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Memory Overcommitment in the Real World

We really think VMware Virtual Infrastructure gives a huge amount of value and features compared to other virtualization solutions on the market. Our customers tell us the ROI is high, the time to recoup their costs is small, and a virtualization-first policy can increase your IT agility and even transform your business.

If you (or your boss) can’t get beyond the price, our new Virtual Reality blog had a post last week doing a back-of-the-envelope calculation on price per VM. The point was not to present a full ROI calculation (go over to our VMware TCO-ROI Calculator for a more comprehensive view), but to point out that the sticker price doesn’t tell the whole story. In this case, because VMware VI3 can share memory pages between VMs, you can actually use more memory than is physically resident on the system, and consequently you can consolidate more VMs per physical server. Fewer servers = less hardware & fewer software licenses = lower cost per VM. That’s the reader’s digest version — read both of Eric’s pieces for more information: Cheap Hypervisors: A Fine Idea — If You Can Afford Them and More on VMware Memory Overcommit, for Those Who Don’t Trust the Numbers.

Well, you would have thought that we said Bill Gates wasn’t curing malaria or Citrix killed a man in Reno just to watch him die. The fuss was incredible. You’d begin to suspect that memory overcommit wasn’t on the product roadmap of either Microsoft or Citrix.

Everything we say here on our blogs doesn’t mean a damn thing compared to what’s going on in your data center. This stuff works in real life. (That doesn’t mean you can’t misconfigure it, but it’s probably working right now in your own data center.)

Mike DiPetrillo comes back with a real-world story. First off, he points to this great comment from a VMware customer about memory overcommit and VDI:

One virtualized Citrix server is handling 50-85 sessions and it’s not
full yet. Each of the sessions is running one of three published
applications that all share the same base PowerBuilder code and .DLLs
(about an 80MB memory footprint for each session). Because each of the
50-85 sessions shares the same code, the VMware’s Content Based Page
Sharing consolidates many of the identical pages into single read only
reference points and discards the redundant copies. The net result is
significant ESX host memory savings. As an example, what I’m seeing
inside the Citrix VM is nearly 4GB of RAM being used, but from the ESX
host perspective, 1GB or less physical RAM is being utilized, leaving
the additional 3GB of physical RAM for another VM in the cluster to
use. Now multiply this memory savings by the number of virtualized
Citrix servers and the memory savings adds up quickly.

And then he goes in detail on another real-world situation with VDI at a bank. Link: Memory Overcommitment in the Real World.

This customer configured their standard Windows XP environment for
their call centers to run in a virtual machine. Each virtual machine is
granted 512 MB of memory and 1 virtual CPU. Each VM runs a series of
applications including Marimba, Microsoft Office, a call recording
application, a customer database application, and a BPO (business
process off-shoring) application. …

Below is a screenshot of their environment showing a total of 178 VMs
running on the system. You can also see in the screenshot that less
than 20 GB of RAM out of the total 64 GB of RAM is being used on the
system. With a total of 178 VMs configured for 512 MB of RAM each they
are currently allocating 89 GB of memory to running VMs which means
they are oversubscribed on the host. …

In order to run the same setup with a competitive solution we would
need to have a server configured with at least 89 GB of RAM – the total
allocated to all of the running virtual machines. …

Compared to the original configuration this is a difference of
$11,800.00 just to add more memory to the server to support a solution
that does not have memory overcommit. The cost of a 4 socket VMware VI3
Enterprise license is $11,500.00 list price. As you can see the cost of
a VMware license is actually $300 less than the cost of adding more
memory. Not much of an advantage on the cost side but it still drives
home the point that the VMware solution is not more than the
competitive "free" solution. What’s more is now you get all of the
enhanced functionality of the VMware solution that the competitive
solutions are lacking.

We can keep on coming up with more of these, or just ask your colleagues over at VMware Communities if this works in real life. Feel free to chime in over at Mike’s post on memory overcommit examples if you want to share your story.