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Hyper-V 2012 R2 changes: What are Generation 2 virtual machines?

Hyper-V 2012 R2 includes some important changes virtualization admins should know about, including the addition of Generation 2 VMs.

What are Generation 2 VMs in Hyper-V 2012 R2, and why are they important?

One of the big changes that Microsoft is making in the forthcoming Windows Server Hyper-V 2012 R2 is a feature known as Generation 2 virtual machines. Generation 2 VMs are an attempt to modernize the virtual machine structure, which has remained relatively unchanged since Windows Server 2008.

Generation 2 VMs will be able to run more efficiently than first generation VMs because they are hypervisor aware and do not rely on synthetic or emulated hardware.

As you can imagine, with this modernization comes a lot of changes and many of the changes will directly impact storage admins.

Probably the biggest change affecting storage admins is that when you create a Generation 2 virtual machine, Hyper-V 2012 R2 uses virtual SCSI disks by default. Previous versions of Windows used virtual IDE disks instead. This means that Generation 2 VMs are not subject to the limits of IDE. It therefore becomes easier to create and attach multiple virtual hard disks to a virtual machine. Doing so was possible in some of the previous versions of Hyper-V, but the virtual SCSI controller was not created by default as it is now.

The move away from IDE also means that Generation 2 VMs will not be able to access physical DVD drives. It is possible, however, to create a virtual SCSI DVD drive. A virtual SCSI DVD drive appears to the guest operating system as a DVD drive but is actually linked to an ISO file rather than to a physical DVD drive.

Another big change with Hyper-V 2012 R2 is that you can boot a Generation 2 VM from a virtual SCSI controller. Previous versions of Hyper-V only supported booting from virtual IDE devices. Of course, since Generation 2 VMs do not support the use of virtualized IDE devices, booting from them is not even an option.

Incidentally, Microsoft has also made it possible to perform a Preboot Execution Environment (PXE) boot from a standard network adapter. Previously, if you wanted to perform a PXE boot, you had to create a legacy synthetic virtual network adapter. This is no longer a requirement for Generation 2 VMs.

One last thing to know about Generation 2 VMs is that they can only host 64-bit editions of Windows Server 2012 R2 or Windows 8.1. These VMs cannot be live migrated to a Windows Server 2012 host, and there is no way to change a VM's generation once you have created the VM.

This was first published in September 2013

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