The default_ephemeral_device in instances table is NULL when deploying a VM

[Sun Mengyun]

Description
===========
I deploy a vm with flavor:
+----------+-----------+------+-----------+------+-------+-------------+---------
| Name | Memory_MB | Disk | Ephemeral | Swap | VCPUs | RXTX_Factor |
+----------+-----------+------+-----------+------+-------+-------------+-----------+-------------+
| smy_test | 512 | 10 | 5 | 1 | 2 | 1.0 | True | - |
+----------+-----------+------+-----------+------+-------+-------------+---------

When the VM is deployed successfully and running normally,the default_ephemeral_device is not updated in instances table in database. Obviously default_swap_device and root_device_name is updated.
+------------------+--------------------------+---------------------+
| root_device_name | default_ephemeral_device | default_swap_device |
+------------------+--------------------------+---------------------+
| /dev/vda | NULL | /dev/vdc |
+------------------+--------------------------+---------------------+

Steps to reproduce
==================
1. create a flavor using ephemeral
2. deploy a VM with this flavor

Expected result
===============
check instances table and default_ephemeral_device is NULL;

Actual result
=============
default_ephemeral_device has a value;

Environment
===========
[root@nail1 ~]# rpm -qa | grep nova
openstack-nova-api-18.0.2-1.el7.noarch
openstack-nova-common-18.0.2-1.el7.noarch
python2-novaclient-11.0.0-1.el7.noarch
openstack-nova-placement-api-18.0.2-1.el7.noarch
openstack-nova-scheduler-18.0.2-1.el7.noarch
openstack-nova-conductor-18.0.2-1.el7.noarch
openstack-nova-novncproxy-18.0.2-1.el7.noarch
python-nova-18.0.2-1.el7.noarch
openstack-nova-compute-18.0.2-1.el7.noarch
openstack-nova-console-18.0.2-1.el7.noarch

hypervisor:
Libvirt + KVM

[Brin Zhang (zhangbailin)]

The current BUG is similar to https://bugs.launchpad.net/nova/+bug/1815476

Their type is 'Integer' in the db model and "" or 'NULL' in the response.

[Balazs Gibizer (balazs-gibizer)]

I can reproduce what the reported described.

Is there any user facing impact of the missing value?