| isfd | isam file identifier |
SUCCESS, or a negative value on error, in which case the error code is found in iserrno
isflush calls fsync(2) on both the index and data files, which instructs the system to flush all cached updates.
isflush will not return until all data has been written.
if supported by your compiler, fdatasync(2) is used, which does not flush non essential meta data such as file access and modification times, for better performance.