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Hyper-V and reserving RAM for the host/root/parent partition

After a long process, I finally have a real-world calculation for determining how much RAM to reserve for a Hyper-V host. The question/answer about it is here. But the summary is that Hyper-V loses RAM to the Nonpaged pool (and all of it is "untagged") in addition to the "standard" stuff that Microsoft has documented.

Be aware that I write MB/GB here, when I actually mean MiB/GiB. I feel it will be more intuitive to see the notation that Windows (incorrectly) uses.

Host Overhead
  1. 300 MB for the Hypervisor services
  2. 512 MB for the Host OS (This is a recommended amount; you have some wiggle-room with this.)
  3. [The amount of physical RAM available to the host OS] multiplied by 0.0425 (result in GB) for the Nonpaged pool (Which means multiply that by 1024 to convert to "MB")
Per-VM Overhead
  1. 24 MB for the VM
  2. 8 MB for each 1 GB of RAM allocated to the VM.

Examples


12 GB RAM, 1 VM @2 GB, 1 VM @4 GB
  • Host: 812 + (0.0425 * 12 * 1024) = 1,334.24 MB
  • VM1: 24 + (2 * 8) = 40 MB
  • VM2: 24 + (4 * 8) = 56 MB
  • Total:  1430.24 MB (~1.4 GB)
24 GB RAM, 2 VM @8 GB, 1 VM @ 4 GB
  • Host: 812 + (0.0425 * 24 * 1024) = 1856.48 MB
  • VM1: 24 + (8 * 8) = 88 MB
  • VM2: 24 + (8 * 8) = 88 MB
  • VM3: 24 + (4 * 8) = 56 MB
  • Total: 2088.48 MB (~2.04 GB)
36 GB RAM, 1 VM @24 GB
  1. Host: 812 + (0.0425 * 36 * 1024) = 2,378.72 MB
  2. VM1: 24 + (24 * 8) = 216 MB
  3. Total: 2,594.72 (~2.53 GB)
You can see why most people just say "2GB for the host" and stop there. Essentially, you can get away with 2 GB until you get up to a 24 GB server. But after that you have to start caring more about the amount of RAM lost to the Nonpaged pool.

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