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From next-generation Windows VMs to better Linux support, there's something for everyone in the next version of Microsoft's virtualization platform
The basic architecture of the virtual machine has not changed in a long time. Because operating systems were built to operate physical devices, all VMs emulate broadly supported hardware, such as a specific NIC card or IDE disk controller. Hyper-V in Windows Server 2012 R2 supports the concept of a totally new architecture based on modern hardware with no emulated devices. This makes it possible to add a number of new features, such as secure boot for VMs and booting off of virtual SCSI or virtual network adapters. The catch is that guest support is limited to 64-bit versions of Windows 8 and Windows Server 2012.
VM Direct Connect
Connecting to a running VM over RDP requires an active network connection, which you can't always count on. In addition to an active network connection, the VM must have an IP address reachable by the system attempting to connect, a requirement with potential management and security issues depending on the environment in which you're running. All this changes in Windows Server 2012 R2 and Hyper-V with the addition of VM Direct Connect. This feature allows a direct remote desktop connection to any running VM over what's now called the VM bus. It's also integrated into the Hyper-V management experience.
Extend replication to a third site
Hyper-V Replica in Windows Server 2012 is currently limited to a single replication target. This makes it difficult to support scenarios like a service provider wanting to act both as a target for a customer to replicate and a source to replicate to another offsite facility. Windows Server 2012 R2 and Hyper-V now provide a tertiary replication capability to support just such a scenario. By the same token, enterprises can now save one replica in-house and push a second replica off-site.
Online VM exporting and cloning
One of the downsides of Hyper-V in Windows Server 2012 is the need to stop a running VM before you can export or clone it. In production environments, this is simply not an option. Windows Server 2012 R2 Hyper-V removes this restriction. It's now possible to export or clone a running VM from System Center Virtual Machine Manager 2012 R2 with a few mouse clicks. As with pretty much anything related to managing Windows Server 2012, you can accomplish the same task using Windows PowerShell.
Online VHDX resizing
In Windows Server 2012 Hyper-V, it is not possible to resize a virtual hard disk attached to a running VM. Windows Server 2012 R2 removes this restriction, making it possible to not only expand but even reduce the size of the virtual disk (VHDX format only) without stopping the running VM. What you can't do with this feature: Compress an online VHD. You can make these adjustments from System Center Virtual Machine Manager 2012 R2, Hyper-V Manager, or the command line using PowerShell. You can use PowerShell to set the disk size to the current disk consumption if you want to reduce it to the absolute minimum.
Windows Server 2012 R2 includes the ability to limit individual VMs to a specific level of I/O throughput. In the screenshot at left, you can see the two settings for minimum and maximum IOPS. In the initial release of Windows Server 2012 R2, the only number that really makes a difference is the maximum limit. The IOPS are measured by monitoring the actual disk rate to and from the attached virtual hard drives. If you have applications capable of consuming large amounts of I/O, you'll want to consider this setting to ensure that a single I/O-hungry VM won't starve neighbor VMs or take down the entire host
Dynamic Memory support for Linux
Microsoft continues to improve support for Linux VMs. In the Windows Server 2012 R2 release, Hyper-V gains the ability to dynamically expand the amount of memory available to a running VM. This capability is especially handy for any Linux workload (notably Web servers) where the amount of memory needed by the VM changes over time. In environments with many Linux VMs, dynamic memory becomes even more critical to efficiently manage the total memory used by all running VMs. Windows Server 2012 R2 Hyper-V also brings Windows Server backups to Linux guests.
With Windows Server R2 Hyper-V, Windows guest clusters (think traditional Windows Server failover clustering but using a pair of VMs) no longer require an iSCSI or Fibre Channel SAN, but can be configured using commodity storage: namely a shared VHDX file stored on a Cluster Shared Volume. Note that while the clustered VMs can be live migrated as per usual, a live storage migration of the VHDX file requires one of the cluster nodes to be taken offline.