Increasing demands on datacenter computing prompts research in energy-efficient warehouse scale systems. In one approach, server activation policies invoke low-power sleep states but the power state transition latency must be small to produce effective energy savings. Chrome OS and Arch Linux require 50ms and 650ms, respectively, to enter sleep states. These states consume merely 4-6% of nominal power. By analyzing the critical path, we propose strategies for selecting hardware components and optimizing kernel resume sequences to make datacenter server activation viable. With fast transitions, server activation can provide better performance at lower energy than dynamic voltage and frequency scaling. © 2013 IEEE.