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Virtualization has changed the face of modern computing; administrators can provision computing resources and operate workloads completely decoupled from the underlying servers. To accomplish this feat, a hypervisor is usually installed directly on top of the server's hardware, and virtual machines can then be established above the hypervisor to run a wide range of operating systems and applications. But a new virtualization model which allows one hypervisor to run within another is emerging, allowing IT professionals to mix hypervisors and develop complex virtualized environments that have not been practical before. Although the technology isn't quite ready for busy production data centers yet, interest in "nested virtualization" is growing, and vendors are demonstrating serious support. Here's what you need to know about nested virtualization today.
Which hypervisors support nested VMs, and are there any hardware requirements?
Nested virtualization -- or nested VMs -- is not a new idea. VMware discussed the issue back to 2008, and a VM created with one hypervisor should ideally work when nested inside another VM. For example, a host hypervisor like ESXi 6.0 will support guest hypervisors, including Hyper-V, Xen and KVM. However, the ability of a host hypervisor to support particular guest hypervisors should never be assumed. It's always best to start your nested virtualization research by checking with hypervisor vendors to determine which specific hypervisors are known to work as guests -- and also check the ability of the host hypervisor to support particular guests. If you cannot find documentation that supports your desired combination of host and guest hypervisors, you can still experiment in a controlled environment and benchmark the results for yourself, which is always a sound practice.
The principle issue with nested virtualization has been the potential performance impact to guest VMs, also known as nested VMs. Hypervisors like ESXi, Hyper-V, Xen and KVM all need access to the processor hardware extensions that enhance virtualization such as Intel VT-x and AMD-V along with Intel's extended page tables (EPT) and VM control structure shadowing as well as AMD's rapid virtualization indexing (RVI) technologies. This isn't a problem for modern servers since both processor extensions were added back in 2006. But once a hypervisor was installed on the server's bare-metal hardware, the host hypervisor typically didn't expose the server's virtualization features to guest hypervisors, resulting in poor guest hypervisor performance, if the nested VM launched at all.
Modern hypervisors like ESXi 5.1 and later are able to virtualize the processor and memory enhancements and make those features available to guest VMs, which can then be nested within other VMs and offer full hardware-accelerated performance.
Although current hypervisors should support nesting, remember that it may be necessary to deliberately enable virtualized hardware-assistance as a feature of the host hypervisor before nested VMs -- and guest hypervisors -- can be deployed properly. For example, ESXi 5.1, 5.5 and 6.0 all require administrators to access the processor settings screen within the Web client and check the "Expose hardware-assisted virtualization to the guest operating system" box while VMware Workstation 8 and Player 4 require administrators to check the "Virtualize Intel VT-x/EPT or AMD-V/RVI" in the processor settings screen. As another example, enabling nested virtualization in Xen may require changes to the Xen configuration file such as:
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