We followed the advice of whomever recommended the Kill-A-Watt tool, and learned that basically a very fast dual-core opteron running 4 BG RAM, 8 SCSI hd's at 15K spin on LSI-Logic RAID ran at 1.7 amps. Up from the 1.2 and 1.4 (conservative) estimate I had made.
You plug the kill-a-watt into the outlet, it has a plug for your server cable. It gives a read in digital or what amperage you draw. 20.00 USD. Handy.
Much better than unshielding a cable to expose it's neutral or positive wire (scary) and using a "clamp ammeter", which may not even work at all because 1 amp is so tiny and they usually tackle 300 amp situations.
Spreading power load across multiple circuits is, and will be the name of the game for a while.
On a 20 amp circuit, the most i'm willing to allow is 12, so it can spike to 16 under heavy load, and only get
to 18 if the math is slightly off. When you add another circuit into the equation, you effectively get to divide by 2. Just enough to offset any bad math.
It was interesting to see it spike up to 1.7. When reading the stickers on the power supply unit, can we safely conclude that the +5V = 0 - 2 is actually saying "this power supply goes between 0 and 2 amps". There are alot of numbers and symbols, but this is the one relevant to our discussion right?
Anyone care to explain the +5V and -5V stuff?
Thanks alot for the advice so far, helped massively.
-karl
Dan Pritts <danno@xxxxxxxxxxxxx> wrote:
Dan Pritts <danno@xxxxxxxxxxxxx> wrote:
On Tue, Aug 01, 2006 at 08:03:58PM -0700, Mark Schoonover wrote:
> get watts. I think a better way is to determine the max current that the
> system will use. If you have a 500 watt PSU, assume 85% efficiency, then by
> using the input voltage you can calculate the max input power the server is
> going to draw. I'd design the datacenter to support the max power level the
> server will need, not just what it takes to run the thing. Start up will
> draw the most power, approaching the max power output of the PSU, then it'll
> lower some once all the drives are up and spinning.
Using this method, you will probably overbuild for power.
Power supplies in good servers are much higher-capacity than is necessary
for the typical application.
Regarding drive spin-up, i think a reasonable number for spin-up of a
15k drive is 30 watts (i couldn't quickly find a spec for spin-up power,
but ~10 watts idle is 'typical' for seagate 15k drives). Three of
those per server is 90 watts. During spin-up time, the CPU(s) will be
idle so will be drawing a lot less power than they would at full load,
so you have a bit of savings there.
Depending on your application, overprovisioning your power might not
be a bad thing - next year's servers will probably use more.
> It's somewhat an art, but you can get reasonably close to the entire power
> draw of your rack. In a pinch, I've added up all the UPSes and told the
> electrician 12000VA. They'll know what to do with that number.
This is good advice.
The "kill a watt" meters referenced in another post are a great
suggestion too.
danno
--
dan pritts - systems administrator - internet2
734/352-4953 office 734/834-7224 mobile
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