The vSphere Web Client indicates whether a DRS cluster is valid, overcommitted (yellow), or invalid (red).
DRS clusters become overcommitted or invalid for several reasons.
A cluster might become overcommitted if a host fails.
A cluster becomes invalid if vCenter Server is unavailable and you power on virtual machines using the vSphere Web Client.
A cluster becomes invalid if the user reduces the reservation on a parent resource pool while a virtual machine is in the process of failing over.
If changes are made to hosts or virtual machines using the vSphere Web Client while vCenter Server is unavailable, those changes take effect. When vCenter Server becomes available again, you might find that clusters have turned red or yellow because cluster requirements are no longer met.
When considering cluster validity scenarios, you should understand these terms.
A fixed, guaranteed allocation for the resource pool input by the user.
The sum of the reservation or reservation used (whichever is larger) for each child resource pool, added recursively.
This nonnegative number differs according to resource pool type.
Nonexpandable resource pools: Reservation minus reservation used.
Expandable resource pools: (Reservation minus reservation used) plus any unreserved resources that can be borrowed from its ancestor resource pools.