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.

6 files changed, 96 insertions, 6 deletions

diff --git a/lib/kernel/doc/src/file.xml b/lib/kernel/doc/src/file.xml
index 50f9722a1c..c94df62d1f 100644
--- a/lib/kernel/doc/src/file.xml
+++ b/lib/kernel/doc/src/file.xml
@@ -1641,6 +1641,33 @@ f.txt:  {person, "kalle", 25}.
       </desc>
     </func>
     <func>
+      <name>datasync(IoDevice) -> ok | {error, Reason}</name>
+      <fsummary>Synchronizes the in-memory data of a file, ignoring most of its metadata, with that on the physical medium</fsummary>
+      <type>
+        <v>IoDevice = io_device()</v>
+        <v>Reason = ext_posix() | terminated</v>
+      </type>
+      <desc>
+        <p>Makes sure that any buffers kept by the operating system
+          (not by the Erlang runtime system) are written to disk. In
+          many ways it's resembles fsync but it not requires to update
+          some of file's metadata such as the access time. On
+          some platforms, this function might have no effect.</p>
+        <p>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.</p>
+        <p>Available only in some POSIX systems. This call results in a
+          call to fsync(), or has no effect, in systems not implementing
+          the fdatasync syscall.</p>
+      </desc>
+    </func>
+    <func>
       <name>truncate(IoDevice) -> ok | {error, Reason}</name>
       <fsummary>Truncate a file</fsummary>
       <type>
diff --git a/lib/kernel/src/file.erl b/lib/kernel/src/file.erl
index 46ffa9d708..5da359fdf1 100644
--- a/lib/kernel/src/file.erl
+++ b/lib/kernel/src/file.erl
@@ -40,7 +40,7 @@
 	 read/2, write/2, 
 	 pread/2, pread/3, pwrite/2, pwrite/3,
 	 read_line/1,
-	 position/2, truncate/1, sync/1,
+	 position/2, truncate/1, datasync/1, sync/1,
 	 copy/2, copy/3]).
 %% High level operations
 -export([consult/1, path_consult/2]).
@@ -472,6 +472,16 @@ pwrite(#file_descriptor{module = Module} = Handle, Offs, Bytes) ->
 pwrite(_, _, _) ->
     {error, badarg}.
 
+-spec datasync(File :: io_device()) -> 'ok' | {'error', posix()}.
+
+datasync(File) when is_pid(File) ->
+    R = file_request(File, datasync),
+    wait_file_reply(File, R);
+datasync(#file_descriptor{module = Module} = Handle) ->
+    Module:datasync(Handle);
+datasync(_) ->
+    {error, badarg}.
+
 -spec sync(File :: io_device()) -> 'ok' | {'error', posix()}.
 
 sync(File) when is_pid(File) ->
diff --git a/lib/kernel/src/file_io_server.erl b/lib/kernel/src/file_io_server.erl
index 3ac35a209d..d0b30192b3 100644
--- a/lib/kernel/src/file_io_server.erl
+++ b/lib/kernel/src/file_io_server.erl
@@ -219,6 +219,14 @@ file_request({pwrite,At,Data},
 	Reply ->
 	    std_reply(Reply, State)
     end;
+file_request(datasync,
+	     #state{handle=Handle}=State) ->
+    case ?PRIM_FILE:datasync(Handle) of
+	{error,_}=Reply ->
+	    {stop,normal,Reply,State};
+	Reply ->
+	    {reply,Reply,State}
+    end;
 file_request(sync, 
 	     #state{handle=Handle}=State) ->
     case ?PRIM_FILE:sync(Handle) of
diff --git a/lib/kernel/src/ram_file.erl b/lib/kernel/src/ram_file.erl
index d996650948..0230cb4e0a 100644
--- a/lib/kernel/src/ram_file.erl
+++ b/lib/kernel/src/ram_file.erl
@@ -24,7 +24,7 @@
 -export([open/2, close/1]).
 -export([write/2, read/2, copy/3,
 	 pread/2, pread/3, pwrite/2, pwrite/3, 
-	 position/2, truncate/1, sync/1]).
+	 position/2, truncate/1, datasync/1, sync/1]).
 
 %% Specialized file operations
 -export([get_size/1, get_file/1, set_file/2, get_file_close/1]).
@@ -60,6 +60,7 @@
 -define(RAM_FILE_TRUNCATE,       14).
 -define(RAM_FILE_PREAD,          17).
 -define(RAM_FILE_PWRITE,         18).
+-define(RAM_FILE_FDATASYNC,      19).
 
 %% Other operations
 -define(RAM_FILE_GET,            30).
@@ -167,6 +168,8 @@ copy(#file_descriptor{module = ?MODULE} = Source,
     %% XXX Should be moved down to the driver for optimization.
     file:copy_opened(Source, Dest, Length).
 
