Rack Cooling: Within the Rack
One of the biggest issues in any high-density server environment is keeping the equipment at safe operating temperatures. This issue grows even larger when the equipment spreads from a small number of racks into the much larger numbers commonly seen in data centers. There are two main areas to be addressed here: the issue of cooling within each individual rack, and the facility-level issue of cooling the floor space the racks occupy. Let's look at cooling within the individual racks first; we'll save the facility-level issue for a future article.
Cooling within a rack starts with the rack itself. It's important to have a rack that allows for good front-to-rear ventilation. That means you want a rack with perforated doors - or no doors at all - and not a rack with solid steel or glass doors. Some racks will also have fans mounted on the top that may help depending on the type of installed equipment in the rack. Once the rack has been selected, the equipment should be laid out properly within the rack. It is advisable to install larger and longer equipment near the bottom of the rack and leave the top area for the smallest devices. This helps to ensure that the upper-level equipment does not trap a pocket of hot air within the central area of the rack where it is most difficult to remove. Next, ensure that all your equipment is setup for the proper front-to-rear airflow pattern. This is usually not an issue with commercially manufactured servers, but it's a common problem with servers that are assembled from individual components by the end user. Some network devices - notably Cisco ASA firewalls - use a "backwards" airflow pattern and should be mounted backwards in the rack to allow the proper airflow. You can think of it as "cold in the front, hot out the back" airflow.
Do you still see equipment running a little too warm? Especially near the top of the rack? There are two possible solutions to this problem: First, if the problem is only near the top of the rack one of the top-mounted exhaust fan options may help. Second, if you have a lot of unused space in the rack it may help to use blank rack plates to fill any unused spaces in the front of the cabinet. Open spaces between servers can sometimes allow for hot air from the rear of the rack to recirculate into the front of the rack resulting in the equipment running hotter than it should. The old trick of leaving a space between each installed server frequently results in hotter-running servers than if they were all installed together in a solid block!
In the back of the rack, it's important to ensure that the cabling does not block the airflow any more than necessary. We generally recommend to our customers that they NOT use the cable management arms that some manufacturers offer for their servers. While these arms make the servers easy to slide out for maintenance, they also severely restrict the airflow in the back of the racks resulting in much less airflow for the servers they support. Cables should be routed from the back of each server to the side of the rack and then vertically along the side of the rack.
Properly handling cooling within a rack well help to ensure your equipment operates reliably over a long period of time. In the next part of this two-part article we'll discuss some of the facility-level cooling issues involved with groups of multiple racks.