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CCSI's Cooling Technology

'The Challenge'

The introduction in June of 2001 of dual Athlon system boards by Tyan not only "raised the bar" of computing performance but also of thermal management. Conventional chassis and cooling solutions can simply no longer handle the thermal requirements. The magnitude of the problem becomes even greater when designing low profile, high-density, servers such as the 1U and 2U rackmount solutions

In conventional tower server chassis and in low-density rack chassis installations, there is ample real estate in the front and rear and on the sides of the chassis for mounting intake and exhaust fans. In contrast, the low-profile 1U and 2U rack space does not provide adequate room for exhaust fans. Furthermore, there is precious little room for intake and exhaust vents, which compromises fan performance, lowering their actual performance to significantly below their "rated" airflow capacity, which is designed for a minimal resistance (to airflow) environment. In fact the only exhaust system available on a conventional 1U or 2U chassis is the power supply exhaust fan which is totally inadequate to provide sufficient exhaust of hot air to allow the system to cool reliably. The result, of course, is an unstable and virtually useless system.

The thermal dilemma becomes even more critical when conventional fan top or blow through CPU coolers are used. The efficiency of all CPU coolers is linked directly to the temperature of the intake air. Every degree rise in intake air temperature to the CPU cooler directly translates into a corresponding rise in the CPU junction temperature.

In the conventional server chassis the warm exhaust air from the CPU cooler is re-circulated inside the chassis causing the CPU temperature to rise until it reaches a thermal equilibrium which is determined by the chassis's exhaust system capacity. In our tests at CCSI, we have seen CPU temperature rises of between 25 and 48 degrees centigrade in several conventional 1U and 2U chassis specimens that we have tested. This is easily observed with a simple cover on and then cover off test while running a CPU loading utility. Another major shortcoming of the conventional fan top CPU coolers is the reduction of airflow due to pressure drop resulting from the airflow obstruction of the chassis cover and the fins of the heat sink itself. Fans performance is rated in cubic feet per minute (CFM) with 0 pressure drop and performance is severely compromised with only minimal air flow obstructions from either the intake or exhaust side of the fan. The CCSI Wind Tunnel™ eliminates these problems.

ONLY THERMAL CORRECTNESS CAN ELIMINATE CPU THROTTLING

As a self-protection feature, some CPU's incorporate an internal temperature monitoring diode and a throttling circuit. In simple terms, you can think of this as a thermally controlled governor. The governor in this case wields a two-edged sword. On the plus side, the governor will prevent the CPU from overheating to the point of system failure, and thus provides for a stable system even when built into a thermally sub-standard chassis. On the minus side, the governor limits the performance of any such system by automatically lowering the CPU speed. In other words, while you get a stable system regardless of the thermal performance of the chassis, you don't "get what you pay for". The really sad thing about this situation is that you have no way of knowing if and how much your system performance has been compromised and performance-monitoring software is no help, because it will of course report that the CPU is fully occupied. While there is undeniable value in having a stable system, we at CCSI feel that insuring system reliability by "hobbling" system performance is a bit like harnessing a racehorse to a sightseeing carriage or perhaps in an extreme situation to a plow. While the racehorse certainly could pull the carriage or the plough, that is not what he was born and bred for!

CPU's that do not have the throttling feature always run at full speed, but require a chassis that provides proper cooling in order to run reliably. The advantage of a thermally correct chassis is that it will not only allow a non-throttled CPU system to run reliably, but will also allow a throttled CPU to run without the governor being invoked, thus optimizing both system reliability and performance.

As can be seen from the above discussion, it is critical when purchasing a chassis to make sure that it is thermally correct in order to avoid CPU throttling. Any statement to the effect that "we have the system running at such and such speed with no problem" should be taken with at least a "grain of salt". Since the system, while operating reliably, may not truly be "running", but "walking" or even "crawling".

Power supply design for 1U and 2U servers is another extremely challenging task due to several opposing design criteria. On the one hand we must provide more power for the higher speed dual CPU processor servers and on the other hand we have far less physical space available for the power supply plus we have to contend with the thermal issues described above. Increasing power output while significantly reducing the size, and providing the required thermal management of high output 1U power supplies is a daunting task, which is well beyond the abilities of the average conventional designer. In fact, with experienced engineering staff with many years of experience in designing high density systems and solving thermal management challenges, CCSI still spent over 2000 man-hours on these solutions and went through three different power supply designs to arrive at the final dual 300 watt power supply utilized in our RC 0103 1U chassis.

To solve these thermal obstacles and build "future proof" thermally correct high-density high performance 1U and 2U solutions, CCSI created a task force of engineering experts with one objective: to design and develop both 1U and 2U server solutions that meet and exceed the thermal requirements of not only today's but also future high performance CPU's.

Our Goals were:

A) To invent a new exhaust system which allows the maximum cross flow possible in these high-density server chassis. Mission Accomplished

B) To invent a new "outside the box" CPU cooling system that draws air from outside the chassis rather than recirculating warm air from within the chassis. This design incorporates adequate volume, velocity and pressure of air to not only cool the CPU's but also the other hot components on the motherboard as well as peripheral cards. Mission Accomplished

C) To invent a new generation of high power low profile 1U power supply to meet the power budget of current and future high performance CPU's. Mission Accomplished

D) To invent an all new server component layout for optimum configuration, fault tolerance and serviceability. Mission Accomplished

It took our team over 2000 man-hours to complete these tasks and finally gain the recommendation of our most stringent partner, AMD, for all their present and planned future speed grades of the Athlon MP processors.

 

 

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