Q:

mesh lines appear in quick rendering

I noticed in my last 2 blender files, that when i use subdivision surface, unwanted lines appear in quick rendering what is this? how can i fix that?

A:

The problem is only present in orthographic view. Changing to perspective or a camera view will fix this problem.
It's also why the issue isn't present in the final render, because most of the time you will be rendering with a perspective camera. You can actually zoom into any mesh in orthographic view and small cracks will start to appear on the mesh.
It is a known issue, with the known issue page stating:

When using Instancing, Motion Blur or Dynamic BVH, self-intersection may occur due to precision issues. Switching to Static BVH may avoid some problems but not all

There are several bug reports mentioning that this might be improved in future here and here

