calculations on VDIs…

August 15th, 2009 by Sebastian Leave a reply »

[For the german/original version - please click here.]

As a fan of saving energy in IT infrastructures, I have calculated how much energy one can save via implementing a VDI.  Below is a detailed VDI solution used as an example to prove exactly how much energy can be saved.   In order to better explain this, I will assume the following:

  • the system-integrator configures an automatic shutdown for the hypervizor overnight
  • no shared storage is used to store VMs because you don’t need high availability functions for all desktops
  • the workloads we have to  virtualize are average office users using a software that does not run on terminal servers but we can serve 4 desktops with one CPU-Core

In VDIs, energy saving can only be achieved if a PC is replaced by a Thin Client (TC).

energy-consumption(PC) – energy-consumption(TC) = energy-savings per workspace

As an example, let’s assume we replace a DELL Optiplex 740n (consuming 60 – 90 watts, using an average of 70 watts for our calculation)[1] with an Igel UD2 (consuming 13 – 20 watts, using an average of 17 watts) [2]. The following equation demonstrates the amount of energy one is able to safe for this workspace:

75 watts – 17 watts = 58 watts

We will now calculate how many PCs can be served using one hypervisor. Based on the above mentioned 3rd assumption – we can serve 4 VMs with one CPU-Core of the hypervisor. With a standard-server that includes 2 Quadcore CPUs we can serve 4×8=32 VMs.

To calculate the energy-consumption of the hypervizor, we will assume, that the hypervizor is installed on the following product:

Poweredge R610
2x Intel Xeon X5560 2,8 GHz
6x 146GB 15k SAS HD

With the DELL Energy-Calculator [3] you can calculate that this server consumes 412 watts watts if it runs on 100%. The energy-savings we can achieve through using this server as a hypervizor (for replacing PCs through TCs) is:

32 VMs x 58 watts = 1856 watts

Since the server itself consumes the following amount of power:

412 watts (Power) + 618 watts (Cooling) = 1030 watts

In the end we have saved the following energy consumption:

1856 watts – 1030 watts = 826 watts (for 32 workspaces)

In total, 9.9 kWh a day have been saved, if the VM’s are running 12 hours per day for 32 workspaces.

Ok, and how much money has one saved? This figure will depend on the price per kWh of electrical power.  In Germany, one pays about 0.14 €/kWh, thus the following can be saved:

9.9 kWh x 0.14 €/kWh = 1.38 € (a day)

Unfortunately, the VDI solution is not free – for a professional VDI-Solution you can assume the following costs:

6000 € for Hardware
4000 € for Software
3000 € for Installation Services

So, how many years does one need to calculate to obtain the return on invest…?

13000 € / 1.38 € (per day) = 9420 days = 25.8 years ;-)

As a result, VDI does not save money?  No, not through energy costs alone. There are other ways to save, some are as follows:

  • centralization of software deployment
  • decrease downtime
  • minimize administration costs

Furthermore, the entire calculation can change dramatically if one small parameter is changed. For instance, if you use the Wyse S10 (6.6 watt power consumption)[4] instead of the Igel UD2 you will get 13.9 kWh of energy savings a day (based on 32 workspaces running for 12 hours). Furthermore, this calculation is done for 32 workspaces – if you were to virtualize 300 or more the whole calculation would be different because the cost for the servers and TCs would decrease dramatically.

PS: Additional cost saving would also occur in buying TC’S rather than PC’s.  Please note, these cost saving figures had not been incorporated in my calculations.
PPS: The most efficient solution would be a mixture of VDI and Terminalservices because the cost per desktop would decrease dramatically (With the Server-Hardware in this article you can easily serve 120 desktops.  For example, this would apply to XenApp on XenServer).
PPPS: If you are truly in search of a greener IT system for your company, I would highly recommend reading the following article: “The five things you should know about Green-IT baselines” [5].




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