I found this interesting sketch which allows logging data on internal memory:
https://github.com/fabiuz7/esp-logger-l ... logger.ino
But I can't figure out how much "mass storage" memory I have available.
Additionally, I need realtime logging, which is said not to be supported by SPIFFS, so I will have to store some data in RAM before writing to "disk"; but, again, I don't know how much memory I have available for variables storage: how long can a string be in ESP32?