The x86 rack server space is very crowded, but is still possible to rise above the crowd. Quite a few data centers have many "gaping holes" in the racks as they have exceeded the power or cooling capacity of the data center and it is no longer possible to add servers. One way to distinguish your server from the masses is to create a very low power server. The x86 rack server market is also very cost sensitive, so any innovation that seriously cuts the costs of buying and managing the server will draw some attention. This low power, cost sensitive part of the market does not get nearly the attention it deserves compared to the high performance servers, but it is a huge market. According to AMD, sales of their low power (HE and EE) Opterons account for up to 25% of their total server CPU sales, while the performance oriented SE parts only amount to 5% or less. Granted, AMD's presence in the performance oriented market is not that strong right now, but it is a fact that low power servers are getting more popular by the day.

The low power market is very diverse. The people in the "cloudy" data centers are - with good reason - completely power obsessed as increasing the size of a data center is a very costly affair, to be avoided at all costs. These people tend to almost automatically buy servers with low power CPUs. Then there is the large group of people, probably working in the Small and Medium Enterprise businesses (SMEs) who know they have many applications where performance is not the first priority. These people want to fill their hired rack space without paying a premium to the hosting provider for extra current. It used to be rather simple: give heavy applications the (high performance) server they need and go for the simplest, smallest, cheapest, and lowest power server for applications that peak at 15% CPU like fileservers and domain controllers. Virtualization made the server choices a lot more interesting: more performance per server does not necessarily go to waste; it can result in having to buy fewer servers, so prepare to face some interesting choices.

Do you go for a server that tries to minimize power and rack space by using low power CPUs and a server form-factor such as the Twin or Blade server? In that case, you may end up with more servers than originally planned. Alternatively, do you use standard 1U/2U servers with standard CPUs? In that case, you may miss the chance to lower your monthly collocation and/or energy bill. While we won't be able to give you a perfectly tailored answer, we'll give you some of the fundamental information that you need to make an educated decision. In this article we will measure how much power low power CPUs, such as the Opteron 2377 EE and the Xeon L5520, save compared to their "normal" siblings. We will use our own virtualization benchmarks to make it a bit more realistic than SPECpowerjbb. Maybe the most important question: is the performance/watt ratio of a more expensive, low power server CPU really better?

Focusing on the CPUs alone would be a missed chance. Whether you need 10 or 8 servers to consolidate your applications does not depend solely on the CPU power, but also on the amount of memory you can place inside your server, the amount of expansion slots, and so on. In addition, power consumption does not depend solely on the CPU but also on how clever the server engineers design the server chassis and power supply. We assembled four different servers from three different manufacturers. Every server represents a different take on how to reduce power and CAPEX costs. Let us see how much sense it makes to invest in low power servers with low power CPUs in a virtualized environment.

Does that mean this article is only for server administrators and CIOs? Well, we feel that the hardware enthusiasts will find some interesting info too. We will test seven different CPUs, so this article will complement our six-core Opteron "Istanbul" and quad-core Xeon "Nehalem" reviews. How do lower end Intel "Nehalem" Xeons compare with the high end quad-core Opterons? What's the difference between a lower clocked six-core and a highly clocked quad-core? How much processing power do you have to trade when moving from a 95W TDP Xeon to a 60W TDP chip? What happens when moving from a 75W ACP (105W TDP) six-core Opteron to a 40W ACP (55W TDP) quad-core Opteron? These questions are not the ultimate goal of this article, but it should shed some light on these topics for the interested.

Making Sense of AMD ACP/TDP and Intel TDP
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