Add file:datasync/1 for syncing file contents only

file:datasync/1 invokes the POSIX system call "int fdatasync(int fd)".
This system call is similar to "fsync" but, unlike fsync, it does not
update the metadata associated with the file (like the access time for
example). It's used by many DBMSs (MySQL and SQLite of example) to
increase disk IO performance, as it avoids disk seeks and disk write
operations compared to fsync.

More details on it at:

http://linux.die.net/man/2/fdatasync

An example, from the MySQL source:

http://bazaar.launchpad.net/~mysql/mysql-server/mysql-5.1-telco-6.1/annotate/head%3A/mysys/my_sync.c#L61

This new function just calls fsync on systems not implementing fdatasync.

1 files changed, 11 insertions, 0 deletions

diff --git a/erts/emulator/drivers/unix/unix_efile.c b/erts/emulator/drivers/unix/unix_efile.c
index 1d094ee613..0cece76221 100644
--- a/erts/emulator/drivers/unix/unix_efile.c
+++ b/erts/emulator/drivers/unix/unix_efile.c
@@ -774,6 +774,17 @@ efile_closefile(int fd)
 }
 
 int
+efile_fdatasync(Efile_error *errInfo, /* Where to return error codes. */
+	    int fd)               /* File descriptor for file to sync data. */
+{
+#ifdef HAVE_FDATASYNC
+    return check_error(fdatasync(fd), errInfo);
+#else
+    return efile_fsync(errInfo, fd);
+#endif
+}
+
+int
 efile_fsync(Efile_error *errInfo, /* Where to return error codes. */
 	    int fd)               /* File descriptor for file to sync. */
 {