One of vSphere's energy-saving features is Distributed Power Management. Virtualization guru Rick Vanover shows how to configure an HP iLO interface for Distributed Power Management.
With the release of VMware vSphere 4.1, administrators can now add a vendor-specific option to add the HP Integrated Lights-Out controller (iLO) to the properties of a host server. It's only available within a host managed by vCenter because it's a licensed feature that comes with a VMware Distributed Resource Scheduler (DRS) cluster. This is why a free ESXi installation does not display this option in the host software configuration section.The Power Management section of the vSphere Client shows this configuration option in the Host | Configuration | Software | Power Management. A sample HP ProLiant ESXi host is shown configured for the Distributed Power Management (DPM) features in Figure A. When a cluster is not fully utilized, you can use DPM to power off or suspend idle host servers to save power management resources. Figure A
IPMI is short for intelligent platform management interface, and BMC is the industry term for baseboard management controller. (Click the image to enlarge.)
I use DPM in lab situations, but I find that the savings of having one or two host systems powered off is not worth the uncomfortable situation if any problems were to arise in production situations. The fundamental issue is that out-of-band monitoring systems can't really interpret the DPM command to suspend the ESXi host. vSphere has some built-in alarm definitions that can alert you to these issues, such as vCenter not being able to communicate to the BMC with the "Host Baseboard Management Controller status" alert.
There are a number of other configuration options, such as designating a host to never or always receive the DPM recommendations. Further, some scripting ahead of time can increase the comfort level to rearrange workloads ahead of DRS if needed.
Tell us in the discussion if you've used this integrated feature of the HP iLO to vCenter.