> I am unsure if the on-off power cycling reduces machine longevity.

Possibly the disks are the most vulnerable components. I wonder if it would be possible in software to shut down the CPUs and have only disks + network running...

In a diurnal cycle like Facebook's, you'd have one start-stop per day, which should be well within the rated specs of hard disks. A few years back I looked at the idea of treating disk lifetime as a resource and explicitly managing it: http://dx.doi.org/10.1109/MSST.2011.5937221

That's assuming that the servers have disks at all, which they probably shouldn't.

A bit late to the party on this one. Check out cpu "c states" and things like intels "speedstep." Modern cpus can reduce/shutdown power to individual packages and cores. This can reduce power consumption from a hundred tdp to tens of tdp.

The downside is latency associated with changing state. Depending on the change it can be hundreds or thousands of micros to go through these states. On a server workload this can introduce huge latency outliers as a request blocks on a core to wake up.

I was more after "shut down everything except the disks" to have the absolute minimum running and the disks spinning to reduce wear on the engines.

If you're shutting down, you have massive latency anyway but if it's possible to at least save all the power not required for keeping disks up that 'd be great.