fsync() - Unix, Linux System Call

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fsync, fdatasync - synchronize a file’s in-core state with storage device
#include <unistd.h> 
int fsync(int fd); 
int fdatasync(int fd); 


int fsync(int fd); 

int fdatasync(int fd); 

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.
On success, zero is returned.  On error, -1 is returned, and
errno is set appropriately.
If the underlying hard disk has write caching enabled, then
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).
POSIX.1-2001
bdflush (2)
bdflush (2)
open (2)
open (2)
sync (2)
sync (2)
sync_file_range (2)
sync_file_range (2)
hdparm (8)
hdparm (8)
mount (8)
mount (8)
sync (8)
sync (8)
update (8)
update (8)

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