@visualzhou , when I apply your pull request to master (commit aff6a0bba39634759e24dfe116a7e393a79e6baf) and run this test, I get the following failure. Is this expected?

```
2014-02-10T10:47:41.705-0500 TypeError: Cannot call method 'hasWriteErrors' of undefined at /data02/workspace/mongo/jstests/core/opcounters.js:27
failed to load: /data02/workspace/mongo/jstests/core/opcounters.js
```