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 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 3.25 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/.