Man Fsync en anglais
FSYNC(2) Linux Programmer's Manual FSYNC(2)
NAME
fsync, fdatasync - synchronize a file's in-core state with storage
device
SYNOPSIS
#include
int fsync(int fd);
int fdatasync(int fd);
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., modi-
fied buffer cache pages for) the file referred to by the file descrip-
tor 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 associ-
ated 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 modifica-
tion; see stat(2)) do not require flushing because they are not neces-
sary 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 synchro-
nization.
CONFORMING TO
4.3BSD, POSIX.1-2001.
AVAILABILITY
On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRO-
NIZED_IO is defined in 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 mod-
ification 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 3.05 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/.
Linux 2007-07-26 FSYNC(2)
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