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There is a speed issue when sending the files from SD card over HTTP.
The problem is, that the SD-specific class
File
inherited fromStream
does not override thevirtual size_t readBytes(char *buffer, size_t length)
method, while this method is used under the hood of
ESP8266WebServer::streamFile
:The default implementation of
readBytes
in theStream
class does not actually use buffered reading, just a loop reading bytes one by one (throughStream::timedRead()
method), which is obviously slow.The proposed fix redirects
readBytes
to buffered read of SD-specific File class.This speeds up the
ESP8266WebServer::streamFile
more than 3 times. Tested on streaming the 800+ Kb file from SD (FAT32), average time without a fix was 9000 ms, with the fix is 2600 ms (maximal possible SPI speed used), which is as fast as streaming the same file from internal SPIFFS. Hardware: WeMos D1 mini.