fsync() transfers ("flushes") all modified in-core data of
(i.e., modified buffer cache pages for) the
file referred to by the file descriptor
fd to the disk device (or other permanent storage device)
where that file resides.
The call blocks until the device reports that the transfer has completed.
It also flushes metadata information associated with the file (see
stat(2)).
Calling
fsync() does not necessarily ensure
that the entry in the directory containing the file has also reached disk.
For that an explicit
fsync() on a file descriptor for the directory is also needed.
fdatasync() is similar to
fsync(), but does not flush modified metadata unless that metadata
is needed in order to allow a subsequent data retrieval to be
correctly handled.
For example, changes to
st_atime or
st_mtime (respectively, time of last access and
time of last modification; see
stat(2))
do not not require flushing because they are not necessary for
a subsequent data read to be handled correctly.
On the other hand, a change to the file size
(st_size, as made by say
ftruncate(2)),
would require a metadata flush.
The aim of
fdatasync(2)
is to reduce disk activity for applications that do not
require all metadata to be synchronised with the disk.
