| isfd | isam file descriptor |
ISTRUE on success, else ISFALSE on error, in which case the error code will be found in the errno global.
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.