The change that should be there is that if you run multiple rails instances against the same log file you should for concurrent requests get interleaved lines s

The change that should be there is that if you run multiple rails instances against the same log file you should for concurrent requests get interleaved lines since it is written line by line immediately to the file (though not necessarily flushed to the HDD). If you get scrambled lines (ie the contents of two lines can mix up) then I personally would see that as a bug. No change to IO#sync, IO#fdatasync or the filesystem settings should make it revert to the old behavior. If you set IO#sync to false it might seem like you get something like the old behviour back, but I think you risk scrambled lines since it will the flush when Ruby's buffer is full.