[mythtv-users] Power outages and UPSs

Brian Wood beww at beww.org
Fri Jul 27 13:55:27 UTC 2007

Preston Crow wrote:
> On Thu, 2007-07-26 at 21:13 -0700, DaveD wrote:
>> You can get away with the cheapest 
>> version available if you just have the CPU plugged into it with a 
>> 2-minute unconditional shutdown (system goes down clean if power not 
>> back within two minutes).  That's how I have mine  configured, but 
>> they're smart enough to calculate percent of battery left and wait
>> until 
>> the last minute to tell the machine to shut itself down (default 
>> configuration).
> I have several problems that I haven't figured out a solution to yet
> concerning using a UPS.
> 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.
> *) 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.
> 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?

For the APC units have a look at apcupsd. Just about everything you have
mentioned is configurable, though I agree your last *) point is perhaps
valid. Obviously every system has some vulnerabilities, and systems that
are not fully up and "aware" are potential victims.

Since you can specify at what battery level the shutdown begins, and you
should know how long that till take, you should be able to ensure that a
shutdown always leaves sufficient battery power for a full bootup, but I
just used the word "should" twice, and any engineer knows that is risky.

A consumer-type UPS VASTLY increases the reliability of a Myth system,
it does not turn it into a commercial-grade high-reliability system.


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