fsync, fdatasync

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);

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.

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.