Resource pools are a tool to aggregate physical host resources and channel the aggregated resources to individual virtual machines. You use resource pools to group virtual machines, either to isolate portions af aggregated resources or to prioritize resource allocations between groups of virtual machines.

Resource pools, like individual virtual machines, can be configured with boundaries for resource allocation. Resource pools can also be configured with priority shares, although shares settings act somewhat differently for resource pools than for virtual machines. See Understanding Fixed Shares and Understanding Scalable Shares.

You configure a reservation value for a resource pool to help protect its virtual machines from the negative impact of virtual machines in other pools. If you configure a fixed reservation for a resource pool, the host will always make at least that much of the resource available for the resource pool to distribute among its children. See Understanding Fixed Reservations.

Note: If a resource pool's virtual machines are not using the resource pool's entire reserved amount of a resource, the host may allocate the unused amount to virtual machines in other pools, on a temporary basis. The host will return the borrowed resource to the resource pool that reserved it, whenever needed to start virtual machines within that resource pool.

You configure a limit value for a resource pool to help prevent its virtual machines from negatively impacting performance of virtual machines in other resource pools. The host will never allocate more than that quantity of the resource to any or all of the virtual machines within the pool, whether immediate children or chidlren of a nested pool. After all of the resource is allocated, a resource shortage might cause virtual machines within the pool to fail to start or fail to progress, but it has no effect on virtual machines outside the pool.

If you configure expandable reservations for a resource pool and its siblings, and you configure no reserved amounts for any of the siblings, then they share the parent's reservation amount on a 'first come first serve' basis. If any siblings are configured for fixed reservations, those siblings are guaranteed their fixed reservations but no more, while the remainder of the parent's reservation is available to share among the pools configured for expandable reservations. See Understanding Exxpandable Reservations.