vSphere 6.7 introduces a new two-pass algorithm to allocate a resource pool’s resource reservation to its children (also known as divvying).

VMware vSphere Distributed Resource Scheduler (DRS), a utility that balances computing workloads with available resources, uses this new two-pass algorithm to allocate a resource pool’s reservation to its child virtual machines. In the first pass, the resource pool reservation is distributed and capped at the virtual machines’s demand, subject to each virtual machine’s fair share. In the second pass, excess reservation is distributed proportionally, capped at the virtual machine’s configured size. Therefore, resource pool reservation is aggressively allocated to its children, giving more buffer to sudden virtual machine demand spikes.

Note: If a resource pool has a high resource reservation value, there is a possibility that it can reserve most of the host memory. In such cases, the ESXi kernel or agents cannot allocate additional memory. This condition can cause virtual machines to suffer from temporary performance issues even if the resource pool is configured with enough reservation. In such cases, the new two-pass DRS algorithm adds more memory stress to the ESXi hosts if a resource pool uses a large memory reservation. This symptom can be more apparent after upgrading to vCenter 6.7. To learn how to correct insufficient available, reservable memory, see Knowledge Base article KB 71044.