS to delay  
powering back up until the battery reaches a certain charge level.   
For example, at work our rack-mount UPSs will not switch back on  
until the batteries have a 15% charge.  This also helps protect  
equipment from the surges that can happen when power is initially  
restored.

>
> What is really needed is to tell the UPS that you're done, and have it
> cut power until it is recharged.  Can any consumer-grade UPSs do that?

Most of these features are only found in higher-end UPSs, like APC's  
"SmartUPS" line.  You have to decide, for your application, if it's  
worth the extra expense to be sure you won't have to go poke the  
RESET button if power comes on at the wrong moment.