我似乎无法在 MQTT 中配置持久性

我正在运行 Raspbian 的 Raspberry Pi 上尝试 MQTT。基本设置有效，但我似乎无法配置持久性。

关注https://tutorials-raspberrypi.com/raspberry-pi-mqtt-broker-client-wireless-communication/

我安装了mosquitto mosquitto-clients

我在后台进程中安装paho-mqtt并运行以下脚本，以从连接的传感器发布温度和湿度读数。

#!/usr/bin/python3
import os
import time
import sys
import Adafruit_DHT as dht
import paho.mqtt.client as mqtt
import json
import datetime
#
# Sensor should be set to Adafruit_DHT.DHT11, Adafruit_DHT.DHT22, or Adafruit_DHT.AM2302.
sensor = dht.DHT11
pin = 4

BROKER = 'localhost'

# Data capture and upload interval in seconds.
INTERVAL=15

sensor_data = {'date': 0, 'temperature': 0, 'humidity': 0}

next_reading = time.time()

client = mqtt.Client()

# Connect to BROKER using default MQTT port and 60 seconds keepalive interval
client.connect(BROKER, 1883, 60)

client.loop_start()

try:
    while True:
        humidity,temperature = dht.read_retry(sensor, pin)
        humidity = round(humidity, 2)
        temperature = round(temperature, 2)
        print(u"Temperature: {:g}\u00b0C, Humidity: {:g}%".format(temperature, humidity))
        sensor_data['temperature'] = temperature
        sensor_data['humidity'] = humidity
        sensor_data['date'] = datetime.datetime.now().replace(microsecond=0).isoformat()

#         client.publish('test_channel', json.dumps(sensor_data), 1)
        client.publish('test_channel', json.dumps(sensor_data), 2)

        next_reading += INTERVAL
        sleep_time = next_reading-time.time()
        if sleep_time > 0:
            time.sleep(sleep_time)
except KeyboardInterrupt:
    pass

client.loop_stop()
client.disconnect()

这有效，我可以在另一个 Pi 上看到消息，mosquitto_sub -h IPaddress -v -t test_channel但这仅在进程运行时有效。我希望代理保存消息，直到订阅者连接。

我在文件中添加了“persistence true”，/etc/mosquitto/conf.d/Milliways.conf日志文件显示如下消息

1547597521: Saving in-memory database to /var/lib/mosquitto/mosquitto.db.

该数据库似乎不包含任何相关数据。

我已阅读https://pypi.org/project/paho-mqtt/#publishing和manfor mosquitto 上的文档。