If you shut down cleanly, do you still get a file size of 0? FYI, files are not written in a single pass, at least so far as caching and buffers going to actual disk are concerned. When you first write a file, and then read a file, you are only guaranteed that the buffer has the content…the content may not have been committed to disk yet. In that case a clean shutdown would guarantee flush of buffer and cache to disk prior to completing shutdown. This is completely unpredictable as the schedule for committing to disk is generally out of our hands. If you still get an error after a clean shutdown, where you know the file size is tested as greater than zero prior to shutdown, but then shows as zero after reboot, then you have a genuine bug.
You wouldn’t want to change an old technology mechanical hard drive to run synchronously due to extreme loss of performance. You wouldn’t want to change eMMC to synchronous due to both a dramatic performance loss and due to death of the eMMC in a short time.
If you have checked that a file is greater than zero size, and then the file reverts to zero after a clean shutdown, then you should note that on the forums because it would be a real bug. All bets are off with loss of power or unclean shutdown. 10 seconds of wait won’t matter since there is a kind of scheduler…the system won’t commit writes on a solid state until it needs to, this is the way the driver protects solid state memory from wearing out too soon. Even on an old tech hard drive you wouldn’t get a “guarantee” at 10 seconds, but eMMC purposely avoids unnecessary writes…the scheduler has no idea you are about to pull the plug.