You can also select systems that cold-boot rapidly. Model to model and brand to brand, servers exhibit wide variances in power-up delay. This metric isn't usually measured, but it becomes relevant when you control power consumption by switching off system power. It needn't take long. Servers or blades that boot from a snapshot, a copy of RAM loaded from disk or a SAN can go from power-down mode to work-ready in less than a minute. The most efficient members of a reserve/disaster farm can quiesce in a suspend-to-RAM state rather than be powered down fully so that wake-up does not require BIOS self-test or device querying and cataloging, two major sources of boot delay.
Suspend-to-RAM I get. That's like a hibernation mode where everything's powered down except the RAM, which is carrying enough power to hold activation instructions? But how do you copy RAM, and from a disk, no less? I can't quite wrap my head around that yet.