fsync, fdatasync — synchronize a file's in-core state with storage device
("flushes") all modified in-core data of (i.e., modified
buffer cache pages for) the file referred to by the file
fd to the
disk device (or other permanent storage device) so that all
changed information can be retrieved even after the system
crashed or was rebooted. This includes writing through or
flushing a disk cache if present. The call blocks until the
device reports that the transfer has completed. It also
flushes metadata information associated with the file (see
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_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
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.
On success, these system calls return zero. On error,
−1 is returned, and
is set appropriately.
fd is not a
valid open file descriptor.
An error occurred during synchronization.
fd is bound
to a special file (e.g., a pipe, FIFO, or socket) which
does not support synchronization.
On POSIX systems on which
fdatasync() is available,
_POSIX_SYNCHRONIZED_IO is defined in
to a value greater than 0. (See also sysconf(3).)
On some UNIX systems (but not Linux),
fd must be a
writable file descriptor.
In Linux 2.2 and earlier,
fdatasync() is equivalent to
fsync(), and so has no performance
in older kernels and lesser used filesystems does not know
how to flush disk caches. In these cases disk caches need to
be disabled using hdparm(8) or sdparm(8) to guarantee safe
This page is part of release 4.07 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
Copyright 1993 Rickard E. Faith (faithcs.unc.edu) and
and Copyright 2006 Michael Kerrisk <mtk.manpagesgmail.com>
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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.
2012-02-27 Various changes by Christoph Hellwig <hchlst.de>