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.

7 files changed, 63 insertions, 1 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, _}}) ->