FSYNC(2)                                            Linux Programmer's Manual                                           FSYNC(2)



NAME
       fsync, fdatasync - synchronize a file's in-core state with storage device

SYNOPSIS
       #include <unistd.h>

       int fsync(int fd);

       int fdatasync(int fd);

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

       fsync(): _BSD_SOURCE || _XOPEN_SOURCE
                || /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
       fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500

DESCRIPTION
       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 require flushing because they are not necessary for a sub-
       sequent data read to be handled correctly.  On the other hand, a change to the file size (st_size, as made by say  ftrun-
       cate(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 synchronization.

CONFORMING TO
       4.3BSD, POSIX.1-2001.

AVAILABILITY
       On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_IO is defined in <unistd.h> 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 modification 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 3.25 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                                                      2008-11-07                                                   FSYNC(2)