man-pages/man2/fsync.2

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.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
.\"   Removed note about old libc (pre-4.5.26) translating to 'sync'.
.\" Modified 15 Apr 1995 by Michael Chastain <mec@shell.portal.com>:
.\"   Added `see also' section.
.\" Modified 13 Apr 1996 by Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"   Added remarks about fdatasync.
.\" Modified 31 Jan 1997 by Eric S. Raymond <esr@thyrsus.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.
.\"
.TH FSYNC 2 2008-11-07 "Linux" "Linux Programmer's Manual"
.SH NAME
fsync, fdatasync \- synchronize a file's in-core state with storage device
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "int fsync(int " fd );
.sp
.BI "int fdatasync(int " fd );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR fsync ():
_BSD_SOURCE || _XOPEN_SOURCE
.br
         || /* since glibc 2.8: */ _POSIX_C_SOURCE\ >=\ 200112L
.\" _POSIX_C_SOURCE\ >=\ 200112L only since glibc 2.8
.br
.BR fdatasync ():
_POSIX_C_SOURCE\ >=\ 199309L || _XOPEN_SOURCE\ >=\ 500
.SH DESCRIPTION
.BR fsync ()
transfers ("flushes") all modified in-core data of
(i.e., modified buffer cache pages for) the
file referred to by the file descriptor
.I 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
.BR stat (2)).

Calling
.BR fsync ()
does not necessarily ensure
that the entry in the directory containing the file has also reached disk.
For that an explicit
.BR fsync ()
on a file descriptor for the directory is also needed.

.BR fdatasync ()
is similar to
.BR 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
.I st_atime
or
.I st_mtime
(respectively, time of last access and
time of last modification; see
.BR 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
.RI ( st_size ,
as made by say
.BR ftruncate (2)),
would require a metadata flush.

The aim of
.BR fdatasync ()
is to reduce disk activity for applications that do not
require all metadata to be synchronized with the disk.
.SH "RETURN VALUE"
On success, these system calls return zero.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor open for writing.
.TP
.B EIO
An error occurred during synchronization.
.TP
.BR EROFS ", " EINVAL
.I fd
is bound to a special file which does not support synchronization.
.SH "CONFORMING TO"
4.3BSD, POSIX.1-2001.
.SH AVAILABILITY
On POSIX systems on which
.BR fdatasync ()
is available,
.B _POSIX_SYNCHRONIZED_IO
is defined in
.I <unistd.h>
to a value greater than 0.
(See also
.BR sysconf (3).)
.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
.\" -1: unavailable, 0: ask using sysconf().
.\" glibc defines them to 1.
.SH 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
.BR fsync ()
immediately in order to ensure that the written data is physically
stored on the harddisk.
Unfortunately,
.BR 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
.BR 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
.BR fsync ()
/
.BR fdatasync ()
return.
.\" See
.\" .BR hdparm (8)
.\" for how to disable that cache for IDE disks.
.LP
When an ext2 file system is mounted with the
.I sync
option, directory entries are also implicitly synced by
.BR fsync ().
.LP
On kernels before 2.4,
.BR fsync ()
on big files can be inefficient.
An alternative might be to use the
.B O_SYNC
flag to
.BR open (2).

In Linux 2.2 and earlier,
.BR fdatasync ()
is equivalent to
.BR fsync (),
and so has no performance advantage.
.SH "SEE ALSO"
.BR bdflush (2),
.BR open (2),
.BR sync (2),
.BR sync_file_range (2),
.BR hdparm (8),
.BR mount (8),
.BR sync (8),
.BR update (8)