Forget the density, just unlock your cloud's power sweet spot

Optimising densities

So, instead of investigating how to implement the highest possible densities, Schneider decided to focus on optimising densities in the light of current technology trends, enduring mixed IT environments in our data centres, and the cost of purchasing and implementing infrastructure at varying densities.

Specifically it looked at the cost per watt as density increases, and claimed that there is a steep decrease in cost per watt from 1kW per rack to 5kW per rack until the curve begins to level off.

Based on these findings, Schneider now suggests the optimal average density per rack in a practical data centre is between 5kW-8kW per rack. Lower densities are too expensive per watt, while higher delivers poor return for the added complexity.

Robert Bunger, director of data centre science and solutions at Schneider, said: “There are solid reasons for the curve. The cost of the actual racks can be a significant capital expenditure, so configuring them with very low densities or indeed anything below 5kW decreases the cost efficiency per rack."

“There is a sweet spot of efficiency from 5kW-8kW, but this again diminishes at higher densities due to the cost of wider racks, more powerful and expensive power distribution units, the costs of cooling, more intensive maintenance, and so on," he added.

"While many data centre managers have the ability to manage one or two racks with very high density if required, they are very rarely able to reach those levels as an average density," he said.

You might think that it would be extremely difficult to characterise a “well-specified density” for an efficient data centre, given that there are such disparities between brownfield and greenfield sites, different geographic locations, legacy equipment investments and many other factors.

Surely, a green-field site optimised for modern racked server blades would benefit from greater density in an ideal world? Well, not necessarily actually.

“True, conceptually high-performance computing scenarios with very high average rack densities in perfect conditions might make higher densities more attractive," Bunger told The Reg, "but the reality we are seeing is that even Chinese tech giants building multiple new premises with few restrictions are still designing data centres with average densities at around 8kW to avoid more complex infrastructure investments.”

“Our findings continue to point to a target overall density of around 5kW per rack, allowing for specific pods housing higher density equipment with dedicated power distribution to limit over-provisioning," he added.

"If there is demand for a few 10kW racks, then it is smart to consolidate those into one area commissioned with larger PDUs and more efficient cooling," he said.

As with rack density, there are also sweet spots for sensible power infrastructure investment. Schneider suggests that data centres even keep their higher density pods at below 11kW per rack to avoid the need to invest in specialist circuit breakers, larger PDUs and higher-grade cabling.