Now that the Service Pack 1 for Windows Server 2008 R2 is available as a public beta, Microsoft has made it clear what their dynamic memory feature is all about.
With Hyper-V Dynamic Memory there are two values: Start-up RAM and Maximum RAM:
Start-up RAM is the initial/startup amount of memory assigned to a virtual machine. When a virtual machine is started this is the amount of memory the virtual machine will be allocated.
The Maximum RAM setting is the maximum amount of memory that the guest operating system can grow to, up to 64 GB of memory. Based on the settings above.
The Memory Buffer property specifies the amount of memory available in a virtual machine for file cache purposes (e.g. SuperFetch) or as free memory. The range of values are from 5 to 95. A target memory buffer is specified in percentages of free memory and is based on current runtime memory usage. A target memory buffer percentage of 20% means that in a VM where 1 GB is used, 250 MB will be ‘free’ (or available) ideally for a total amount of 1.25 GB in the virtual machine. By default, Hyper-V Dynamic Memory uses a default buffer allocation of 20%. If you find this percentage is too conservative or not conservative enough, you can adjust this setting on the fly while the virtual machine is running without downtime.
Memory Priority: By default, all virtual machines are created equal in terms of memory prioritisation. However, it’s very likely you’ll want to prioritise memory allocation based on workload. For example, I can see a scenario where one would give domain controllers greater memory priority than a departmental print server. Memory Priority is a per virtual machine setting which indicates the relative priority of the virtual machine’s memory needs measured against the needs of other virtual machines. The default is set to ‘medium’. If you find that you need to adjust this setting, you can adjust this setting on the fly while the virtual machine is running without downtime.
Thanks to Alessandro Perilli for the details.