Although Dynamic Memory can help you consolidate more virtualized workloads on a host, it’s important that the host itself have enough memory. If the host has insufficient memory, the management operating system might hang or crash, resulting in all virtual machines being stopped unexpectedly, with a possible loss of data occurring.
To prevent this from happening, Hyper-V automatically reserves some of the host’s physical memory for the management operating system. With Hyper-V in Windows Server 2008 R2 SP1, you could also manually specify how much memory should be reserved for the host by using Registry Editor to create a REG_DWORD registry value named MemoryReserve in the following location:
MemoryReserve could then be used to specify the amount of physical memory in MB that should be reserved for the parent partition, with 2 GB (2048 MB) being the recommended value. The host then had to be rebooted for the change to take effect.
With Hyper-V in Windows Server 2012, however, the management operating system on the host now reserves more memory than in previous versions of Hyper-V to ensure greater reliability and responsiveness. In addition, manually configuring the MemoryReserve registry value as I just described can have undesirable results in some circumstances. For example, it might happen that a combination of management activity, background processes, scheduled tasks, and other activity on the host cause memory usage by the management operating system to temporarily rise above the value you manually configured using the MemoryReserve registry value. Should this occur, the management operating system might hang or crash, resulting in service interruption and possible data loss. As a result, Microsoft no longer recommends manually configuring the MemoryReserve registry value. Instead, you should allow Hyper-V itself to ensure enough memory is always reserved for the host.
The above tip was excerpted from Mitch Tulloch's book Training Guide: Installing and Configuring Windows Server 2012 from Microsoft Press.
Mitch is a nine-time recipient of the Microsoft Most Valuable Professional (MVP) award and a widely recognized expert on Windows administration, deployment and virtualization. For more information see http://www.mtit.com.