Whether we're designing, building, or maintaining data centers, we need to pay attention to a lot more than how and where the bits flow. We also need to know about the physical environment. We need to keep all the gear at a reasonable temperature, and we need to protect it from all kinds of possible mayhem -- from unauthorized access to burst pipes.
Environmental monitoring is one of the elements of data center construction and maintenance that seems to be missed at many levels. In numerous cases, it may be that data center design consultants were brought in for the overall design and then left, resulting in a data center that has been well equipped with fire suppression and cooling systems, but has no remote telemetry or monitoring systems. In other cases, it may be because the server crew is monitoring the servers, the network staff is monitoring the network, the storage group is monitoring the storage, but nobody's keeping tabs on the room itself.
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The first order of business for a new data center (or to retrofit an existing data center) is to implement remote telemetry and environmental monitoring systems -- not in just one area of the room, but in multiple places around the room. APC and others make environmental monitoring systems for exactly this type of distributed deployment, and no data center should be without something like it.
Taking the temperature
As far as what you monitor, and how it's monitored, my overall thought is that you can never gather too many statistics and data points. Obviously we need to monitor temperature, but we need to do so at the ceiling and floor of the room, in multiple locations, and at the rack inlet. Ideally every few racks will have a temperature sensor placed in the front of the rack, where it can measure the temperature of the air entering the hardware.