Another data point.
We had some down time on our system today to complete some maintenance
work. It took the opportunity to rebuild the 700GB file system using
XFS instead of Reiser.
One iostat output for 30 seconds is
avg-cpu: %user %nice %sys %iowait %idle
1.58 0.00 19.69 31.94 46.78
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdd 343.73 175035.73 277.55 5251072 8326
while doing a select count(1) on the same large table as before.
Subsequent iostat output all showed that this data rate was being
maintained. The system is otherwise mostly idle during this measurement.
The sequential read rate is 175MB/s. The system is the same as earlier,
one cpu is idle and the second is ~40% busy doing the scan and ~60%
idle. This is postgresql 8.1rc1, 32KB block size. No tuning except
for using a 1024KB read ahead.
The peak speed of the attached storage is 200MB/s (a 2Gb/s fiber channel
controller). I see no reason why this configuration wouldn't generate
higher IO rates if a faster IO connection were available.
Can you explain again why you think there's an IO ceiling of 120MB/s
because I really don't understand?
-- Alan