+datasync(#file_descriptor{module = ?MODULE, data = Port}) ->
+    call_port(Port, <<?RAM_FILE_FDATASYNC>>).
 
 sync(#file_descriptor{module = ?MODULE, data = Port}) -> 
     call_port(Port, <<?RAM_FILE_FSYNC>>).
diff --git a/lib/kernel/test/file_SUITE.erl b/lib/kernel/test/file_SUITE.erl
index 1d170790a3..2e6c3aba16 100644
--- a/lib/kernel/test/file_SUITE.erl
+++ b/lib/kernel/test/file_SUITE.erl
@@ -52,7 +52,7 @@
 	 old_modes/1, new_modes/1, path_open/1, open_errors/1]).
 -export([file_info/1, file_info_basic_file/1, file_info_basic_directory/1,
 	 file_info_bad/1, file_info_times/1, file_write_file_info/1]).
--export([rename/1, access/1, truncate/1, sync/1,
+-export([rename/1, access/1, truncate/1, datasync/1, sync/1,
 	 read_write/1, pread_write/1, append/1]).
 -export([errors/1, e_delete/1, e_rename/1, e_make_dir/1, e_del_dir/1]).
 -export([otp_5814/1]).
@@ -377,7 +377,7 @@ win_cur_dir_1(_Config) ->
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
-files(suite) -> [open,pos,file_info,consult,eval,script,truncate,sync].
+files(suite) -> [open,pos,file_info,consult,eval,script,truncate,sync,datasync].
 
 open(suite) -> [open1,old_modes,new_modes,path_open,close,access,read_write,
 	       pread_write,append,open_errors].
@@ -1355,6 +1355,30 @@ truncate(Config) when is_list(Config) ->
     ok.
 
 
+datasync(suite) -> [];
+datasync(doc) -> "Tests that ?FILE_MODULE:datasync/1 at least doesn't crash.";
+datasync(Config) when is_list(Config) ->
+    ?line Dog = test_server:timetrap(test_server:seconds(5)),
+    ?line PrivDir = ?config(priv_dir, Config),
+    ?line Sync = filename:join(PrivDir,
+			       atom_to_list(?MODULE)
+			       ++"_sync.fil"),
+
+    %% Raw open.
+    ?line {ok, Fd} = ?FILE_MODULE:open(Sync, [write, raw]),
+    ?line ok = ?FILE_MODULE:datasync(Fd),
+    ?line ok = ?FILE_MODULE:close(Fd),
+
+    %% Ordinary open.
+    ?line {ok, Fd2} = ?FILE_MODULE:open(Sync, [write]),
+    ?line ok = ?FILE_MODULE:datasync(Fd2),
+    ?line ok = ?FILE_MODULE:close(Fd2),
+
+    ?line [] = flush(),
+    ?line test_server:timetrap_cancel(Dog),
+    ok.
+
+
 sync(suite) -> [];
 sync(doc) -> "Tests that ?FILE_MODULE:sync/1 at least doesn't crash.";
 sync(Config) when is_list(Config) ->
diff --git a/lib/kernel/test/prim_file_SUITE.erl b/lib/kernel/test/prim_file_SUITE.erl
index 6badbb5090..842945e83c 100644
--- a/lib/kernel/test/prim_file_SUITE.erl
+++ b/lib/kernel/test/prim_file_SUITE.erl
@@ -34,7 +34,7 @@
 	 file_info_times_a/1, file_info_times_b/1, 
 	 file_write_file_info_a/1, file_write_file_info_b/1]).
 -export([rename_a/1, rename_b/1, 
-	 access/1, truncate/1, sync/1,
+	 access/1, truncate/1, datasync/1, sync/1,
 	 read_write/1, pread_write/1, append/1]).
 -export([errors/1, e_delete/1, e_rename/1, e_make_dir/1, e_del_dir/1]).
 
@@ -380,7 +380,7 @@ win_cur_dir_1(_Config, Handle) ->
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
-files(suite) -> [open,pos,file_info,truncate,sync].
+files(suite) -> [open,pos,file_info,truncate,sync,datasync].
 
 open(suite) -> [open1,modes,close,access,read_write,
 	       pread_write,append].
@@ -1064,6 +1064,24 @@ truncate(Config) when is_list(Config) ->
     ok.
 
 
+datasync(suite) -> [];
+datasync(doc) -> "Tests that ?PRIM_FILE:datasync/1 at least doesn't crash.";
+datasync(Config) when is_list(Config) ->
+    ?line Dog = test_server:timetrap(test_server:seconds(5)),
+    ?line PrivDir = ?config(priv_dir, Config),
+    ?line Sync = filename:join(PrivDir,
+			       atom_to_list(?MODULE)
+			       ++"_sync.fil"),
+
+    %% Raw open.
+    ?line {ok, Fd} = ?PRIM_FILE:open(Sync, [write]),
+    ?line ok = ?PRIM_FILE:datasync(Fd),
+    ?line ok = ?PRIM_FILE:close(Fd),
+
+    ?line test_server:timetrap_cancel(Dog),
+    ok.
+
+
 sync(suite) -> [];
 sync(doc) -> "Tests that ?PRIM_FILE:sync/1 at least doesn't crash.";
 sync(Config) when is_list(Config) ->