Hot aisle/cold aisle became popular in the early 2000s. This data center best practice required lining up server racks in alternating rows with cold-air intakes all facing one aisle, hot-air exhausts the other. Over the past several years, it became the de facto standard. But as server density has increased, efficiency gains have eroded.
Hot aisle/cold aisle "looks neat in drawings, but in practice it's unmanageable," said Mukesh Khattar, the energy director at Oracle Corp., in a presentation at the SVLG event. "Up to 40% of the air doesn't do any work. Hot air goes over the tops of racks and around the rows."
Hot-aisle/cold-aisle containment systems use a physical barrier that separates the hot-or cold-aisle airflow through makeshift design solutions like vinyl plastic sheeting used in meat lockers, ducted plenum systems and other commercial offerings.
The combination of hot-aisle/cold-aisle containment and variable fan drives (VFDs) can create significant energy savings. The separation of hot and cold air can provide much better uniformity of air temperature from the top to the bottom of the rack. That uniformity of temperature enables data center pros to raise the set point temperature more safely.
William Tschudi, a project manager in the Energy Technologies Division at Lawrence Berkeley National Laboratory, has studied the effectiveness of hot-aisle/cold-aisle containment. Tschudi said that, together with VFDs, the combination can reduce fan energy use by 75%.
Higher-density server configurations can easily be handled with hot-aisle/cold-aisle isolation, and it's not terribly important how you do it, according to Tschudi. The video below is an interview with Tschudi from the SVLG event:
Companies implementing hot-aisle/cold-aisle containment
Numerous data centers employ hot-aisle/cold-aisle containment, including Sun Microsystems Inc.'s. Dean Nelson, the senior director of global data center design services, showed off a hot-aisle containment system on a recent data center tour. Storage vendor NetApp has employed vinyl curtains similar to those you see in meat lockers to contain the air in the hot aisles in its Silicon Valley data center. These curtains alone save NetApp 1 million kWh of energy per year.
Yahoo Inc. has also employed vinyl curtains in one of its data centers for airflow containment.
While the number of companies employing airflow containment strategies continues to grow, there is little agreement on whether it is better to contain the hot or cold air. According to Nelson at Sun, Kingston, R.I.-based infrastructure vendor APC promotes hot-aisle containment, while Emerson Liebert's product line supports cold aisle.
Jeremy Hartley, in technical support at U.K. rack vendor Data Racks, said cold-aisle containment is a better strategy, especially for existing data centers. According to Hartley, companies are deploying newer servers that are deeper than the racks. These servers extend out the back of the rack into the hot aisle and can reduce data center space, which is often at a premium. [See Chuck Goolsbee's blog post on the server depth issue.]
Reduced space in the hot aisle is further exacerbated by obstructions like cable ways, ducting, pipe work, etc., and the server fans are not powerful enough and the CRAC units not strong enough to pull hot air efficiently. Hartley says containing cold air is a more manageable solution and the benefit is that you have cold air only where you want it.
Are there potential downsides to containment? One concern with hot-aisle isolation is fire suppression. If you've put plastic sheets over your racks, how do sprinklers put out potential flames? Luckily, the plastic sheeting used by most companies melts at 130 degrees Fahrenheit, dropping to the floor in the event of a fire and allowing sprinklers to work.
Beyond the fire-suppression issue, hot-aisle/cold-aisle containment seems like a no-brainer and the next evolutionary step for data center cooling strategies.