It would be hard to complain that virtualization is contributing to any kind of “global warming” hysteria. In fact and by its very consolidating nature, virtualization offers many advantages over “traditional” computing models that make it “green” even in its most basic format. VMware reinforces this argument with the claims that “every server that is virtualized saves 7,000 kWh of electricity and four tons of carbon dioxide emissions per year.”
However, virtualization’s promise was born out of the recognition that x86 servers were commonly operating with enormous “excess capacity.” In typical deployments, virtual servers are driven to only 30-40% of capacity, and where excess capacity abounds, there exists a potential for slashing available resources in a quest for limiting on-line power consumption.
Enter VMware’s power saving extension to its Distributed Resource Scheduler (DRS) that scrubs VMware clusters for unused capacity, automatically consolidates stray virtual machines away from near-idle members and shuts those members down to conserve power. This magic green genie is called Distributed Power Management (DPM) and, while simple to configure, has a few quirks that not only hinder its effectiveness but actually make no sense (more on that later).
“VMware DPM monitors the cumulative demand of all virtual machines in the cluster for memory and CPU resources and compares this to the total available resource capacity of all hosts in the cluster. If sufficient excess capacity is found, VMware DPM places one or more hosts in standby mode and powers them off after migrating their virtual machines to other hosts. Conversely, when capacity is deemed to be inadequate, DRS brings hosts out of standby mode (powers them on) and migrates virtual machines, using VMotion, to them. When making these calculations, VMware DPM considers not only current demand, but it also honors any user-specified virtual machine resource reservations.”
vSphere Resource Management Guide, Managing Power Resources