 shared buffers appears to be
      insufficient, the background writer writes some dirty buffers to the
      file system and marks them as clean.  This reduces the likelihood
      that server processes handling user queries will be unable to find
      clean buffers and have to write dirty buffers themselves.
      However, the background writer does cause a net overall
      increase in I/O load, because while a repeatedly-dirtied page might
      otherwise be written only once per checkpoint interval, 