10 power-saving myths debunked

Is powering a server off and on bad for the system? We disprove this legend and others

Companies are finding themselves embroiled in a power crisis as they struggle to find ways to rein in soaring energy costs -- as well as do their part to address global climate change. However, how can you be certain that the power-saving strategies your company has adopted are, in fact, the best ones? After all, there are plenty of myths out there about saving energy that are patently false. In this report, we examine 10 such myths and bring the truth to light.

Myth No. 1: Powering a computer or server up and down limits its life span. The extreme temperature and current swings of power cycling can stress electronic components (especially capacitors and diodes) in a machine.

Fact: Power cycling healthy electronics is not a source of stress. The same electrical components that are used in IT equipment are used in complex devices that are routinely subjected to power cycles and temperature extremes, such as factory-floor automation, medical devices, and your car.

There is a kernel of truth in this myth, however: Cycling power on a sick system is going to bring attention to latent component weaknesses that go unnoticed in operation. Power-on diagnostics are brief yet rigorous and can be performed remotely on servers with dedicated management controllers. Power cycling doesn't just save energy. It's a zero-cost aid to maximizing server availability.

Myth No. 2: It takes too long to cold-start servers to react to spikes in demand. If customers are made to wait, they'll go elsewhere.

Fact: Idling servers at zero workload as hot spares is an egregious waste of energy and an administrative burden. If customers need to wait while you spin up cold spares to handle rising workload, brag about it. For a Web site, put up a static page asking users to wait while additional resources are brought online. As for the wait, people will stay on hold if they know their call will be answered. Build power management into your services architecture and make it part of the message that you send to users and customers.

You can also select systems that cold-boot rapidly. Model to model and brand to brand, servers exhibit wide variances in power-up delay. This metric isn't usually measured, but it becomes relevant when you control power consumption by switching off system power. It needn't take long. Servers or blades that boot from a snapshot, a copy of RAM loaded from disk or a SAN can go from power-down mode to work-ready in less than a minute. The most efficient members of a reserve/disaster farm can quiesce in a suspend-to-RAM state rather than be powered down fully so that wake-up does not require BIOS self-test or device querying and cataloging, two major sources of boot delay.

Myth No. 3: The power rating (in watts) of a CPU is a simple measurement of the system's efficiency.

Fact: Efficiency is measured in percentage of power converted, which can range from 50 to 90 percent or more. The AC power not converted to DC is lost as heat, which increases the cooling burden of the system, adding even more to the overall energy loss. Unfortunately, it's often difficult to tell the efficiency of a power supply, and many manufacturers don't publish the number. You can either look for systems with published efficiency numbers or measure the actual power draw of various systems at idle and full load, then make your decisions based on that.

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Logan G. Harbaugh

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