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With Update Rollup 6 for System Center 2012 R2 Virtual Machine Manager, you can now overcommit clouds and host groups in your VMM environment.

This brings significant savings for the cloud and host group resources and helps you manage your datacenter in a more economical way.

After applying Update Rollup 6, to enable overcommit of replica VMs to a cloud or host group you simply need to configure the following registry key on the VMM Server:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft System Center Virtual Machine Manager Server\Settings\Placement

Name: IgnoreMemoryForStoppedReplicaVM

Type: DWORD

Value: 1

Note: If the Placement key doesn’t exist you will need to create it.

Later, if you need to disable overcommit simply change the Registry Value for IgnoreMemoryForStoppedReplicaVM to 0

VMM Replica Overcommit

You can read more here: http://blogs.technet.com/b/scvmm/archive/2015/04/29/vmm-2012-r2-ur6-now-offers-the-option-to-overcommit-cloud-and-host-group-capacity-for-replica-vms.aspx