Subscription QoS should be 2

[haphut]

At least in MQTT v3.1.1, the QoS of a received message never surpasses the QoS of a published message: "The QoS of Payload Messages sent in response to a Subscription MUST be the minimum of the QoS of the originally published message and the maximum QoS granted by the Server."

Therefore for archival purposes its sensible to always try to subscribe with QoS 2. Otherwise it is almost meaningless to store the QoS of a received message.