Using dashboard launching VMs, the setting is that 'Instance Count : 2, Instance Boot Source : Boot from image'. After clicking the button of 'launch', the simultaneous launch of two VMs failed.
the nova-api.log shows as follows.
"/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1356, in create
2014-08-05 21:47:46.117 25518 TRACE nova.api.openstack self._check_multiple_instances_neutron_ports(requested_networks)
2014-08-05 21:47:46.117 25518 TRACE nova.api.openstack File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1327, in _check_multiple_instances_neutron_ports
2014-08-05 21:47:46.117 25518 TRACE nova.api.openstack for net, ip, port in requested_networks:
2014-08-05 21:47:46.117 25518 TRACE nova.api.openstack ValueError: too many values to unpack
beacause of the error log 'too many values to unpack', I modifed the code of 'for (net, ip, port) in requested_ networks: ' to 'for (net, ip, port, vnic_type) in requested_ networks: ' in nova/compute/ api.py, and the simultaneous launch of two VMs successed.