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.

13 files changed, 159 insertions, 7 deletions

diff --git a/erts/configure.in b/erts/configure.in
index a14b10adbf..f11aba1b15 100644
--- a/erts/configure.in
+++ b/erts/configure.in
@@ -1757,6 +1757,12 @@ fi
 dnl Need by run_erl.
 AC_CHECK_FUNCS([openpty])
 
+dnl fdatasync syscall (Unix only)
+AC_CHECK_FUNCS([fdatasync])
+
+dnl Find which C libraries are required to use fdatasync
+AC_SEARCH_LIBS(fdatasync, [rt])
+
 dnl ----------------------------------------------------------------------
 dnl Checks for features/quirks in the system that affects Erlang.
 dnl ----------------------------------------------------------------------
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));
diff --git a/erts/emulator/drivers/common/erl_efile.h b/erts/emulator/drivers/common/erl_efile.h
index 9aa941e550..6821a0e2ee 100644
--- a/erts/emulator/drivers/common/erl_efile.h
+++ b/erts/emulator/drivers/common/erl_efile.h
@@ -126,6 +126,7 @@ int efile_readdir(Efile_error* errInfo, char* name,
 int efile_openfile(Efile_error* errInfo, char* name, int flags,
 		   int* pfd, Sint64* pSize);
 void efile_closefile(int fd);
+int efile_fdatasync(Efile_error* errInfo, int fd);
 int efile_fsync(Efile_error* errInfo, int fd);
 int efile_fileinfo(Efile_error* errInfo, Efile_info* pInfo,
 		   char *name, int info_for_link);
diff --git a/erts/emulator/drivers/common/ram_file_drv.c b/erts/emulator/drivers/common/ram_file_drv.c
index 4a39a156e6..d4e547ade6 100644
--- a/erts/emulator/drivers/common/ram_file_drv.c
+++ b/erts/emulator/drivers/common/ram_file_drv.c
@@ -35,6 +35,7 @@
 #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
@@ -558,6 +559,13 @@ static void rfile_command(ErlDrvData e, char* buf, int count)
 	    numeric_reply(f, 0); /* 0 is not used */
 	break;
 
+    case RAM_FILE_FDATASYNC:
+	if (f->flags == 0)
+	    error_reply(f, EBADF);
+	else
+	    reply(f, 1, 0);
+	break;
+
     case RAM_FILE_FSYNC:
 	if (f->flags == 0)
 	    error_reply(f, EBADF);
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. */
 {
diff --git a/erts/emulator/drivers/win32/win_efile.c b/erts/emulator/drivers/win32/win_efile.c
index 89aaad31da..cdf3bb424c 100644
--- a/erts/emulator/drivers/win32/win_efile.c
+++ b/erts/emulator/drivers/win32/win_efile.c
@@ -764,6 +764,15 @@ int fd;				/* File descriptor for file to close. */
 }
 
 int
+efile_fdatasync(errInfo, fd)
+Efile_error* errInfo;		/* Where to return error codes. */
+int fd;				/* File descriptor for file to sync. */
+{
+    /* Not available in Windows, just call regular fsync */
+    return efile_fsync(errInfo, fd);
+}
+
+int
 efile_fsync(errInfo, fd)
 Efile_error* errInfo;		/* Where to return error codes. */
 int fd;				/* File descriptor for file to sync. */
diff --git a/erts/preloaded/src/prim_file.erl b/erts/preloaded/src/prim_file.erl
index 43e6f6cd88..53c741e66e 100644
--- a/erts/preloaded/src/prim_file.erl
+++ b/erts/preloaded/src/prim_file.erl
@@ -25,7 +25,7 @@
 %%% Interface towards a single file's contents. Uses ?FD_DRV.
 
 %% Generic file contents operations
--export([open/2, close/1, sync/1, position/2, truncate/1,
+-export([open/2, close/1, datasync/1, sync/1, position/2, truncate/1,
 	 write/2, pwrite/2, pwrite/3, read/2, read_line/1, pread/2, pread/3, copy/3]).
 
 %% Specialized file operations
@@ -96,6 +96,7 @@
 -define(FILE_IPREAD,           27).
 -define(FILE_ALTNAME,          28).
 -define(FILE_READ_LINE,        29).
+-define(FILE_FDATASYNC,        30).
 
 %% Driver responses
 -define(FILE_RESP_OK,          0).
@@ -292,6 +293,9 @@ pwrite(#file_descriptor{module = ?MODULE}, _, _) ->
     {error, badarg}.
 
 
+%% Returns {error, Reason} | ok.
+datasync(#file_descriptor{module = ?MODULE, data = {Port, _}}) ->
+    drv_command(Port, [?FILE_FDATASYNC]).
 
 %% Returns {error, Reason} | ok.
 sync(#file_descriptor{module = ?MODULE, data = {Port, _}}) ->
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) ->