And, of course, you need cameras. No corner of the data center should be out of sight of at least one camera. Fixed-position and pan-tilt-zoom cameras should be used together, and at least a few should have infrared features to allow for visibility in the dark.
What to do with all that data
With all of these data collection points, we have high visibility into the data center itself, not just into the servers and other hardware in the room. All this data should be maintained, tracked, and trended. Using SNMP and tools such as Cacti, or through vendor solutions, you should be able to recall and view data from any sensor from any time. You will be able to see if the ambient temps have been rising over months due to the addition of new gear, or to verify when a particular rack door was opened.
As far as alerting goes, it will take some time to find and set acceptable alert thresholds for some sensors, and for escalating alert messages delivered by email and text. In a high-traffic data center you may not want to know every time someone enters the room, but in a low-traffic data center, you probably do. Your cameras should be taking pics or video whenever motion sensors are triggered, and those images and video should be shipped to a server for storage, ideally synchronized to an offsite system.
It's not a bad idea to make sure you have at least one analog phone line and a modem hooked up to a Linux box somewhere, or a 3G/4G/LTE data connection that can be turned up automatically when needed. In a real emergency situation, that might be the only way you can check on the data center if the data circuits are down.
If you think that's overkill, ask anyone who was responsible for the care and feeding of a data center in the New York and New Jersey area when Hurricane Sandy came through. They'll tell you different. It takes only one significant, unexpected environmental problem to justify the expense and management of data center monitoring systems. I hope you never find that justification, but at the same time, you cannot assume you won't.
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