Comment 1 for bug 1390033

With some added debug logs, the most possible root cause is around the function "get_host_availability_zone()" in the module "nova.availability_zones".
In particular, in this function, if the variable "host" is None, then the returned "aggregates" and "metadata" will also be None. Consequently, the final retrieved value of "az" will be the default one of "nova" that is hard-coded in the module.

During the process of launching new VM instance, this function is called multiple times. At the first time of being called, the passed value of "host" is always None (and the task_state is SCHEDULING). However, for subsequent calls, the "host" is returned correctly (no longer None) and everything becomes correct again.

So, the question is why the "host" value passed is None when the task_state is SCHEDULING??

Further investigation is being carried out.

Horizon is not guilty in this story but it is affected.
Nova is the guilty one.