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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 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 syn-
       chronized 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  hard-
       disk.   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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-07                          FSYNC(2)