Q: I notice Windows Server 2012 virtual machines have a Smart Paging File Location--what is the Smart Paging File?

A: Windows Server 2008 R2 SP1 introduced Dynamic Memory, which allowed memory to be added to a running virtual machine (VM) based on the memory currently being used by processes running in the guest OS. Memory could also be removed from a VM using a kernel mode mini-device driver balloon driver.

Using these two capabilities, the memory assigned to a VM could go up and down based on the VM's current needs, giving memory when needed, and removing and allocating to other VMs when not needed, allowing a higher density of VMs on a Hyper-V host. The amount the memory could go up and down was based on two values, Startup RAM and Maximum RAM, which were the amounts the VM was allocated at startup and the maximum amount that could be allocated to a VM, respectively.

Windows Server 2012 introduces a third configuration value, Minimum RAM, which, as the name suggests, is the minimum amount of RAM a VM can ever be allocated.

When a VM starts, it might need a certain amount of RAM, the Startup RAM, to be able to successfully boot and start services, for example, 1GB. However, after the VM has started, this amount of memory is no longer required.

This is when the Minimum RAM value is used. It allows memory to be removed from the VM down to the Minimum RAM value, such as 512MB (see screen shot below).

Hyper-V memory settings

So, to summarize, these are the types of RAM a VM can use:

The new minimum RAM capability introduces a potential problem. Consider the scenario below on a host that's fully utilized from a memory perspective.

This one-and-only scenario is where the new Smart Paging feature is utilized:

At this time a Smart Paging file is created for the VM in the location specified in the VM's configuration and is used by the VM as memory to complete startup. As soon as possible, that memory mapped to the Smart Paging will be ballooned out and the Smart Paging file no longer used and deleted. The target time to stop using the Smart Paging is as soon as possible and no longer than 10 minutes, as documented at Microsoft's website.

The Smart Paging feature is used only to provide reliable restart of VMs and isn't used in any other circumstance, including if a machine is being started from off state instead of being restarted, or any other memory overcommit scenario.