WAL performance with wal_sync_method = open_sync

Hi,

I'm playing with wal parameters and found that wal_sync_method =
open_sync enormously enhance the performance on my machine. Without it
(using default fsync) I got only 90 tps at the best using pgbench (-s
2). However if I set wal_sync_method = open_sync, I get ~200 tps. I
have checked PostgreSQL uses O_SYNC flag when it opens WAL log files
using strace. Can anybody tell me why? I am afraid this is just a
dream:-) Linux kernel 2.2.17.
--
Tatsuo Ishii