Hyper-V Guide: Configuring Dynamic Memory

Dynamic Memory is a feature of Hyper-V that allows memory to be added to and removed from a virtual machine on demand.

All of the Hyper-V Exam Guide and Notes are available.

Reference

Some of the references used are below:

Overview

  • Allows increased consolidation – more virtual machines on the same host due to increased memory use efficiency which is especially useful for VDI environments;
  • Introduces a minimum memory setting for a virtual machine which allows memory to be reclaimed from a virtual machine;

Supported Operating Systems

Dynamic Memory Changes to a Running Virtual Machine

  • Minimum memory can be decreased;
  • Maximum memory can be increased;

Dynamic Memory Settings

  • The following settings can be configured for a virtual machine:

Startup RAM

  • The amount of memory the virtual machine just to start.
  • Optimal value should be high enough to start and low enough for efficient memory utilisation;

Minimum RAM

  • This is the minimum amount of memory to be allocated to the virtual machine while running;
  • Must be between 32MB and Startup RAM;

Maximum RAM

  • Maximum memory that can be allocated to the virtual machine while running;
  • Must be between Startup RAM and 1TB;
  • Remember a virtual machine will only use the maximum amount of memory the guest operating system supports;

Memory Buffer

  • The percentage of memory that Hyper-V will allocate to the virtual machine over and above what it is currently demanding.
  • Helps cope with memory demand spikes;
  • When there is not enough physical memory to give each virtual machine it’;s Memory Buffer, then they are not allocated;

Memory Weight

  • Allows Hyper-V to prioritise which virtual machines should be allocated memory when there is not enough available for all of them;

Smart Paging

  • Smart Paging uses disk as a temporary ‘memory storage’ when memory is demanded but is not available;
  • Smart Paging can / does degrade the performance of the virtual machine;
  • To minimise the performance impact, Smart Paging is only used when all of the following occurs:
    • The virtual machine is being restarted;
    • There is no available physical memory on the host;
    • No memory can be reclaimed from other virtual machines running on the host;
  • Smart Paging is not used when:
    • A virtual machine is being cold-started (ie. started from a shutdown state);
    • Oversubscribing memory for a running virtual machine is required;
    • A virtual machine is failing over in a cluster;
  • When host memory is oversubscribed Hyper-V relies on the paging features of the guest operating system because it is more efficient than Smart Paging;
  • Hyper-V removes the Smart Paging memory, and the Smart Paging memory files, after the virtual machine has started. Smart Paging cannot be used again until the virtual machine is restarted;
  • Smart Paging is temporary and not expected to last more than 10 minutes;

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