Nested VMs

In the slipstream of container technology, which becomes available to the Windows world with Server 2016, Microsoft has introduced a feature that many admins have wanted for a long time: nested virtualization. Nested virtualization means that you can run a Hyper-V host, which is itself a VM, in Hyper-V. Now, you might be wondering why the IT world needs something like this, and this is precisely what stopped Microsoft from doing so in Hyper-V, thus far. There are hardly any practical scenarios for it. People who use this technology do so mostly for lab or training purposes, because you can simulate a whole virtualization cluster with just one computer. This scenario worked with VMware and others, but not with Hyper-V.

Meanwhile, Microsoft has developed a requirement for nested virtualization – the Hyper-V container (Figure 4). As a special feature, Windows Server 2016 now not only offers container technology, but also an advanced container security mode. Lifting the lid on these "containers" reveals virtualization that does not simulate a complete machine but that runs multiple virtual servers on the same operating system base. Each of these "containers" has its own data space, network configuration, and process list – but the base is the same for all of them. For certain purposes, this is significantly more efficient than a full-blown VM. The principle is widely used in the Linux world for web and cloud applications, and Hyper-V containers are a variant.

Figure 4: Installation via Server Manager does not work in the preview yet, but the container technology itself does. It provides for modular web applications, thanks to lean and efficient virtual servers.

In contrast to the (likewise new) Windows containers, the virtual servers do not share the operating system; rather, a rudimentary Hyper-V VM is built for each container, which requires more resources than a normal container, but fewer resources than a full VM. Microsoft needed to develop nested virtualization for this technology to work when the container host itself is a VM, which is thus now available for other scenarios.

Conclusions

Windows Server 2016, and thus the next Hyper-V version, was finally released on October 1. On the basis of the Technical Preview used for this review, the complex container technology with its Docker integration and ambitious storage features, which need to be mature when they come on the market, might have been the reason for the delay. However, the many innovations revealed in Hyper-V was likely worth the wait for many admins.