driver-legacy/src/main/com/mongodb/Mongo.java
@Deprecated
    public CommandResult fsync(final boolean async) {
        DBObject command = new BasicDBObject("fsync", 1);
        if (async) {
            command.put("async", 1);
        }
        return getDB(ADMIN_DATABASE_NAME).command(command);
    }
@Deprecated
    public CommandResult fsync(final boolean async) {
        DBObject command = new BasicDBObject("fsync", 1);
        if (async) {
            command.put("async", 1);
        }
        return getDB(ADMIN_DATABASE_NAME).command(command);
    }
Forces the master server to fsync the RAM data to disk This is done automatically by the server at intervals, but can be forced for
better reliability.

@param async if true, the fsync will be done asynchronously on the server.
@return result of the command execution
@throws MongoException if there's a failure
@mongodb.driver.manual reference/command/fsync/ fsync command