Energy consumption

The key constraint in data centers now is not rack space but power supply. Manufacturers have increased server density over the past few years by producing physically smaller servers. The smallest, a Blade server, stands upright in a chassis containing up to 8 Blades. But it is all for nothing. Smaller servers still use a lot of power and produce a lot of heat, so there is a limit to how many you can fit in one place, regardless of the rack density. You may well see a rack containing only one blade chassis. At that point the effort to reduce server size is redundant. It may as well be a full size server with more space and more airflow.

As the processing power of servers has increased, so has the energy consumption. It is close to a linear relationship. We are familiar with the heat and battery life problem from laptops. But the more fundamental problem is that computers are using more power, and power is becoming the constraining factor in computing capcity. It is a constraint because of cost, cooling capacity and standby generation capacity.

Here is a table of a small selection of HP Compaq servers over the past few years:

Model         Year      BTU      Power
1850R         1995      1010      225
DL380         2000      1475      275
DL380 G5    2006      4150      1000

The newer server is doing more work, and is possibly a little more energy efficient. But whereas you might not notice a 200W power consumption, a 1KW consumption is like having a small electric heater running. Imagine if your office contained several electric heaters running summer and winter. And as you need air-conditioning to reduce the temperature, your overall power consumption is even larger.

What should we do?

  1. Add energy costs into any cost-benefit that involves buying computers: the power consumed and the power for cooling.
  2. Increase the output per unit of computing capacity, by consolidating applications onto fewer servers. The traditional and simple way to run applications has been to give each major application a dedicated server. This has some real advantages, for example when downtime is required. Also, like town traffic, a lot of computing wants resources at the same time and it is difficult to work out what can share with what. Even if you could work it out, some vendors make software that is unintentionally incapable of sharing a server because of conflicts. The answer to this is Virtualization. Several virtual servers can run independently on one larger physical server. The virtual servers can be allocated resources based on usage, and can be moved between physical servers if required. This is the best way to make use of capacity, and hence power consumption.
  3. Don’t bother to switch PC’s off at night. They use hardly any power on standby. The HP DC7700 uses around 3W on standby. Monitors use almost none. At night they are carrying out background tasks like backup, virus scanning and updating. You can switch the PC off if you want, but you would save more by switching off all the lights.


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