I’ve had a few conversations with my peers around virtualisation and power saving. As we know, both Microsoft and VMware provide green power savings.

In the same way as Live Migration and vMotion both provide the seamless movement of virtual machines between hosts in different ways (Microsoft is one at a time and VMware is many at once), power savings are also the same…..but different

Ok, so in VMware, very simply, many virtual machines collapse (vMotion) onto a lesser number of physical hosts to conserve power. The free hosts are then shutdown reducing power and cooling etc.

In Microsoft, the machines don’t necessarily Live Migrate onto one but instead make use of Core Parking. Have a look on Matt's blog here for more details on Core Parking.

So on the face of it, switching off is the best power saving you can have……isn’t it?

After all, you turn your TV off at night and not on standby….

But…look at it this way....

I'm going to use BTUs as a measure as cooling is usually the biggest issue in a datacentre. A very typical host in any virtual environment is an HP DL380 and lets go for a G7 performance model (with plenty of RAM) – latest and greatest.

So, with VMware, you collapse your virtual machines onto a smaller number of physical hosts. Although some hosts are now switched off, the others have gone to full tilt to support the extra requirements of the VMs. When the trusty DL380 G7 is on full revs, it will produce somewhere in region of 4500BTUs. To put this into context, the largest fan heater at B&Q (other good hardware stores are available!) kicks out around 8000BTU so if you have two servers running all VMs, you will end up having this kind of heat!

With Microsoft and Core Parking, the hosts reduce the BTU output to somewhere around 1000BTU, and with Server Core, this is around 700BTU. The CPU is the component that needs the most cooling as it generates the most heat so if we can shut parts of these off, then less power draw and less cooling.

Also, what’s faster to start when required? A server booting or a processor core starting? And servers aren't designed to be switched on and off like a desktop or laptop - that's usually when failures occur.