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, 23 insertions, 0 deletions

diff --git a/erts/emulator/drivers/common/efile_drv.c b/erts/emulator/drivers/common/efile_drv.c
index d2b916000e..b64d27de68 100644
--- a/erts/emulator/drivers/common/efile_drv.c
+++ b/erts/emulator/drivers/common/efile_drv.c
@@ -53,6 +53,7 @@
 #define FILE_IPREAD             27
 #define FILE_ALTNAME            28
 #define FILE_READ_LINE          29
+#define FILE_FDATASYNC          30
 
 /* Return codes */
 
@@ -883,6 +884,15 @@ static void invoke_chdir(void *data)
     invoke_name(data, efile_chdir);
 }
 
+static void invoke_fdatasync(void *data)
+{
+    struct t_data *d = (struct t_data *) data;
+    int fd = (int) d->fd;
+
+    d->again = 0;
+    d->result_ok = efile_fdatasync(&d->errInfo, fd);
+}
+
 static void invoke_fsync(void *data)
 {
     struct t_data *d = (struct t_data *) data;
@@ -1919,6 +1929,7 @@ file_async_ready(ErlDrvData e, ErlDrvThreadData data)
       case FILE_RMDIR:
       case FILE_CHDIR:
       case FILE_DELETE:
+      case FILE_FDATASYNC:
       case FILE_FSYNC:
       case FILE_TRUNCATE:
       case FILE_LINK:
@@ -2209,6 +2220,18 @@ file_output(ErlDrvData e, char* buf, int count)
 	    goto done;
 	}
 
+    case FILE_FDATASYNC:
+    {
+	    d = EF_SAFE_ALLOC(sizeof(struct t_data));
+
+	    d->fd = fd;
+	    d->command = command;
+	    d->invoke = invoke_fdatasync;
+	    d->free = free_data;
+	    d->level = 2;
+	    goto done;
+    }
+
     case FILE_FSYNC:
     {
 	d = EF_SAFE_ALLOC(sizeof(struct t_data));