[Baylisa] Q: Estimating DC Power

Wed Jun 4 15:10:23 PDT 2014

Hello,

I have inherited an environment and some nice ServerTech PDUs.  I've run
stress tests on some servers and measured the power draw through the PDUs.
In this way I can come up with a value, in Watts, of what a server will
draw at peak.  I can then figure out how many of what size circuits I need
to power a given rack . . .

When doing the measurements, I'll observe, say, the *average* peak draw
averaging out to say, 4.4A, but with *spikes* recorded at 5A, &c.  (The
peak draw in that case being 88% of the highest measurement..)

We are running Hadoop.  So, most of the machines in a given rack will hit
peak load around the same time.  This is somewhat different from other
environments I have managed with more variable workloads across a rack.
So, I feel more pressure to Get This Right. :)

As best I can tell, the Commonly Accepted Best Practice is that you aspire
to not exceed 80% capacity on any given circuit.  But then Management wants
to know why.  And my answer is:
1) Estimating power consumption is not a precise science
2) Power fluctuates, you want some room for error
3) Getting it wrong means blowing a fuse on the PDU, losing a rack, and a
prolonged outage...
4) ... But, hey, I honestly don't understand electricity ...

Back to 1: I figure that for the past century, if a guy is standing there
watching a power meter, he's going to see the meter fluctuating around a
certain high value under peak draw.  The little needle will jump higher and
lower, but where the needle rests is his 80% of circuit capacity.

So, I have competing strategies:
1) Most Conservative: Take the *max* momentary measurement observed as your
peak power consumption.  That's your 80% baseline.
2) Take the *average* peak consumption as your peak consumption.  That's
your 80% baseline.
OR...
3) Take the max momentary measurement as your 100% of circuit capacity, or
90% ... as long as your average peak doesn't exceed 80% of circuit capacity
...

I am of course comfortable with Most Conservative, but I'm not the one
writing the checks, and I'd rather not spend money we don't need to spend
... how do you folks estimate power needs?

Thanks,
-danny

-- 
http://dannyman.toldme.com