Which UPS for two server?

Alessandro FAGLIA a.faglia at farmol.it
Thu Dec 21 06:52:33 CST 2006

Robert D. Holtz - Lists wrote:
>> My concern is about redundant power suppliers. I think that during 
>> normal operations, only one of them is actually working, and in case the 
>> first breaks, the second takes over the faulty one and stars working. 
>> What is going to happen in the switching moment? Will I have a current 
>> spike?
> The power supplies run in active/active mode, i.e., they're both feeding
> power to the system.  There is no switch period.  On failure more current
> starts flowing through the one that's still okay.

So the total absorbed power I read in data sheets refers to both 
suppliers working? Or should I double the number?

>> My question is: which is the best (in terms of available power) 
>> rack-mounting UPS that I can buy to serve these two babies?
>> If the total power is 1420W, I could go for the APC Smart-UPS 3000i, 
>> 2700 watt (probably the APC Smart-UPS 2200i, 1600 watt is enough, but 
>> the load will be tight to 100%). What about the current spike I 
>> mentioned before?
> Buy two APC rack mount units.  Model is your preference of features.  Take
> one power supply from each server and feed it from the first UPS and take
> the second power supply from each server and feed them from the second UPS.

Nice tip, but: if I do so, how am I going to drive the shut-down of 
servers if batteries are runnign out of power? How can I manage this 
aspect? In the worst case, the second UPS may fail, the first supplier 
of a server may fail, and the server finds itself without power... (this 
can be the n-th Murphy's law!)


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