bulkdata: Build up rows and write to disk all at once

nilmdb/server/bulkdata.py

@@ -221,9 +221,11 @@ class File(object):
         # An optimized verison of append, to avoid flushing the file
         # and resizing the mmap after each data point.
         try:
+            rows = []
             for i in xrange(count):
                 row = dataiter.next()
-                self._f.write(packer(*row))
+                rows.append(packer(*row))
+            self._f.write("".join(rows))
         finally:
             self._f.flush()
             self.size = self._f.tell()