RAMDirectory doesn't win over FSDirectory all the time, why?

[zhoucheng2008]

I read the lucene in action book and just tested the
FSversusRAMDirectoryTest.java with the following uncommented:

 

    //    /**

    //    // change to adjust performance of indexing with FSDirectory

        writer.mergeFactor = 100;

        writer.maxMergeDocs = 999999;

        writer.minMergeDocs = 1000;

//    */

 

Here is the output:

RAMDirectory Time: 805 ms

FSDirectory Time : 728 ms