What`s wrong with JFS configuration?

Hello!

I have strange situation. I`m testing performance of PostgreSQL database
at different filesystems (ext2,ex3,jfs) and I cant say that JFS is as
much faster as it is said.
My test look`s like that:

Server: 2 x Xeon 2,4GHz 2GB ram 8 x HDD SCSI configured in RAID arrays
like that:

Unit UnitType Status %Cmpl Stripe Size(GB) Cache AVerify
IgnECC
------------------------------------------------------------------------------
u0 RAID-10 OK - 64K 467.522 ON - -
u6 RAID-1 OK - - 298.09 ON - -

Port Status Unit Size Blocks Serial
---------------------------------------------------------------
p0 OK u0 233.76 GB 490234752 Y634Y1DE
p1 OK u0 233.76 GB 490234752 Y636TR9E
p2 OK u0 233.76 GB 490234752 Y64VZF1E
p3 OK u0 233.76 GB 490234752 Y64G8HRE
p4 NOT-PRESENT - - - -
p5 OK - 233.76 GB 490234752 Y63YMSNE
p6 OK u6 298.09 GB 625142448 3QF08HFF
p7 OK u6 298.09 GB 625142448 3QF08HHW

where u6 stores Fedora Core 6 operating system, and u0 stores 3
partitions with ext2, ext3 and jfs filesystem.
Postgresql 8.2 engine is intalled at system partition (u6 in raid) and
run with data directory at diffrent FS partition for particular test.
To test I use pgBench with default database schema, run for 25, 50, 75
users at one time. Every test I run 5 time to take average.
Unfortunetly my result shows that ext is fastest, ext3 and jfs are very
simillar. I can understand that ext2 without jurnaling is faster than
ext3, it is said that jfs is 40 - 60% faster. I cant see the difference.
Part of My results: (transaction type | scaling factor | num of clients
| tpl | num on transactions | tps including connection time | tps
excliding connection time)

EXT2:

TPC-B (sort of),50,75,13,975|975,338.286682,358.855582
TPC-B (sort of),50,75,133,9975|9975,126.777438,127.023687
TPC-B (sort of),50,75,1333,99975|99975,125.612325,125.636193

EXT3:

TPC-B (sort of),50,75,13,975|975,226.139237,244.619009
TPC-B (sort of),50,75,133,9975|9975,88.678922,88.935371
TPC-B (sort of),50,75,1333,99975|99975,79.126892,79.147423

JFS:

TPC-B (sort of),50,75,13,975|975,235.626369,255.863271
TPC-B (sort of),50,75,133,9975|9975,88.408323,88.664584
TPC-B (sort of),50,75,1333,99975|99975,81.003394,81.024297

Can anyone tell me what`s wrong with my test? Or maybe it is normal?

Pawel Gruszczynski