This morning, I’ve been reading about using input from server temperature sensors to adjust cooling system response. I think this idea goes right some ways and wrong others.
On the plus side, you can never have too much data about what’s happening to the data center environmental conditions. Using the sensors already built in to the servers makes perfect sense, as does the use of standard SNMP to collect the information. That using this input to attempt to make fine control responses to compensate for inefficient cooling systems is the most effective response to the cooling problems listed is questionable.
The first point they raise is hot spots at the top of cabinets in a raised floor cooling environment. In raised floor cooling, the cool air is pumped under the raised floor and up to the rack. This is counterproductive, since it requires forcing the air against the natural tendency to sink. Feed the cold air from above and not only do you eliminate the uneven cooling, you also spend less energy pumping the air around, no heroic measures required.
The next point is that they assume an environment with no hot aisle containment, hence the worry about excess ventilation driving the hot exhaust air up and over the top of the cabinet and back into the cold aisle.
If you’re going to use hot aisle/cold aisle, it’s an exercise in imperfection to attempt to separate hot and cold without a physical containment. It’s a total waste of effort and lots of unnecessary complication to control this by varying fan speeds. The easy and effective answer is to use a hot aisle containment system or, as I’ve said before, use chimney type cabinets and duct the hot exhaust air for perfect control and separation.
Finally, presuming you have efficient delivery and exhaust that works WITH the natural movement of the air and you maintain an appropriate reserve to supply the servers. the servers themselves will take care of adjusting their own air needs. This assumes that you’re running modern servers that are capable of adjusting their own fan speeds. This means that the cabinet fans don’t need to be speed adjusted at all, since they will never put through more than the total airflow that the servers are putting through.
So, what does this all mean? Over complexity is no substitute for dumping an obsolete cooling design for an efficient one. It’s as simple as using the natural movement of the air to move itself where it’s needed and an easy, fool proof, way to separate hot and cold.
Sometimes it really IS just that simple.
Vern, SwiftWater Telecom
data center, web hosting, Internet engineering