Windows Server Technical Preview – Hot Memory Resizing

Dynamic Memory was added in W2008 R2 SP1 to allow Hyper-V to manage the assignment of memory to enabled virtual machines based on the guest OSs demands. Today in WS2012 R2, a VM boots up with a start-up amount of RAM, can grow, based on in-guest pressure and host availability, up to the maximum amount allowed by the host administrator, and shrink to a minimum amount.

But what if we need more flexibility? Not all workloads are suitable for Dynamic Memory. And in my estimation, only about half of those I encounter are using the feature.

The next version of Hyper-V includes hot memory resizing. This allows you to add and remove memory from a running virtual machine. The operation is done using normal add/remove administration tools. Some notes:

  • At this time you need a vNext guest OS
  • You cannot add more memory than is available on the host
  • Hyper-V cannot remove memory that is being used – you are warned about this if you try, and any free memory will be de-allocated.

This new feature will save a lot of downtime for virtualised services when administrators/operators need to change the memory for a production VM. I also wonder if it might lead to a new way of implementing Dynamic Memory.

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