2024-08-03
A surprise with the temperature distribution in our machine room
Our primary machine room is quite old and is set up in an old fashioned way, so that we don't really have separate 'hot aisles' and 'cold aisles'; the closest we come is one aisle where both sides are the the front of servers. We have some long standing temperature monitoring in this machine room, and recently (for reasons outside the scope of this entry) we put a second (trustworthy) temperature monitoring unit into the room. The first temperature sensor is relatively near the room's AC unit, while the second unit is about as far away from it as you can get (by our rack of fileservers, not entirely coincidentally).
Before we set up the second temperature unit and started to get readings from it, I would have confidently predicted that it would report a higher temperature than the first unit, given that it was all the way diagonally across the room from the AC unit, and that row of racks sort of backs on to one of the room's walls (with space left for access and air circulation). Instead, it consistently reads lower than the first unit; how much lower depends on where the room is in the AC's cycle, because the second unit sees lower temperature swings than the first one.
(At their farthest apart, the two readings can be over 2 degrees Celsius different; at their closest, they can be only 0.2 C apart. Generally they're closest when the AC is on and the room temperature is at its coolest, and furthest apart when the room is at its warmest and the AC is about to come up for another cycle. Our temperature graphs also suggest that the cold air from the AC being on takes a bit longer to reach the far unit than the near unit.)
Temperature sensors can be fickle things, but this is an industrial unit with a good reputation (and an external sensor on a wire), so I believe the absolute numbers shown by its readings. So one of the lessons I take from this is that I can't predict the temperature distributions of our machine room (or more generally, any of our machine rooms and wiring closets). If we ever need to know where the hot and cold spots are, we can't guess based on factors like the distance from the AC units; we'll need to actively measure with something appropriate.
(I'm not sure what we'd use for relatively rapid temperature readings of the local ambient air temperature, but there are probably things that can be used for this.)