ESP32 支持 SPIFF 了，但是用 ESP8266 的代码测试确无法使用。
查了下资料，需要给 begin 添加一个参数。之后工作正常。

```
if(!SPIFFS.begin(true)){‌‌‌‌‌

Serial.println("SPIFFS Mount Failed");

return;

}
```

比较完整的示例：
```
#include "SPIFFS.h"

void setup() {‌‌‌‌

Serial.begin(115200);

if (!SPIFFS.begin(true)) {‌‌‌‌
Serial.println("An Error has occurred while mounting SPIFFS");
return;
}

File file = SPIFFS.open("/test.txt", FILE_WRITE);

if (!file) {‌‌‌‌
Serial.println("There was an error opening the file for writing");
return;
}
if (file.print("TEST")) {‌‌‌‌
Serial.println("File was written");
} else {‌‌‌‌
Serial.println("File write failed");
}

file.close();
}

void loop() {‌‌‌‌}
```

https://github.com/espressif/arduino-esp32/blob/master/libraries/SPIFFS/examples/SPIFFS_Test/SPIFFS_Test.ino

https://www.tutorialspoint.com/esp32_for_iot/esp32_for_iot_spiffs_storage.htm