One of the top questions that I keep hearing from our customers is to explain the ‘over-committed’ status and why we sometimes display this status. An example of when this status might be displayed is when attempting to migrate a VM to a particular host, depending on what is already on the destination the ‘over-committed’ status might be displayed.
HAVM Placement and “Over-committed” Status
When placing a highly available virtual machine, the placement process in VMM calculates whether adding a new virtual machine to a host cluster will over-commit the cluster based on the cluster reserve configured for the host cluster in VMM. The
specifies the number of node failures a cluster must be able to sustain while still supporting all virtual machines that are currently deployed on the clustered hosts. If a host cluster cannot withstand the specified number of node failures and still keep all of the virtual machines running, the cluster is placed in an Over-committed state, and the hosts are not available for placement. An administrator can override this and place an HAVM on a host in an over-committed cluster during manual placement. Cluster reserves are a unique feature of VMM.
For example, if you specify a node failure reserve of 2 for an 8-node cluster, the rule is applied in the following ways:
· If all 8 nodes of the cluster are functioning, the host cluster is marked Overcommitted if any combination of 6 nodes (8-2) in the cluster lacks the capacity to accommodate existing virtual machines.
· If only 5 nodes in the cluster are functioning, the cluster is marked Overcommitted if any combination of 3 (5-2) nodes in the cluster lacks the capacity to accommodate existing virtual machines.
VMM’s cluster refresher updates the host cluster’s Over-committed status after each of the following events:
· A change in the cluster reserve value
· The failure or removal of nodes from the host cluster
· The addition of nodes to the host cluster
· The discovery of new virtual machines on nodes in the host cluster