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Agile Insider Blog

Hyper-V Dynamic Memory (aka OverCommit) Configuration Guide

Before you can get Dynamic memory, you must upgrade the integration components. Four new virtual machine settings in Hyper-V Manager allow you to configure Dynamic Memory. Review the following descriptions to understand how the features work and how they are related. (Instructions on configuring these settings are presented later in this guide, in “Step 3: Configure Dynamic Memory.”)

Setting Description
Startup RAM Specifies the amount of memory required to start the virtual machine. The value needs to be high enough to allow the guest operating system to start, but should be as low as possible to allow for optimal memory utilization and potentially higher consolidation ratios.
Maximum RAM Specifies the maximum amount of memory that this virtual machine is allowed to use. The value can be set from as low as the value for Startup RAM to as high as 64 GB. However, a virtual machine can use only as much memory as the maximum amount supported by the guest operating system. For example, if you specify 64 GB for a virtual machine running a guest operating system that supports a maximum of 32 GB, the virtual machine cannot use more than 32 GB.
Memory buffer Specifies how much memory Hyper-V will attempt to assign to the virtual machine compared to the amount of memory actually needed by the applications and services running inside the virtual machine. Memory buffer is specified as a percentage because the actual amount of memory that represents the buffer changes in response to changes in memory usage while the virtual machine is running. Hyper-V uses performance counters in the virtual machine that identify committed memory to determine the current memory requirements of the virtual machine and then calculates the amount of memory to add as a buffer. The buffer is determined using the following formula: Amount of memory buffer = how much memory the virtual machine actually needs / (memory buffer value / 100). For example, if the memory committed to the guest operating system is 1000 MB and the memory buffer is 20%, Hyper-V will attempt to allocate an additional 20% (200 MB) for a total of 1200 MB of physical memory allocated to the virtual machine.

Note
The buffer is not maintained when there is not enough physical memory available in the computer to give every virtual machine its requested memory buffer.
Memory weight Provides Hyper-V with a way to determine how to distribute memory among virtual machines if there is not enough physical memory available in the computer to give every virtual machine its requested amount of memory.

 

Note
You can change RAM amounts only when the virtual machine is off, but you can change the buffer and weight at any point.

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