logger

#https://www.home-assistant.io/integrations/logger/

logger:
  default: info
  logs:
      # log level for HA core
    homeassistant.core: debug
    # Log level for Homekit
    homeassistant.components.homekit_controller: debug
    homeassistant.components.bluetooth: debug

    #https://github.com/ha0y/xiaomi_miot_raw
    custom_components.xiaomi_miot_raw: debug
    miio: debug

logger

If you set the logging for recorder to debug,

logger:
  default: warn
  logs:
    homeassistant.components.recorder: debug

you will see what the purge does.

2019-04-10 16:22:22 DEBUG (Recorder) [homeassistant.components.recorder.purge] Purging events before 2019-04-02 14:22:22.370017+00:00
2019-04-10 16:22:24 DEBUG (Recorder) [homeassistant.components.recorder.purge] Deleted 5147 states
2019-04-10 16:22:30 DEBUG (Recorder) [homeassistant.components.recorder.purge] Deleted 5520 events

https://community.home-assistant.io/t/recorder-purge-and-repack/110552/4?u=msly