Cluster over-committed with 50+GB of RAM Remaining?

Question

I have 2 2-node Hyper-V R2 clusters.  When I add a guest with 4GB of RAM, I get an over-committed status in VMM 2008 R2 even though there is 50GB of RAM left on one node and 60GB on the other node.  If I reduce the RAM to 2 GB the clusters are
no longer over-committed.  The behavior is the same on both nodes.  These guests are all test, not connected to any virtual switch and running no workloads. Just an OS sitting there doing nothing.  There are 20 of them, but the host
Ratings seem to be very low and available resources seem to be very high to get any over-committed status.  Have you seen anything like this?  I suspect that the resource that is overloaded is RAM, but are there any details scripts that will tell
me exactly what resource is causing the over-committed status?  I have R1 clusters with production workloads and using much more RAM that have a higher rating and are not over-committed.
Thanks.

Answer 1

The issue that you’ve outlined sounds like the cluster nodes are already at maximum capacity before you add the new guest with 4GB RAM. 
Based on the host reserves, VMM calculate the available resources and then will use the “over-committed” status. 
We have a great
TechNet article outlining this status and why it occurs.
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
cluster reserve 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
The cluster reserve is set on the
General tab of the host cluster properties. For a procedure, see
How to View and Modify the Properties of a Host Cluster (http://go.microsoft.com/fwlink/?LinkID=162986).
Thanks,
Mike Briggs [MSFT]
http://blogs.technet.com/scvmm
http://blogs.technet.com/mbriggs
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