Keeping memory free for VMkernel

The amount of memory VMkernel will try to keep free can be set through the Mem.MemMinFreePct parameter. MemMinFreePct determines the amount of memory that VMkernel should keep free. vSphere 4.1 introduced a dynamic threshold for the Soft, Hard, and Low states to set appropriate thresholds and prevent VM performance issues while protecting VMkernel. The different states, based on %pRAM, which is still free, determines what type of memory reclamation techniques would be used.

For MemMinFreePct, using a default value of 6 percent can be inefficient when 256 gigabytes, 512 gigabytes, or even 1 TB hosts are becoming more mainstream. Having a 6 percent threshold on 512 gigabytes results in 30 gigabytes idling most of the time. However, not all customers use large systems; some prefer to scale out rather than scale up. If you choose to scale out and have more servers with less RAM in each system, a 6 percent MemMinFreePct threshold might be suitable. To have the best of both the worlds, vSphere 5 and above uses a sliding scale to determine the Mem.MemMinFreePct threshold, based on the amount of RAM installed in the hosts.