Power Math

Greenness aside, its difficult to develop truly dense computing when you’re limited to x amount of power in a cabinet or cage.  Many data centers talk about engineering x watts per square foot.  With air cooling, 450-550 watts per square foot is supposedly the max.  One data center claims 1500 watts per square foot using air cooling.  Power converts to heat, more power = more heat.  While a facility may have the electrical capacity, most are engineering for the cooling capacity.

4800 watts is the limit at a particular data center for 1 cabinet.  Remember watts and amps and volts?

Watts = Volts * Amps

120Volt 20Amp service = 2400 Watts.

208Volt 20Amp service = 4160 Watts.

So, if you have a cabinet with 2 120V@20A circuits, and consider that dual quadcore Xeon servers come with 500 Watt power supplies, one cabinet with ten 500 watt machines exceeds the power available in a cabinet.  Exceeding 80% of the circuit is considered bad form, some companies specify a 75% maximum.

Supermicro Blade servers are slightly more efficient with 10 dual quadcore Xeon servers using 2000 watts, leaving room for support equipment.  The same blade chassis uses 1500 watts if you use 208V.

Yes, I know a server doesn’t use the max wattage all the time, but, the higher the CPU utilization, the closer to that max you get.  Startup power drain also approaches that max.  There are power strips that do staggered power-on, set max amp draw with shutoff, etc.  These aren’t problems you run into with the breadrack and minitower data centers because you just plug machines in until the circuit blows, then, remove the last one.

How much power does a device use:

  • 10/100/1000 48 port switch with 4 fiber uplinks, 45 watts (peak .4 amps)
  • P4/3.0ghz, 2gb RAM, 320gb SATA Hard Drive, 73.2 watts (peak 1.2 amps)
  • Core2Duo E6600, 2gb RAM, 320gb SATA Hard drive, 114.8 watts (peak 1.0 amps)
  • Core2Quad Q6600 64bit mode, 2gb RAM, 320gb SATA Hard drive x 2, 103.8 watts (peak 1.2 amps)
  • Core2Quad Q6600 32bit mode, 2gb RAM, 320gb SATA, 110.9 watts (peak 1.0 amps)

So, even though most of these machines have 260-320 watt power supplies, their in-use wattage is less than half the stated max which allows a little more density.  So, without that metering, if you were to spec everything based on the books, you’d be seriously underutilizing your racks.

Raritan and APC both make Power Strips that allow you to monitor power with the Raritan being a little more pricey, but, allowing metering down to the individual outlet.

Granted, you still need to plan ahead, without knowing the true power use and efficiency of your power supplies, you could severely overbudget or underbudget your power needs.

What does my day have in store?  Doing a ton more power math for our next expansion.

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