How to run Tinylicious with inMemory: false?

Question

1. I Launch demo app from https://github.com/microsoft/FluidExamples
2. I Use "inMemory": false in "config.json"

     "db": {
         "inMemory": false,
         "path": "/var/tmp/db"
     }

3. I Launch the "Tinylicious" server

4. When I try to create a new document, I get an error:

error: Error writing checkpoint to MongoDB: {} {"messageMetaData": {"documentId": "1610173896601", "tenantId": "tinylicious"}, "label": "winston", "timestamp": "2021-01- 09T06: 32: 34.968Z "}

what am I doing wrong? thks!