[rescue] Deciding how to place machines in a rack.

Robert Darlington rdarlington at gmail.com
Fri Mar 7 11:19:31 CST 2008

Hey, just a side note about a fun experience a few years back.  I was
called in to try to figure out why this small compute cluster was
having issues and shutting down all the time due to thermal problems.
No amount of fans would help.  Here was the setup when I walked into
the room.  There were 5 or 6 rows of racks, about 5 racks wide each.
One rack in front of the other, pretty standard stuff.  Well, the
computer scientists that designed it thought it would look nice to
have them all face the front of the room.  The hot air from the first
rack was drawn right into the second row of racks, then into the third
and so on.  They just didn't get it.

The "big iron" ASCI Q machine was built similarly, but fronts of racks
faced fronts with a row of tiles between them with cold air coming up
through the floor, and backs faced backs.  Those hot isles were nice
to walk down in the winter.  You were guaranteed to be hot by the time
you got to the end of the row of racks!


On Fri, Mar 7, 2008 at 9:51 AM, Earl Baugh <earl at baugh.org> wrote:
> Ok, I've parsed all this advise, and am going to place the machines as close
>  as possible.
>  I do want to understand one thing a bit better regarding the cooling.....
>  all of the machines I'll have
>  in the rack do front-to-back air flow (fans in the front and back which pull
>  air thru the case front to back)
>  It seems that the recommendation is to keep the back door off, and either
>  keep the front door off, or get one
>  that has a lot of ventilation.    This all makes sense with the front to
>  back air flow.
>  But I'm curious about two things.  First, I'm trying to understand the
>  purpose of the fans at the top of the rack.
>  If this approach is to move the hot air out the back, I'm thinking I'll just
>  have a hot spot behind the rack and that the top fans will
>  just be pulling air from the front and back near the top.   Is this really
>  assisting the cooling?  I can believe with the doors off
>  that it's going to move that much air from the bottom of the rack.  Or is it
>  to pull heat from the "stack" of machines?
>  I'm just trying to understand this....  In my head I was thinking having the
>  doors would help the top fans pull air from the
>  bottom.  But I hadn't thought about the way they'd also hold in heat from
>  all the machines, and that you'd get that hot air
>  exhaust coming around and re-circulating thru the machines.  Very very
>  helpful to know that that happens....
>  Also, if I have any open space in the rack, I should bunch it up... I'm
>  thinking it should be near the top.... or does
>  it really matter where it is?
>  In all cases, I really appreciate the advise, real world data, and modeling
>  results!  Way cool... and really helps to
>  bust the myths that we all build up in our heads...
>  Earl
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