[geeks] Need refs for the clue-impaired on redundant PSUs

[Scott Howard]

On Wed, Sep 07, 2005 at 08:03:45PM -0400, velociraptor wrote:
> Anyone have any good references out there regarding how
> redundant PSUs generally work?  I need something, preferrably
> from "experts," to give to the clue-impaired.
> 
> Some *ahem* people (I use this term loosely) are having issues
> with hitting the ceiling on their UPS and incoming power and seem
> to think removing the redundant PSUs on their existing servers will
> give them room to add more servers.

It will give you a little, but not much.  Pretty much all systems now
days use active-active power supplies. So say your system itself needs
200 watts of power, then :

With 1 power supply, you will draw (say) 220 watts, being the 200 the
system needs, and 20 watts worth of losses in the power supply
itself.

With 2 power supplies, you will draw (say) 240 watts, being the 200 the
system needs, plus 2x 20 watts of losses for the 2 power supplies. In
this case, each power supply will be drawing about 120 watts in normal
use, but if one fails then the remaining one will jump to 220 watts
(as per the one PS case).

The important thing to remember is that you need to spec it for the case
when all systems lose 1 feed (ie, if a breaker in the rack goes). I've
seen too many setup where ppl have 2x 16A feeds coming into a rack, and
each is running at 10 Amps - they think they are OK as they are well below
16A, until one feed goes and they end up trying to draw 19-20A from
the remaining one...

  Scott.