fsync copies all in-core parts of a file to disk, and waits until the
device reports that all parts are on stable storage. It also updates metadata
stat information. It does not necessarily ensure that the entry in the
directory containing the file has also reached disk. For that an explicit
fsync on the file descriptor of the directory is also needed.
fdatasync does the same as fsync but only flushes user data, not
the meta data like the mtime or atime.
In case the hard disk has write cache enabled, the data may not really be on
permanent storage when fsync/fdatasync return.
When an ext2 file system is mounted with the sync option, directory
entries are also implicitly synced by fsync.
On kernels before 2.4, fsync on big files can be inefficient. An
alternative might be to use the O_SYNC flag to open(2).