Use subnetpool to create subnet failed when the min_prefixlen of subnetpool is not set

Creating a subnet is failed with a subnetpool when the min_prefixlen of subnetpool is not set.

[root@EXTENV-10-254-8-11 ~]# neutron subnetpool-create ww_v4_subnetpool1 --pool-prefix 172.1.0.0/16
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
Created a new subnetpool:
+-------------------+--------------------------------------+
| Field | Value |
+-------------------+--------------------------------------+
| address_scope_id | |
| created_at | 2019-02-19T02:07:29Z |
| default_prefixlen | 8 |
| default_quota | |
| description | |
| id | cc490736-d912-4ccd-9919-cfd4f92f34d0 |
| ip_version | 4 |
| is_default | False |
| max_prefixlen | 32 |
| min_prefixlen | 8 |
| name | ww_v4_subnetpool1 |
| prefixes | 172.1.0.0/16 |
| project_id | f0561ceca7874b188e43266c60a65128 |
| revision_number | 0 |
| shared | False |
| tags | |
| tenant_id | f0561ceca7874b188e43266c60a65128 |
| updated_at | 2019-02-19T02:07:29Z |
+-------------------+--------------------------------------+
[root@EXTENV-10-254-8-11 ~]# neutron subnet-create --subnetpool ww_v4_subnetpool1 ww_v4_net1
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
Failed to allocate subnet: Insufficient prefix space to allocate subnet size /8.
Neutron server returns request_ids: ['req-63d8430c-fa6e-4113-a59e-62a2bd97bb7e']

I think when the min_prefixlen is not set,it is better to set it to be the prefixken(16) of pool_prefix (172.1.0.0/16) by default.And when the min_prefixlen is set(as the following test result),it should be checked whether it is bigger than the min prefixlen of pool prefix.

[root@EXTENV-10-254-8-11 ~]# neutron subnetpool-create --min-prefixlen 16 --pool-prefix 172.2.0.0/24 ww_v4_subnetpool2
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
Created a new subnetpool:
+-------------------+--------------------------------------+
| Field | Value |
+-------------------+--------------------------------------+
| address_scope_id | |
| created_at | 2019-02-18T09:33:12Z |
| default_prefixlen | 16 |
| default_quota | |
| description | |
| id | 9da2fc9c-dade-4399-a4a9-fd22df45d3b7 |
| ip_version | 4 |
| is_default | False |
| max_prefixlen | 32 |
| min_prefixlen | 16 |
| name | ww_v4_subnetpool2 |
| prefixes | 172.2.0.0/24 |
| project_id | f0561ceca7874b188e43266c60a65128 |
| revision_number | 0 |
| shared | False |
| tags | |
| tenant_id | f0561ceca7874b188e43266c60a65128 |
| updated_at | 2019-02-18T09:33:12Z |
+-------------------+--------------------------------------+
[root@EXTENV-10-254-8-11 ~]#
[root@EXTENV-10-254-8-11 ~]# neutron subnet-create --subnetpool ww_v4_subnetpool2 ww_v4_net1
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
Failed to allocate subnet: Insufficient prefix space to allocate subnet size /16.
Neutron server returns request_ids: ['req-09744170-b8fb-42e9-ba77-1d63fa75de29']