FSYNC(2)		   Linux Programmers Manual		     FSYNC(2)



NAME
       fsync,  fdatasync  -  synchronize  a  files in-core state with storage
       device

SYNOPSIS
       #include 

       int fsync(int fd);

       int fdatasync(int fd);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fsync(): _BSD_SOURCE || _XOPEN_SOURCE
       fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500

DESCRIPTION
       fsync() transfers ("flushes") all modified in-core data of (i.e., modi
       fied  buffer cache pages for) the file referred to by the file descrip
       tor 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  associ
       ated 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 modifica
       tion; see stat(2)) do not require flushing because they are not	neces
       sary  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() is to reduce disk activity for applications that
       do not require all metadata to be synchronized with the disk.

RETURN VALUE
       On success, these system calls return zero.  On error, -1 is  returned,
       and errno is set appropriately.

ERRORS
       EBADF  fd is not a valid file descriptor open for writing.

       EIO    An error occurred during synchronization.

       EROFS, EINVAL
	      fd  is  bound  to a special file which does not support synchro
	      nization.

CONFORMING TO
       4.3BSD, POSIX.1-2001.

AVAILABILITY
       On POSIX systems on which  fdatasync()  is  available,  _POSIX_SYNCHRO
       NIZED_IO is defined in  to a value greater than 0.  (See also
       sysconf(3).)

NOTES
       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 mod
       ification time is not a part of the transaction concept fdatasync() can
       be used to avoid unnecessary inode disk write operations.

       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).

       In  Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so
       has no performance advantage.

SEE ALSO
       bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8),  mount(8),
       sync(8), update(8)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  2007-07-26			      FSYNC(2)