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In my last post I mentioned something about using intuition in a data center design. In that context it was about how the DC is cooled in general: The idea being that how one, as a human being, goes about cooling oneself is not necessarily the best way to cool down a room full of computers.

 

I have an item I wanted to bring up, related to the same general idea about humans intuition:

 

    Rackable Servers cool front to back.

 

That's it. Pretty simple. Open up a server and you'll find a number of fans, and they are all pulling air in the front, and shoving it out the back.

 

Not top to bottom. There was a time when networking gear did that, but hopefully you do not have any of that around any more. Even if the server had top-looking vents in it like these:

 

hp-top-vent.jpg

 

 

These are not servers that need to be spaced out. They are not top-cooling. Here is where the natural human reaction gets you into trouble. These look like top vents. These look like you can not put anything else directly on them. But you can. In fact, I am not even sure why these vents are there. Here are two of these same model mounted with no spacing:

 

hp-vent-space.jpg

 

Not only does that blue handle not block all the air flow, look at all the holes across the back. The fans will be able to keep the air moving.

 

Because of that odd design, and the human reaction to it, some of the servers were mounted like this:

 

front-spacing.jpg

1/3 of a U between each server. Now the air not only flows front to back, it flows back to front, and hot air leaks out into the cold aisle.

 

Since this picture was taken, blanking plates were installed below the systems, but nothing can be installed between them because the smallest blanking plate is 1U. I guess we could stuff socks in there, but the last posts experiment was probably as far as I should take unusual air containment exercises. 

 

Also note that above the top server there is a 2/3's of a U gap. No blanking plate for that either, and as heat rises, and hits the ceiling, it then pushes out more at the top of the rack than at the bottom. That is literally the worst place to not be able to put a blanking plate.

 

A different person racked up the same server types this way, in the rack right next to this one:

 

hp-proper-space.jpg

This rack has proper blanking plates all around it now, and it does not leak hot air back into the cold aisle like the one right next to it does.

 

These servers are the same age, and have been kept at the same temperature all their lives. The ones mounted right next to each other have not had any heat related failures of any sort, but because of the air leakage I have less efficient HVAC.

 

This is not to say that you can not install with spaces. On 1 U servers, inserting 1 U blank spots between each one makes it far easier to do wire management, and therefore have clean airflow across the back. It also brings more of the servers up higher in the rack where they are easier to work on. There are 1U blanking plates to put between each one, so you can keep the hot air where it should be.

 

The Green IT intution should be this: Keep your hot air hot, and your cold air cold.