Thanks for your  reply!
//Were all of those 20 nodes running real hardware (i.e. NOT VMs)?
Yes, there are 20 real servers running in the cluster, and one Casssandra instance runs on each server.
//Did your driver application(s) run on "real" hardware and how many threads did you use?
The clients run on one server of the 20 servers, I used 10 threads to run the write and read tasks. How many threads can make Cassandra get good throughput?
 Thanks!
2010/5/23 Mark Robson <markxr@gmail.com>
On 23 May 2010 13:42, ʷӢ <shiyingjie1983@gmail.com> wrote:
Hi, All
   I am now doing some tests on Cassandra, and I found that both writes and reads on 15 nodes are faster than that of 20 nodes, how many servers does one Cassandra system contains during the real applications?
   Thanks a lot !
                                                                           Yingjie

I'd ask

Were all of those 20 nodes running real hardware (i.e. NOT VMs)?

and

Did your driver application(s) run on "real" hardware and how many threads did you use?

Cassandra can only get good throughput with a lot of client threads, not just a few.

Mark