Large Maximum Memory Impact for VMs

Large Maximum Memory Impact for VMs

Q: Is there a performance impact to specifying a very large maximum memory value, compared to a lower value, for a Hyper-V virtual machine (VM) that's using dynamic memory?

A: I've had this question recently so decided it warranted a deeper explanation of why the actual maximum memory value you configure for a Hyper-V VM using dynamic memory has no impact on the performance of the virtual machine.

When using dynamic memory, three values can be configured:

  • Startup RAM - the memory allocated to the VM when it starts
  • Minimum RAM - the minimum amount of memory the VM can ever be allocated
  • Maximum RAM - the maximum amount of memory the VM can be allocated within the limits of the memory physically available in the host

When the VM starts, it's allocated the Startup RAM value. Then, once the Integration Services load, the Hyper-V management processes can monitor the amount of committed memory. If the VM is running low on memory, then if memory is available in the host, additional memory is allocated to the VM up to the Maximum RAM value.

The key point is the VM has no knowledge of what that Maximum RAM value is. The OS in the VM uses its memory and will see additional memory added where needed, but if you configure the Maximum RAM as 8GB or 1TB, there's no difference to the VM if it's only using 4GB. It only sees the memory allocated to it.

I am not advocating setting every VM with a 1TB maximum value. Applications can leak memory and be greedy, so setting a real Maximum value is always preferred.

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