Benchmarking results, questions

I recently acquired a new (used) machine to be used as a Postgres
server: Compaq DL380 G2 (2 x 1.44ghz, 1gb RAM) with 6 18gb 10k rpm
drives and a 32mb Smart Array 5i RAID controller. It's a clean install
of CentOS-4.2 with non-critical daemons disabled. After reading some
comments by Josh in the archives about the particularly poor performance
of 3-disk RAID-5 with PG, I ran some benchmarks today with pgbench with
1, 2, 5 and 10 concurrent connections. I dropped the testdb and
recreated it each time before running the test including reinitializing
it with pgbench (scaling set to 10). Each test ran 500 transactions 3
different times and the results were averaged. This is my first time
benchmarking postgres and I'm trying to be reasonably methodical.

My original plan was to run a 2-disk RAID-1 for the OS, a 3-disk RAID-5
and have a hot spare handy to minimize emergency trips to the colo. I
also considered 3 RAID-1 mirrors. Here are my baseline transactions per
second with the pg_xlog and data all on the 3-disk raid-5, ext3 mounted
with default settings:

c=1 c=2 c=5 c=10
79.6 68.6 74.0 74.4

It's too bad that postmaster -F doesn't seem to be reliable enough
because those same settings without sync were able to push through 620
tps with 2 users. Mounting ext3 with data=writeback was an improvement,
but still not great:

90.1 106.2 151.9 113.5

data=journal was predictably slow:

30.1 28.1 32.9 34.2

The next big change was keeping my data on the 3-disk RAID-5 and moving
the transaction log to the 2-disk RAID-1 for the OS. I'll only cover
the writeback case since it's the best (50% improvement):

c=1 c=2 c=5 c=10
137.6 150.1 251.1 160.5

I scrapped the RAID-5 and created 2 RAID-1 mirrors for a total of 3
RAID-1s. First I tried PG like this with everything on one array
(translog and data) and then on separate, dedicated RAID-1 arrays:

on one array:
127.1 152.1 258.5 190.8

on separate arrays:
146.8 155.6 227.2 198.2

I find the fairly small margin of difference between everything on one
RAID-1 and across two RAID-1s to be a bit surprising. It's oddly
close/slower/faster than the RAID5+RAID1 setup. Herein lies my questions:

* Why the closeness between RAID-1 with transaction log on the same
array vs. on a separate array? Not running enough transactions to see
the difference?

* Does it seem probable that in real-world use the RAID5 data + RAID1
transaction log would not perform as well as in this test? Having a hot
spare standby isn't such a bad thing if the trade-off is 3-5%.

* These are all with a default postgresql.conf. My plan was to test it
out of the box to determine filesystem/array setup first and then tune
the configuration after that. Does this approach make sense or am I
potentially masking some underlying issues?

I read an (oldish from Bill Moran) filesystem comparison with 7.x and it
seems like JFS was a speed demon (not done with pgbench):
<http://www.potentialtech.com/wmoran/postgresql.php#results>.
Unfortunately CentOS doesn't come with reiser, JFS or XFS by default.

Based on these results, I might try a 4-disk RAID-10 for the data and
put the transaction log on the OS RAID-1 to get minimal striping for the
data, although in real-world use, the OS disk would presumably be busier
and it may reduce some of the performance.

I know tuning is moot if you don't design the database well or properly
index but with a fresh server, it seemed like a good opportunity to
learn a thing or two. Any constructive input is appreciated.

Cheers,

Brian