SPIFFS

Peter Andersson built it for microcontrollers with little RAM and an external SPI flash chip. SPIFFS has no directories — the slash in /config/wifi.json is just another character in the filename — and it spreads writes across the whole chip to even out wear. It became the default on the ESP8266 and early ESP32 projects, usually as the place where the built-in web server kept its HTML.

Its weak spots were performance that degraded as the chip filled up and limited protection against power loss. The project is no longer maintained, ESP-IDF marks it deprecated, and the ESP8266 Arduino core has moved to LittleFS. Devices already in the field still run it, naturally; nobody wants to attempt a filesystem migration over the air.