fdatasync flushes all data buffers of a file to disk (before the system
call returns). It resembles fsync but is not required to update the
metadata such as access time.
Applications that access databases or log files often write a tiny data fragment
(e.g., one line in a log file) and then call fsync immediately in order
to ensure that the written data is physically stored on the harddisk.
Unfortunately, fsync will always initiate two write operations: one for
the newly written data and another one in order to update the modification
time stored in the inode. If the modification time is not a part of the
transaction concept fdatasync can be used to avoid unnecessary inode
disk write operations.