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Microsoft Releases Windows 10 Test Build with 'Nested Virtualization' Preview

• By Kurt Mackie
• 10/14/2015

Microsoft this week released a new Windows 10 test build that includes support for the company's "nested virtualization" feature.

Nested virtualization, which implies running a hypervisor on top of another hypervisor, is new for Microsoft's Hyper-V. A preview of the capability can be tried out using build 10565 of Windows 10, which Microsoft released this week to Windows Insider Program "first release" testers.

Build 10565 supports a few new capabilities, which are described by Microsoft here. The nested virtualization capability, though, is described in this announcement.

Right now, Windows 10's nested virtualization capability is at an "early preview" stage, but Microsoft is billing it as a first look at that capability for its Hyper-V Containers solution, which is an emerging Windows Server 2016 feature. Microsoft plans to deliver both Windows Server Containers and Hyper-V Containers in its Windows Server 2016 product, which is planned for release next year.

Hyper-V Containers, in particular, will permit a "greater density of isolated applications running on a single host," according to a Microsoft blog post explaining Windows Server Containers. The use case is kind of vague, but presumably, testers can run apps on different Windows versions using the same machine and be able to test them all without conflict. Microsoft's announcement showed Cortana running on different virtual machines on a single Windows 10 machine, for instance.

On the Windows Server 2016 side, nested virtualization might be used more in a lab environment, perhaps for testing advanced Hyper-V features, according to virtualization expert Rick Vanover, who noted that running the feature can bog down hardware resources. Windows Server 2016 currently is released as Technical Preview 3, and so far there's no Microsoft description of nested virtualization support available in its TechNet library description.

Trying out nested virtualization on the latest first-release Windows 10 build comes with a lot of caveats. It only works with the latest version of that hypervisor. Testers can't nest a non-Microsoft hypervisor on top of Hyper-V. In addition, the underlying machine has to support Intel VT-x or AMD-V hardware to run the virtual machines. The feature also taxes a system's RAM, with 4GB minimum needed for the host alone. Microsoft lists plenty of known issues in its announcement.