nova boot from volume在多主机zone下的坑

@require_admin_context
def service_get_all_by_topic(context, topic, disabled=None):
    query = model_query(
        context, models.Service, read_deleted="no").\ # models在这里cinder/db/sqlalchemy/models.py
        filter_by(topic=topic)                        # topic默认传过来的是cinder-volume
    if disabled is not None:
        query = query.filter_by(disabled=disabled)
    return query.all()

# 这里是taskflow，只需关注add方法里面的东东
def get_flow(db_api, p_w_picpath_service_api, availability_zones, create_what,
             scheduler_rpcapi=None, volume_rpcapi=None):
    """Constructs and returns the api entrypoint flow.
    This flow will do the following:
    1. Inject keys & values for dependent tasks.
    2. Extracts and validates the input keys & values.
    3. Reserves the quota (reverts quota on any failures).
    4. Creates the database entry.
    5. Commits the quota.
    6. Casts to volume manager or scheduler for further processing.
    """
    flow_name = ACTION.replace(":", "_") + "_api"
    api_flow = linear_flow.Flow(flow_name)
    api_flow.add(ExtractVolumeRequestTask(
        p_w_picpath_service_api,
        availability_zones,
        rebind={'size': 'raw_size',
                'availability_zone': 'raw_availability_zone',
                'volume_type': 'raw_volume_type'}))
                
    api_flow.add(QuotaReserveTask(),
                 EntryCreateTask(db_api),
                 QuotaCommitTask())
                 
    if scheduler_rpcapi and volume_rpcapi:
        # This will cast it out to either the scheduler or volume manager via
        # the rpc apis provided.
        api_flow.add(VolumeCastTask(scheduler_rpcapi, volume_rpcapi, db_api))
    # Now load (but do not run) the flow using the provided initial data.
    
    return taskflow.engines.load(api_flow, store=create_what)

def execute(self, context, size, snapshot, p_w_picpath_id, source_volume,
                availability_zone, volume_type, metadata, key_manager,
                source_replica, consistencygroup, cgsnapshot):
        utils.check_exclusive_options(snapshot=snapshot,
                                      p_w_picpathRef=p_w_picpath_id,
                                      source_volume=source_volume)
        policy.enforce_action(context, ACTION)
        # TODO(harlowja): what guarantee is there that the snapshot or source
        # volume will remain available after we do this initial verification??
        snapshot_id = self._extract_snapshot(snapshot)
        source_volid = self._extract_source_volume(source_volume)
        source_replicaid = self._extract_source_replica(source_replica)
        size = self._extract_size(size, source_volume, snapshot)
        consistencygroup_id = self._extract_consistencygroup(consistencygroup)
        cgsnapshot_id = self._extract_cgsnapshot(cgsnapshot)
        self._check_p_w_picpath_metadata(context, p_w_picpath_id, size)
        availability_zone = self._extract_availability_zone(availability_zone,  # 关心的在这里
                                                            snapshot,
                                                            source_volume)

# _extract_availability_zone函数                                                          
def _extract_availability_zone(self, availability_zone, snapshot,
                                   source_volume):
        """Extracts and returns a validated availability zone.

        This function will extract the availability zone (if not provided) from
        the snapshot or source_volume and then performs a set of validation
        checks on the provided or extracted availability zone and then returns
        the validated availability zone.
        """

        # Try to extract the availability zone from the corresponding snapshot
        # or source volume if either is valid so that we can be in the same
        # availability zone as the source.
        if availability_zone is None:
            if snapshot:
                try:
                    availability_zone = snapshot['volume']['availability_zone']
                except (TypeError, KeyError):
                    pass
            if source_volume and availability_zone is None:
                try:
                    availability_zone = source_volume['availability_zone']
                except (TypeError, KeyError):
                    pass

        if availability_zone is None:
            if CONF.default_availability_zone:    # default_availability_zone判断
                availability_zone = CONF.default_availability_zone 
            else:
                # For backwards compatibility use the storage_availability_zone
                availability_zone = CONF.storage_availability_zone 

        if availability_zone not in self.availability_zones:     # self.availability_zones就是上述cinder-volume的zone外加cinder.conf中两个配置参数
            if CONF.allow_availability_zone_fallback:    # allow_availability_zone_fallback这个很关键
                original_az = availability_zone
                availability_zone = (
                    CONF.default_availability_zone or
                    CONF.storage_availability_zone)
                LOG.warning(_LW("Availability zone '%(s_az)s' "
                                "not found, falling back to "
                                "'%(s_fallback_az)s'."),
                            {'s_az': original_az,
                             's_fallback_az': availability_zone})
            else:
                msg = _("Availability zone '%(s_az)s' is invalid.")
                msg = msg % {'s_az': availability_zone}
                raise exception.InvalidInput(reason=msg)             #  没启动allow_availability_zone_fallback的话，就抛异常了