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       fsync,  fdatasync  -  synchronize  a  file’s in-core state with storage


       #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


       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.


       On success, these system calls return zero.  On error, -1 is  returned,
       and errno is set appropriately.


       EBADF  fd is not a valid file descriptor open for writing.

       EIO    An error occurred during synchronization.

              fd   is   bound  to  a  special  file  which  does  not  support


       4.3BSD, POSIX.1-2001.


       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).)


       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()

       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.


       bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8),  mount(8),
       sync(8), update(8)


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