I have just tried the resize. The hypervisor is KVM. There are errors but nothing that points to a failure. What options should exist in nova.conf to make it happen. No way I have more space because I just resized it up then tried to downsize it.

So it resizes it down based on the flavor, but then the disk just stays the same. Is this the outcome you are seeing?

libvirt: logs it cannot resize to smaller and just keeps the larger disk
xen: tries to copy contents to a smaller disk, fails if too large
hyperv: always errors out if new disk size is smaller
powervm: silently keeps the larger disk
vmware: (couldn't find where migration code handled resize)

It does nothing but error with instance not resizing, skipping migration. This happens very quickly and does not register in dashboard.

Makes sense. You need to edit a few settings in your .conf to allow same host migrations, otherwise it's trying to migrate to other hosts. If you aren't using distributed storage then this fails due to an SSH session opening. Check your error logs for nova on the compute node the instance is on.

allow_resize_to_same_host = False

Set to True. Restart service. It's trying to migrate the system to another host when you do a resize.

No go. Same error. I cannot understand why.

What errors do you see in /var/log/nova/compute.log on the compute node where the instance lives?

Your setup will not let you resize down then. Is there a reason you are wanting to resize down? Even with KVM if it went through successfully the disk size stays the same. If you want to actually change the hardware requirements you could look at making flavors that match the disk size but have different vcpu/memory.

Returning exception Resize error: Unable to resize disk down

I want to reduce the ram and vcpu's. Can I create a flavor to do that? I am not certain why my KVM setup will not allow. Nothing special about the setup.