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Why doesn't Openstack free resources when instance is shelved

Hi,

If I have a tenant that is allowed 100 vCPUs but only 4GB of RAM.

If I create an instance that runs on shared storage (using ceph on a remote server), using 2 vCPU but all 4GB RAM, I cannot create any more instances, as my quota is reached.

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This is expected.

However if I shutdown the instance my quota is still exceeded, even though technically the VM is not using any resources.

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If I now shelve the Instance, I also see no change.

The only way I can release the resources, but retain the data is by snapshotting and deleting the instance. The problem with snapshotting is that I don't retain any of the VM properties such as the assigned IP address.

Is this a bug, or intentional. Is there any way to avoid deleting instances to free up resources?