dbt2 performance

Hi,

I have a couple of questions about dbt2 performance.

1. I tested dbt2+postgresql 8.4.2 on my server, but the NOTPM is around only
320~390 with 10 connections and 30 warehouses. Increasing the number of
connections did not improve the throughput? The NOPTM number does not seem
very high to me. Should I try more configurations to see if it can be
improved? Are there any numbers I can compare with (NOPTM and response
time)?

2. Moreover, the disk utilization was high and the "await" time from iostat
is around 500 ms. Could disk I/O limit the overall throughput? The server
has 2 SATA disks, one for system and postgresql and the other is dedicated
to logging (pg_xlog). As far as I understand, modern database systems should
be CPU-bound rather than I/O-bound, is it because I did not perform adequate
performance tuning?

3. From "vmstat", the cpus spent around 72% of time idle, 25% waiting for
I/O, and only 2~3% left doing real work. I was surprised that the cpu
utilization was so low. Is that normal or could it be due to
misconfiguration? In my opinion, even if disk I/O may have been stressed,
70% of idle time was still too high.

Below are some specs/configurations that I used. Any suggestion is welcome.
Thanks!

server spec:
4 cores (2*Dual-Core AMD Opteron, 800MHz), 12GB ram
2 SATA disks, one for system and postgresql and the other is dedicated to
logging (pg_xlog)

postgres configuration:
30 warehouses
256MB shared_buffer
768MB effective_cache_size
checkpoint_timeout 1hr (All my tests are within 10 minutes interval, so
checkpointing should not interfere the performance)
I turned off fsync to see whether the performance could be improved.

Yu-Ju