Symptoms
When a DRS cluster becomes resource over-committed the Total Reservation Capacity value displayed on its "Monitor->Resource Reservation" page in Web Client could be larger than the "Cluster Total Capacity" value on the same page.A message like below will also be displayed on the cluster Summary page indicating the over-committed state."Insufficient capacity in cluster XXX to satisfy resource configuration in XXX"
Purpose
The purpose of this article is to explain this behavior to avoid possible confusions.
Cause
In the current design the "Total Reservation Capacity" value is simply the sum of "Used Reservation" and "Available Reservation". This is correct in normal cases."Used Reservation" includes the reservation configured at the direct child VMs and the child resource pools as well as resource reserved for HA Admission Control policy."Available Reservation" computes differently in normal cases and over-committed cases. In normal cases, it is result of the total available resource for reservation subtracting "Used Reservation" above.When a DRS cluster becomes over-committed due to cluster member hosts number change (i.e. some hosts are moved out of cluster) the existing resource pool reservation settings is no longer valid under the new capacity(In this case, the "Insufficient capacity ..." message is displayed on the cluster Summary page for vCenter administrator to intervene and correct). In this case we would still want to allow VMs to be powered on at the cluster root node to reduce the impact from such invalid configuration. Hence the calculation of "Available Reservation" is changed to override the reservation configured on the child resource pools, which makes it a result of the total available resource for reservation subtracting reservation configured at the direct child VMs and resource reserved for HA Admission Control policy."Used Reservation" on the other hand remains unchanged in over-committed case. The sum of "Used Reservation" and "Available Reservation" actually counts in the resource pool reservation settings twice resulting "Total Reservation Capacity" larger than "Cluster Total Capacity".
Impact / Risks
The behavior of the displayed value is merely a cosmetic issue. The over-committed state of the DRS cluster do need administrator to correct to resume normal DRS functionality.
Resolution
vCenter administrator should take this behavior as a sign of invalid resource pool configuration and correct it by reallocating the resources under the new cluster resource capacity.VMware is working on improving the UI to reduce the confusion in future updates.
