My openstack all-in-one environment is setup by devstack. I created a server-group with anti-affinity policy. 

Then an instance was created with this server-group successfully with command[1]. Another instance was also 

created with this server-group and with force_host successfully with command[2]. But if I did not specify the 

force_host, the instance failed to create[3].

I think the second instance had broken the anti-affinity server-group.

So my question is that whether the result is within design scope or it is a issue and needs to be discussed : )

[1] nova boot <instance-name> --flavor <flavor> --image <image> --availability-zone <az> --security-groups <sec-group> --nic net-id=<net-id> --hint group=<server-group>

[2] nova boot <instance-name> --flavor <flavor> --image <image> --availability-zone <az>:<host> --security-groups <sec-group> --nic net-id=<net-id> --hint group=<server-group>

[3] nova boot <instance-name> --flavor <flavor> --image <image> --availability-zone <az> --security-groups <sec-group> --nic net-id=<net-id> --hint group=<server-group>

Best Regards

Boxiang