Select resource allocation settings (reservation, limit and shares) that are appropriate for your ESXi environment.
The following guidelines can help you achieve better performance for your virtual machines.
Use Reservation to specify the minimum acceptable amount of CPU or memory, not the amount you want to have available. The amount of concrete resources represented by a reservation does not change when you change the environment, such as by adding or removing virtual machines. The host assigns additional resources as available based on the limit for your virtual machine, the number of shares and estimated demand.
When specifying the reservations for virtual machines, do not commit all resources (plan to leave at least 10% unreserved). As you move closer to fully reserving all capacity in the system, it becomes increasingly difficult to make changes to reservations and to the resource pool hierarchy without violating admission control. In a DRS-enabled cluster, reservations that fully commit the capacity of the cluster or of individual hosts in the cluster can prevent DRS from migrating virtual machines between hosts.
If you expect frequent changes to the total available resources, use Shares to allocate resources fairly across virtual machines. If you use Shares, and you upgrade the host, for example, each virtual machine stays at the same priority (keeps the same number of shares) even though each share represents a larger amount of memory, CPU, or storage I/O resources.