Operating systems serve three basic jobs: organize the computer's resources, launch and manage applications, and...
provide a user interface to those resources and applications. Any disruptions or changes to the operating system can potentially have catastrophic effects on any workload installed there. This challenge is multiplied in virtualized enterprise environments where an OS on a production server may impact dozens of virtual machines. So IT professionals understand that OS patches and upgrades should never be automatic -- it takes planning and testing to vet each change before implementation.
The system requirements for Windows Server 2012 R2 are relatively light. At a minimum, it demands one 1.4 GHz 64-bit processor, 512 MB of RAM, 32 GB of storage, a Gigabit Ethernet adapter, and a DVD or other drive capable of installing the media. Secondary requirements include a display device capable of SVGA resolutions of 1024 x 768 or better, a keyboard and mouse, and Internet access (for downloading drivers, patches and updates during installation).
Remember that these are the absolute minimum requirements to run the OS software only. Installing the OS on a system of this type would not leave any resources for applications. In fact, additional resources may be needed to optimize performance or install certain server roles. Installing the OS in a virtual machine will certainly require more than the minimum resources.
The fact is that Windows Server 2012 R2 should install on almost any modern server. The problem is not whether the OS will have enough resources. The real consideration is "resource creep." As the OS footprint gets bigger, there are slightly fewer resources left for the workload. You normally won't see this on a physical server, but it can be a real issue on a virtualized server with dozens of workloads upgrading to Windows Server 2012 R2. The subtle increases in resource demands can push a fully loaded server to resource exhaustion, lowering the performance of some VMs and forcing IT staff to perform workload balancing to free additional server resources.
Find out the upgrade limitations for the latest version of Windows Server
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Prepare for the upgrade to Windows Server 2012 R2