How much memory should be free for VMkernel?

Recently I have made a small research to see how much free RAM does VMkernel need to work without any hiccups due to:

  • Memory Reclamation Techniques
  • Memory Reservation for the VMkernel itself

I have gathered this data from live environment. However one very important metric is not included in the below measures and graphs, and that is the Virtual Machine overhead that is individual for each environment and is dependant on the VMs’ Memory and vCPU amount.

A quick explanation:

  • RAM [GB]: How many Gigabytes of RAM are installed in the Server.
  • VMKernel [MB]: How many MB are reserved for the VMkernel itself (you can find this value in Configuration -> System Resources Tab).
  • Reclamation [MB]: Calculated with a Memory Reclamation Formula (900 MB + 1% of memory above 24GB).
  • Total [MB]: Sum of VMKernel & Reclamation values. This should be the governing baseline value.
  • Free [%]: How much % of the total server’s memory should be free.
RAM [GB] VMKernel [MB] Reclamation [MB] Total [MB] Free [%]
8 1393 900.0 2293.0 28.0
16 1749 900.0 2649.0 16.2
24 2378 900.0 3278.0 13.3
48 2682 1145.8 3827.8 7.8
64 2745 1309.6 4054.6 6.2
96 3514.5 1637.3 5151.8 5.2
128 3612 1965.0 5577.0 4.3
192 5218.5 2620.3 7838.8 4.0
384 8220 4586.4 12806.4 3.3
512 9985 5897.1 15882.1 3.0

And a graph is below:

Memory Reservation Graph

A Graph representing the GB Installed vs. MB reserved memory.

I hope this table comes in useful when deciding how much RAM there is in your environment for the hosts to use.

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