vSphere allows administrators to prioritize virtual machines by configuring their importance using a feature called shares. The more shares a VM has, the more resources it is allowed to consume when competing with other virtual machines. Shares is of relative importance to VMs or pools and they apply within the same parent.
Note
Shares are usually specified as high, normal, and low, which configures VM priority as 4:2:1 ratio.
This feature is one of the few approaches to resource distribution and performance tuning available to administrators. Shares are taken into account only when there is resource contention. In other words, the feature settings make sense only when two or more virtual machines compete for resources.
The actual number of resources a VM gets may change each time it is powered on. If a host is upgraded with more memory, each virtual machine will get a larger portion of RAM according to the ratio.