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.
This page is part of release 3.14 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/.
