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Green Datacenter » ExpertSpeak
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The Whys & Hows of Measuring Datacenter Power - II |
| By CXOtoday Staff, Jan 21, 2010 1416 hrs IST |
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What to Do with the Data Gathered
Depending on the measurement locations and method of measurement chosen, various energy efficiency initiatives may be taken. Individual outlet-level metering is recommended for IT equipment because it provides useful, actionable information.
Monitoring the power consumed at a rack allows data center managers to determine if their original power allocations make sense today. Quite often power is allocated to IT equipment on the basis of nameplate ratings which are conservatively high. Even when a percentage, say, 70 percent, of nameplate power is used, power is often over-allocated. This means more power is going to an IT equipment rack than what will actually be consumed. This "stranded power" could be deployed elsewhere, but how do you know you're not leaving the rack vulnerable to running out of power in a peak load situation?
Monitor each individual device at regular intervals, the shorter the better, to ensure that no peak periods are overlooked. With individual device power consumption figure, it is possible to set up racks such that equipment power consumption patterns compliment each other and thus more IT equipment can be supported with the same amount of power. If a rack is close to consuming all the power allocated to it, and therefore at risk of tripping a breaker, having individual IT equipment power consumption data allows IT staff to remove equipment in a logical manner so as to minimize the risk of a breaker tripping while maintaining useful loading levels.
Through tests in its own data center, Raritan determined that rules-of-thumb percentages of nameplate ratings simply don't work. Across 59 servers, 15 had average power consumption of 20% or less, 29 had 21 to 40%, 9 had 41 to 60%, 4 had 61 to 80% and 2 had 81% or more. Even at peak power consumption, 49 of the servers were 60% or less of their nameplate rating. Many data center planners use 70% of nameplate, which means there is a lot of stranded power in many data centers.
On the other hand, at peak power consumption, 5 of the 59 servers were at 81% or more of nameplate and therefore at risk of shutting down. The message is that in terms of power consumption, it is important to know what is going on at the individual device, not some aggregated average which may mask problems both on the high and low side.
Environmental Sensors: Their Impact on Power and Cooling Efficiency
Environmental sensors make an important contribution to power efficiency. It is common for cooling to consume 30% or more of a data center's total power. IT equipment vendors provide inlet temperature specifications. As long as the inlet temperature is within the specification the server will perform fine. These specifications are often substantially higher than what is typically provided in data center cold aisles. Thus, the temperature can often be turned up, which leads to less power consumption by the cooling equipment.
Temperature sensors should be placed at the bottom, middle and top third of racks on the cool air inlet side. Cooling IT equipment to temperatures lower than required consumes a lot of power without any beneficial effects. Due to a lack of at-the-rack instrumentation, data center managers often overcool to be confident IT equipment won't fail.
New Technologies Available
Taking an individual snapshot of power consumption at one point in time is not sufficient. IT devices may consume a lot less power at 2 a.m. than they do at 8 a.m. and may hit peak power consumption at 4 p.m. on Thursday. Power consumption can also vary by time of year such as online sales during December.
There are hardware devices that can take snapshots of power consumption at user-defined intervals as often as once every few seconds. Software programs are available to turn these data points into calculations of power usage where the unit of measure is kilowatt hours (kwh). Sophisticated tools can calculate carbon footprints based on energy usage. With actual individual device information data center staff can know the biggest contributors to carbon generation and therefore what needs to be most closely managed.
What to Look For In Power Measurement
Accurate: As carbon caps, credits and trading are adopted, accuracy becomes important. +/- 5% accuracy, assuming perfect sine waves which rarely occur in the real world, may be acceptable to determine if a rack is operating with about a 25% margin before circuit breakers trip. It is not acceptable when dealing with regulations and carbon credits to be verified and traded on exchanges. Nor is it accurate enough for billing or charge backs.
Open and interoperable: Many data centers have deployed an IT management system. To tie such a system to power measurement, look for open standards for integration and interoperability with existing equipment. Ease of use is a key consideration so power management does not become a time-consuming project for already busy IT staff.
Secure: Power is the life blood of data centers. It is important that access to the power management system be secure. Look for systems with high levels of encryption such as 256-bit AES and the ability to set authentication, authorization and permissions.
We hope that if the CIO calls you tomorrow and asks, "What are we doing about power consumption in our data centers?" you'll refer to this article and outline a plan beginning with a program to gather information to establish some baselines. Collecting data now, and taking a stab at data center metrics such as a PUE calculation, will put you on a path to more efficiently manage power and power costs .And to take calls from CIOs.
(Herman Chan is the director for Raritan's Power Solutions Business Unit, and Greg More is the senior product marketing manager for the same unit.)
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