Add MDB_NOMETASYNC env option.

Just a trial. This may not make sense if we decide to split the
meta pages into their own separate file, to allow meta traffic to
reside on a separate spindle.

libraries/libmdb/mdb.c

@@ -3037,7 +3037,7 @@ mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mode_t mode)
 	}
 
 	if ((rc = mdb_env_open2(env, flags)) == MDB_SUCCESS) {
-		if (flags & (MDB_RDONLY|MDB_NOSYNC)) {
+		if (flags & (MDB_RDONLY|MDB_NOSYNC|MDB_NOMETASYNC)) {
 			env->me_mfd = env->me_fd;
 		} else {
 			/* synchronous fd for meta writes */
@@ -6096,7 +6096,7 @@ mdb_put(MDB_txn *txn, MDB_dbi dbi,
  *	at runtime. Changing other flags requires closing the environment
  *	and re-opening it with the new flags.
  */
-#define	CHANGEABLE	(MDB_NOSYNC)
+#define	CHANGEABLE	(MDB_NOSYNC|MDB_NOMETASYNC)
 int
 mdb_env_set_flags(MDB_env *env, unsigned int flag, int onoff)
 {

libraries/libmdb/mdb.h

@@ -159,6 +159,8 @@ typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *rel
 #define MDB_NOSYNC		0x10000
 	/** read only */
 #define MDB_RDONLY		0x20000
+	/** don't fsync metapage after commit */
+#define MDB_NOMETASYNC		0x40000
 /** @} */
 
 /**	@defgroup	mdb_open	Database Flags
@@ -334,6 +336,13 @@ int  mdb_env_create(MDB_env **env);
 	 *		at risk is governed by how often the system flushes dirty buffers to disk
 	 *		and how often #mdb_env_sync() is called. This flag may be changed
 	 *		at any time using #mdb_env_set_flags().
+	 *	<li>#MDB_NOMETASYNC
+	 *		Don't perform a synchronous flush of the meta page after committing
+	 *		a transaction. This is similar to the #MDB_NOSYNC case, but safer
+	 *		because the transaction data is still flushed. The meta page for any
+	 *		transaction N will be flushed by the data flush of transaction N+1.
+	 *		In case of a system crash, the last committed transaction may be
+	 *		lost. This flag may be changed at any time using #mdb_env_set_flags().
 	 *	<li>#MDB_RDONLY
 	 *		Open the environment in read-only mode. No write operations will be allowed.
 	 * </ul>
@@ -392,8 +401,7 @@ void mdb_env_close(MDB_env *env);
 	/** @brief Set environment flags.
 	 *
 	 * This may be used to set some flags that weren't already set during
-	 * #mdb_env_open(), or to unset these flags. Currently only the
+	 * #mdb_env_open(), or to unset these flags.
-	 * #MDB_NOSYNC flag setting may be changed with this function.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] flags The flags to change, bitwise OR'ed together
 	 * @param[in] onoff A non-zero value sets the flags, zero clears them.