Cassandra write performance test (Why cassandra's write is so slow)

You know, one of the biggest advantage of Cassandra is the high writing speed. So I wrote a test on a single Cassandra node to see what's happening. What I found is reaaaaaaaally out of my expect. Based on test result, the write speed of Cassandra is much slower than Oracle. I don't know why.

Oracle and Cassandra are both running on the same server and same disk.

In Cassandra, each row has 20 columns. The row data is like:
cassandra> get Keyspace1.Standard2['key1']                   
=> (column=name9, value=value19, timestamp=1279794828940)    
=> (column=name8, value=value18, timestamp=1279794828940)    
=> (column=name7, value=value17, timestamp=1279794828940)    
=> (column=name6, value=value16, timestamp=1279794828940)    
=> (column=name5, value=value15, timestamp=1279794828940)    
=> (column=name4, value=value14, timestamp=1279794828940)    
=> (column=name3, value=value13, timestamp=1279794828940)    
=> (column=name20, value=value120, timestamp=1279794828941)  
=> (column=name2, value=value12, timestamp=1279794828940)    
=> (column=name19, value=value119, timestamp=1279794828941)  
=> (column=name18, value=value118, timestamp=1279794828941)  
=> (column=name17, value=value117, timestamp=1279794828941)  
=> (column=name16, value=value116, timestamp=1279794828941)  
=> (column=name15, value=value115, timestamp=1279794828941)  
=> (column=name14, value=value114, timestamp=1279794828941)  
=> (column=name13, value=value113, timestamp=1279794828941)  
=> (column=name12, value=value112, timestamp=1279794828941)  
=> (column=name11, value=value111, timestamp=1279794828940)  
=> (column=name10, value=value110, timestamp=1279794828940)  
=> (column=name1, value=value11, timestamp=1279794828903)    

I use Java Thrift API batch_insert to insert rows. You can download my code from the attachments. The Cassandra test result is:

Number of Thread	rows per-thread	Time esapsed	Rows per-second
1	2000	8927 ms	224
1	5000	13854 ms	360
1	10000	20286 ms	492
1	20000	31974 ms	625
1	50000	62965 ms	794
5	1000	12401	403
5	2000	17571 ms	569
5	4000	26202 ms	763
5	10000	50364 ms	992
10	1000	22200 ms	450
10	2000	32712 ms	611
10	5000	65625 ms	761

The Oracle test result is:

50000 rows are inserted
JDBC batch insert speed: 4000 rows persecond
JDBC　single isnert speed: 330 row　persecond

Compare the test result above, it's obviously that the write speed of Oracle is much faster than Cassandra . But everybody is talking the Cassandra is very fast... I am really confused. Did I make some mistake about Cassandra setting? I only changed the path of data and commit log.

Can anybody help me?