Debunking the myths of why Vmware is better than Hyper-V – Disk footprint…

I’m continuing my posts tackling the myths that I meet when talking to customers about Hyper-V. I’ve previously written about Transparent Page Sharing and Memory overcommit.
For today’s post, I’m gonna talk about hypervisor disk footprint.

One of the things I hear mentioned from customers when comparing ESXi to Hyper-V is disk footprint. I’ve heard this mentioned numerous times at various customers, but none has gotten the terms right hence I can only speculate that this argument comes from the marketing machine at a certain vendor 😉

The argument stands as this:

ESXi only consumes 144MB of disk space, while Hyper-V consumes 5GB of disk space.

So, is this true?

Well, yes it is but only if you do the comparison of the two hypervisor structures as you would compare apples to bananas. While it is true that the hypervisor kernel in Vmware only consumes 144MB of disk space it is not true for Hyper-V. Here the kernel only consumes about 600KB of space. So why the claim that Hyper-V uses 5GB of disk space.
Well, Vmware counts the management partition in this calculation while forgetting to do the same for their own operating system.

To compare the two, we need to have some understanding of the two hypervisor structures.
Vmware uses a hypervisor type called monolithic, like shown in this picture:
ESXi kernel typeIn this kernel type, drivers are included in the hypervisor kernel.

If we take a look at Hyper-V, it uses a kernel type called micro-kernelized:
Hyper-V kernel typeIn this type, only the hypervisor runs in the kernel and all drivers, management and so forth are located in the parent (management) partition.

So, as shown above Vmware is both right and wrong when claiming that ESXi consumes 144MB of disk space and Hyper-V uses 5GB. It depends on how you look at it. But to do the comparison on a fair basis, then when Vmware claims that their hypervisor only takes 144MB of disk space, then they should tell that Hyper-V uses only 600KB.

Furthermore, when comparing these two designs there are some distinct differences that are worth mentioning.

  • As drivers are not loaded in the hypervisor kernel, the need for specialized drivers are removed in Hyper-V. All drivers working with Windows Server will work with Hyper-V as opposed to Vmware where they need to be specifically written for it.
  • All drivers in Hyper-V run in the parent partition, thus “isolating” them from acting directly in the hypervisor layer. Looking at the Vmware approach where drivers run in the kernel, this could cause a malfunctioning driver to impact virtual machines or allow a malicious driver to gain access to the virtual machines (for example through the vShield API).
  • The amount of code running in the kernel is 600KB as opposed to 144MB in ESXi

Lastly, another selling point that derives from this footprint discussion is the matter of security. Vmware states that their product is more safe due to the so-called smaller disk footprint, based on the argument that a smaller code base equals a more secure product.
If that statement was to hold up, then Windows 95 is to be considered more secure than Windows 7 as the first one only consumes about 80MB of disk space while the last one uses several GB.
Today, most attackers focus on getting in the same ways at the admins of the given product does. This is a side-effect of the products getting more and more secure, and as so it’s your security policies and processes that keep your infrastructure secure and not the amount of disk space (lines of code).

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