Migrating an instance fails as nova reports that there is not enough available disk space. It reports a negative value.
This is very similar to: https://bugzilla.redhat.com/show_bug.cgi?id=1485427

However, "disk_allocation_ratio" is left at default (value=1) thus seemingly not the same issue.

using the "--disk-overcommit" still fails:
~~~
openstack server migrate 6143cff1-3306-4e90-b49a-c5faaa70ef94 --live  overcloud-compute-28.localdomain --disk-overcommit --debug
(...)
BadRequest: Migration pre-check error: Unable to migrate 6143cff1-3306-4e90-b49a-c5faaa70ef94: Disk of instance is too large(available on destination host:-1398011854848 < need:1638400) (HTTP 400) (Request-ID: req-70cf074b-caba-46fd-8d19-6f2923af5b44)
clean_up MigrateServer: Migration pre-check error: Unable to migrate 6143cff1-3306-4e90-b49a-c5faaa70ef94: Disk of instance is too large(available on destination host:-1398011854848 < need:1638400) (HTTP 400) (Request-ID: req-70cf074b-caba-46fd-8d19-6f2923af5b44)
~~~

~~~
[stack@txwlvcpdirector02 ~]$ nova hypervisor-stats
+----------------------+---------+
| Property             | Value   |
+----------------------+---------+
| count                | 23      |
| current_workload     | 0       |
| disk_available_least | -14395  |
| free_disk_gb         | -14576  |
| free_ram_mb          | 2116671 |
| local_gb             | 6394    |
| local_gb_used        | 20970   |
| memory_mb            | 9041737 |
| memory_mb_used       | 6925066 |
| running_vms          | 284     |
| vcpus                | 920     |
| vcpus_used           | 2025    |
+----------------------+---------+
[stack@txwlvcpdirector02 ~]$ cinder get-pools --detail
+-----------------------------+-----------------------------------+
|           Property          |               Value               |
+-----------------------------+-----------------------------------+
|         QoS_support         |               False               |
|        driver_version       |               1.3.0               |
|       filter_function       |                None               |
|       free_capacity_gb      |           26874.2961426           |
|      goodness_function      |                None               |
| max_over_subscription_ratio |                20.0               |
|             name            | hostgroup@tripleo_nfs#tripleo_nfs |
|   provisioned_capacity_gb   |                0.0                |
|     reserved_percentage     |                 0                 |
|      sparse_copy_volume     |                True               |
|       storage_protocol      |                nfs                |
|  thick_provisioning_support |               False               |
|  thin_provisioning_support  |                True               |
|          timestamp          |     2017-09-26T21:26:11.123650    |
|      total_capacity_gb      |           30841.1833496           |
|         vendor_name         |            Open Source            |
|     volume_backend_name     |            tripleo_nfs            |
+-----------------------------+-----------------------------------+
~~~