c7000 FANs - server consuming too much cool air

[chuckk281]

Steve was looking for answers to a customer question:

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All,

Customer has asked, why does the enclosure (c7000m with 16 x xBL460c G6 servers installed) move so much air when the servers are OFF.  And the temperature is higher….

I think it’s worth answering why the fans on encl-4 are at:

          36% with NO blades active        (with an ambient temp reported of 40c),

          39% with ALL blades active       (with an ambient temp reported of 22c).

Whatever causes this ultimately results in an AIR-hog situation (server consuming too much cool air).

Comments?

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Monty provided some insight:

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The Onboard Administrator controls the enclosure fans to cool both servers and interconnects. 

• Server thermal logic is quite involved with iLO analyzing a ‘sea of sensors’ to develop a VFAN value used by the OA to control server temperature in four zones
• Interconnect module thermal logic is different from server thermal logic and is typically based on a couple of simple setpoints (warm, hot, shutdown).  If the servers are off, the OA only has a couple of fan speeds to use to attempt to control the interconnect temperature – which may result in fan speeds that are higher than those used when the servers are powered on.

We designed the OA Thermal Logic to minimize the enclosure fan power consumption for the use case of servers powered on.

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Other comments or input?

[chuckk281]

Additional info from Monty:

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The air inlet channels that provide cooling for the interconnect bays and OA modules are located on the left and right sides of the c7000 enclosure (see the c7000 Enclosure Tech Brief).  Since these channels pass by the servers on the left and right sides of the enclosure - there is some preheating of that air.

This results in the inlet air temperature to the interconnect modules and OA modules to be slightly higher than the ambient temperature in front of the enclosure.

The OA report of enclosure ambient temperature is based on the ambient temp sensors on all the server blades that are powered on.

When all servers are powered off, the OA report of enclosure ambient temperature is based on the active OA module temp sensor - and due to the preheated inlet air channels this will result in a higher temp reading than when any of the servers are powered on.

If you leave the servers powered off for some time, the fans will cool the entire enclosure and the OA temp sensor will report much closer to the ambient temperature in front of the enclosure.

The OA Thermal Logic does not use the OA report of enclosure ambient temperature for running the fans - all the server 'sea of sensors' and interconnect module temp thresholds are used to determine the fan speed in each zone, and the ambient temp sensor (among all the other temp sensors) on each blade is used by iLO in computing fan request sensor used by the OA for controlling the enclosure fans.

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[chuckk281]

Hoa also had some info:

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Check if  "increased cooling" mode is enable under RBSU per this Customer Advisory: http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?locale=en_US&objectID=c01756204&printver=true.      Switch back to "optimal cooling" see if the fans calm down.     Advanced Option > Thermal Configuration > Optimal Cooling (Default) / Increase Cooling.

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