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);
[Note] Note
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., 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 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 2.79 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/.


Copyright 1993 Rickard E. Faith (faithcs.unc.edu) and
and Copyright 2006 Michael Kerrisk <mtk.manpagesgmail.com>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
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permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
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the use of the information contained herein.  The author(s) may not
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Formatted or processed versions of this manual, if unaccompanied by
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Modified 21 Aug 1994 by Michael Chastain <mecshell.portal.com>:
  Removed note about old libc (pre-4.5.26) translating to 'sync'.
Modified 15 Apr 1995 by Michael Chastain <mecshell.portal.com>:
  Added `see also' section.
Modified 13 Apr 1996 by Markus Kuhn <mskuhncip.informatik.uni-erlangen.de>
  Added remarks about fdatasync.
Modified 31 Jan 1997 by Eric S. Raymond <esrthyrsus.com>
Modified 18 Apr 2001 by Andi Kleen
  Fix description to describe what it really does; add a few caveats.
2006-04-28, mtk, substantial rewrite of various parts.