I think you are over looking the effect of having memory act as disk cache. Having 8GB of extra memory in the system may use extra power but may also cause the harddisk to use less power because it does less seeking, and in the future (dare to dream) possibly go into a sleep or low power use mode.

The latests cpu can adjust there clock speed and thus power usage based on work load, having more memory probably allows the cpu to run at a slower speed longer thus further reducing power consuption. If a processor is waiting an extra second or two while data is fetched from the disk instead of disk cache, the cpu could possibly run at a higher clock rate even though it is actually doing little or no work.

Of course in the case of virtualization, having more memory in the box allows it to do more work, and thus replace more machines. And while the memory uses power, Disks and powersupplies and CPU's, fans add up to much more power usage. So I would look at memory power consumption with a lower priority. Even if you have to use 3times the memory in a system to replace 2 boxes, it still turns out to be a win because you removed at least 2 disks, 1-2 cpus, 4 fans, and the cooling requirements of one box in the process... those can easily be 5000 watts of savings per box replaced for the addition of 160watts of power used by the increased memory.

Posted by James Dickens on January 08, 2008 at 11:09 AM PST #

Not to mention that in the future it will be possible to move workloads to a smaller number of physical machines at times of lower utilization, and then power down the unused physical machines. The flexibility that comes with a fully virtualized environment has the capacity to bring much more in the way of power management in the future.

Posted by Paul Slater on January 08, 2008 at 12:36 PM PST #